*FM 3-6 ● *AFM 105-7 ● FMFM 7-11-H ●

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Preface

Primary users of this manual are NBC staff officers, staff weather officers, firesupport coordination personnel, artillery officers, and others involved in planningNBC operations. These soldiers must understand what effect weather and terrainhave on nuclear, biological, and chemical (NBC) operations and smoke. This manualcontains general information and the basic principles on how to get the best results.Commanders and staffs involved in planning for use of incendiaries or smokeoperations will also benefit from the use of this manual along with other referencessuch as FM 3-50, FM 3-100, FM 3-3, FM 3-4, and FM 3-5.

On the battlefield, the influences of weather and terrain on NBC operationsprovide opportunities to both sides. To retain the initiative, friendly forces leaders andstaff officers must understand how weather and terrain can be used to theiradvantage.

FM 3-6 implements International Standardization Agreement (STANAG) 2103,Reporting Nuclear Detonations, Radioactive Fallout, and Biological and ChemicalAttacks and Predicting Associated Hazards.

This manual explains how weather and terrain influence nuclear, biological, andchemical operations and discusses the following topics for use when planningoperations:

Basic principles of meteorology as they pertain to NBC operations.Influence of weather on the use and behavior of NBC agents.Local weather predictions and their use.Influence of terrain on the behavior of NBC agents.US Air Force Air Weather Service (AWS) forecasts and their use in planning for

operations in an NBC environment. (The Navy gets meteorological forecasts fromcomponents of the Naval Oceanography Command. Meteorological reportinformation is in the NAVOCEANCOMINST 3140.1 publications series. It alsocontains information on the behavior of smoke clouds and incendiaries. In addition, itdiscusses the influences of weather and terrain on the thermal, blast, and radiationeffects of a nuclear detonation.)

Staffs planning the use of chemical weapons and commanders approving strikesmust understand basic weather characteristics. Therefore, weather analysessignificantly influence the selection of agents and munitions for employment. Thetarget analyst must know his or her weather data needs and where to get thisinformation in a combat environment. Chapter 1 covers meteorology and the impact

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of weather on chemical agent use. The remaining chapters address the impact ofweather on smoke, incendiaries, biological agents, and nuclear detonations.

Users of this publication are encouraged to recommend changes and submitcomments for its improvement. Key each comment to the specific page and paragraphin which the change is recommended. Provide a reason for each comment to ensureunderstanding and complete evaluation. To send changes or comments, prepare DAForm 2028 (Recommended Changes to Publications and Blank Forms) and forward itto Commandant, USACMLS, ATTN: ATZN-CM-NF, Fort McClellan, AL 36205-5020.Air Force comments go to HQ USAF/XOORF, Washington, DC 20330. Marine Corpscomments go to Commanding General, Marine Corps Development and EducationCommand (CO93), Quantico, VA 22134. Navy comments go to Chief of NavalOperations (OP-954), Navy Department, Washington, DC 20350.

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The fielddependent ontemperature,precipitation.

CHAPTER 1

Chemical Agents

behavior of chemical agents is understand the impact of chemical agents on theweather variables such as wind, battlefield, the soldier must also understand howair stability, humidity, and these agents are affected by weather and terrain.The influence of each variable The following paragraphs give an overview of the

depends upon the synoptic situation and is locally basic characteristics of chemical agents and howinfluenced by topography, vegetation, and soil. weather and terrain influence and have specific

Chemical agents may appear in the field in effects on them.different forms: vapors, aerosols, or liquids. To

Basic CharacteristicsVapors and small particles are carried by the

winds, while any large particles and liquid dropsfall out in a ballistic-like trajectory and are quicklydeposited on the ground. Many agents give offvapors that form vapor clouds. The speed at whichan agent gives off vapors is called volatility.Agents may be removed naturally from the air byfalling out (large particles fall out much morequickly), by sticking to the ground or vegetation, orby being removed by precipitation. Once depositedupon vegetation or other ground cover, volatileagents may be re-released to the atmosphere forfurther cycles of travel and present a hazard untilsufficiently diluted or decontaminated.

During approximately the first 30 seconds, thesize and travel of an agent are determinedprimarily by the functioning characteristic of themunition or delivery system. Thereafter, the traveland diffusion of the agent cloud are determinedprimarily by weather and terrain. For example, inhigh temperatures, volatile agents producemaximum agent vapor in 15 seconds. Light windsand low turbulence allow high localconcentrations of agents. High winds and strongturbulence reduce the concentration and increasethe area coverage by more quickly carrying awayand diffusing the agent cloud.

VaporsWhen a chemical agent is disseminated as a

vapor from a bursting munition, initially the cloud

expands, grows cooler and heavier, and tends toretain its form. The height to which the cloud rises,due to its buoyancy, is called the height of thethermally stabilized cloud. If the vapor density ofthe released agent is less than the vapor density ofair, the cloud rises quite rapidly, mixes with thesurrounding air, and dilutes rapidly. If the agentforms a dense gas (the vapor density of thereleased agent is greater than the vapor density ofair), the cloud flattens, sinks, and flows over theearth’s surface. Generally, cloud growth duringthe first 30 seconds is more dependent upon themunition or delivery system than uponsurrounding meteorological conditions.

Nevertheless, the height to which the cloudeventually rises depends upon air temperature andturbulence. These determine how much cooler,ambient air is pulled into the hot cloud (and, hence,determines its rate of cooling). The agentconcentration buildup is influenced by both theamount and speed of agent release and by existingmeteorological conditions.

Shortly after release, the agent cloud assumesthe temperature of the surrounding air and movesin the direction and at the speed of the surroundingair. The chemical cloud is subjected to turbulenceforces of the air, which tend to stretch it, tear itapart, and dilute it. The heavier the agent, thelonger the cloud retains its integrity. Underconditions of low turbulence, the chemical agentcloud travels great distances with little decrease inagent vapor concentration. As turbulence

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increases, the agent cloud dilutes or dissipatesfaster.

AerosolsAerosols are finely divided liquid and/or solid

substances suspended in the atmosphere.Sometimes dissolved gases are also present in theliquids in the aerosols. Chemical agent aerosolclouds can be generated by thermal munitions andaerosol spray devices or as by-products of liquidspray devices and bursting munitions.

Airborne aerosols behave in much the samemanner as vaporized agents. Initially, aerosolclouds formed from thermal generators have ahigher temperature than clouds formed from othertypes of munitions. This may cause some initialrise of the cloud at the release point. Aerosol-generated clouds are heavier than vapor clouds,and they tend to retain their forms and settle backto earth. Being heavier than vapor clouds, they areinfluenced less by turbulence. However, as theclouds travel downwind, gravity settles out thelarger, heavier particles. Many particles stick toleaves and other vegetative surfaces they contact.

LiquidsWhen a chemical agent is used for its liquid

effect, evaporation causes the agent to form intovapor. Depending upon volatility, vapor clouds areusually of low concentration, have about the sametemperature as the surrounding air, and tend tostay near the surface because of high vapordensity. Additionally, vapor density governs theextent that the vapor will mix with the air. Liquidagents with high vapor density impact at groundlevel with very little evaporation of the agent.These agents are termed persistent agents. Whiledrops are airborne, and after impacting, the liquidcontinues to evaporate. Agent vapor pressure willgovern the rate at which the liquid will evaporateat a given temperature and pressure. Initialconcentrations are lower, since the vapor source isnot instantaneous as a vapor agent is but evolvesover a long period (until the liquid source is gone).Liquid agents may be absorbed (soaked into asurface) and adsorbed (adhered to a surface), andthey may also evaporate. Once the liquid is nolonger present on the surface, desorption (goingback into the air) begins. The vapor concentrationover areas contaminated with a liquid agent tends

to be less than with newly formed vapor clouds,and downwind agent concentrations are notnearly as great as with other types of agents.

Atmospheric StabilityOne of the key factors in using chemical

weapons is the determination of the atmosphericstability condition that will exist at the time ofattack. This determination can be made from ameteorological report or by observing fieldconditions.

When a meteorological report is available, itshould contain a description of the current orprojected atmospheric stability condition. If thedata given are based on an atmosphericdescription, Figure 1-1 may be used to convert thedata into traditional atmospheric stabilitycategories/conditions. When meteorologicalreports are not readily available, the stabilitycondition can be derived by using the stabilitydecision tree shown in Figure 1-2. Figure 1-2 isentered at the top with the current observedweather conditions (or estimated weatherconditions). Follow the decision tree to determinethe stability condition. The stability conditionplus the wind speed indicates the dispersioncategory of an agent vapor cloud.

Unstable conditions will cause lowerconcentrations and/or poorer target coverage.Stable conditions will cause greater agent stabilityand higher concentrations. Use Figure 1-2 asguidance for employing an agent by starting inupper left corner at the word START. Followarrowed line to the first question. Answerquestion “Is it nighttime?” by selecting,

thethethein

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accordance with the facts, the yes or no arrowindicating your decision. At each branch in thearrows, follow the arrow most nearly correct forthe conditions under which the stability categoryis required. As questions are encountered alongyour path, answer each and proceed along themost nearly correct path until a dispersioncategory is identified. The result from Figure 1-2 isthe stability category. An example of the use ofFigure 1-2 is if you are inland one hour beforesunset and the winds are calm, the stabilitycategory is neutral (N) (category 4).

The dispersion category, the wind speed inknots, and the wind direction are the mostimportant meteorological data for deciding theinfluence of weather on vapor cloud dispersion.For any given dispersion category, a lower windspeed will produce higher dosages, smaller areacoverage, and, consequently, higher toxic effects.This is because when the wind speed is lower, thecloud moves more slowly past the individual in thetarget area; and the individual is in the cloudlonger, yielding a higher dose of the agent. SeeTable 1-1 for the dispersion categories and windspeeds during which atmospheric conditions areeither generally favorable, marginal, orunfavorable for employment of chemical agents.Factors such as agent toxicity, targetvulnerability, and the amount of the agentreleased will determine the actual doses,casualties, and other effects. Elevated agentreleases will alter the table results somewhat, butthe same trends occur. The main effect to beconsidered for elevated release effectiveness over aspecific target is that the agent must be releasedfurther upwind to compensate for the drift as theagent comes down.

Table 1-1 is a general reference tool to providean estimate, based on dispersion category andwind speed, when it would generally be mosteffective to employ a chemical agent vapor. Table1-2 indicates the typical cloud widths at givendownwind distances from a point source releasefor a chemical agent vapor cloud. Note that thecloud width depends upon dispersion category andnot directly upon wind speed. The cloud widthdistances represented in Table 1-2 are the dosagecontours for 0.01 milligram-minutes per cubicmeter (mg-min\M³). If the agent is released from aline source (spray system), the line length shouldbe added to the cloud width (Table 1-2) to determine

total cloud width for travel distances up to 1kilometer. For longer travel distances, the lengthof the line source loses its importance (due todissipation), and the total cloud width isrepresented by the values in Table 1-2. Thechemical cloud widths listed in Table 1-2 areestimates. The widths will vary depending on theweather and terrain of a specific area.

The following examples are cited to explainfurther the use of Table 1-2. Based on a chemicalagent vapor being released from a point source indispersion category 4, the chemical cloud width at7 kilometers downwind would be approximately2.3 kilometers. Based on a chemical agent vaporbeing released from a line source that is 0.1kilometer in length (dispersion category 2), thechemical cloud width at a 0.5 kilometer downwinddistance would be .850 kilometer (0.75+0.1).

Table 1-3 presents the relative center linedosages (mg-min/M³) at different distancesdownwind for different dispersion categories andwind speeds. Remember, low wind speeds at thesame dispersion category give higher dosages. Thedosages listed in Table 1-3 are estimates and willvary depending on the estimated category andwind speed in the target area. The dosage values inTable 1-3 are based on 100 kilograms of thenonpersistent nerve agent (GB) being released atground level from a point source.

The information reflected in Table 1-3 is thedosage that would be incurred if the target werestationary. The dosage would decrease if the targetwere moving through the downwind cloud hazardarea. Additionally, in general, if the sourcestrength (100 kg) were doubled, the dosage wouldalso double, and if the source strength were halved,the dosage would also decrease approximatelyone-half.

To aid in using Table 1-3, the followingexample is provided. With dispersion category 4,wind speed 8 knots, and a downwind distance of 2kilometers, the center line dosage would be 18.91mg-min/M³. With dispersion category 2, windspeed 3 knots, and at a downwind distance of 4kilometers, the center line dosage would be 1.030mg-min\M³.

Vapor Concentration and DiffusionAgent concentration is governed by the

volume of the agent cloud. Since clouds

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continually expand, agent concentration levelsdecrease over time. Wind speed determines thedownwind growth of the cloud. Vertical andhorizontal turbulence determines the height andwidth of the cloud. The rate at which thedownwind, vertical, and horizontal componentsexpand governs the cloud volume and the agentconcentration.

To be effective the agent cloud, at a specificconcentration level, must remain in the target areafor a definite period. Wind in the target area mixesthe agent and distributes it over the target afterrelease. For ground targets, high concentrationsand good coverage can best be achieved with lowturbulence and calm winds when the agent is

delivered directly on target. A steady, predictablewind drift over the target is best when the agent isdelivered on the upwind side of the target.Conditions other than these tend to produce lowerconcentrations and/or poorer target coverage.However, unless weather conditions are knownwithin the target area, the effects of the agent ontarget will be approximations.

The concentration and diffusion of a chemicalagent cloud are also influenced by the factors ofhydrolysis, absorption, adsorption, lateral spread,drag effect, and vertical rise.

Hydrolysis is the process of the agent reactingwith water vapor in the air. It does not influencemost agent clouds in tactical use because the rate

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of hydrolysis is too slow. However, hydrolysis canbe important for smoke screens. See the discussionof the effect of humidity on increasing smokescreen effectiveness in Chapter 2.

Absorption is the process of the agent beingtaken into the vegetation, skin, soil, or material.Adsorption is the adding of a thin layer of agent tovegetation or other surfaces. This is important indense vegetation. Both absorption and adsorptionof chemical agents may kill vegetation, thusdefoliating the area of employment.

When a chemical cloud is released into the air,shifting air currents and horizontal turbulenceblow it from side to side. The side-to-side motion ofthe air is called meandering. While the agent cloudmeanders, it also spreads laterally. Lateralspreading is called lateral diffusion. Figure 1-3shows a cloud with lateral spread andmeandering. Table 1-2 indicates the amount oflateral spread that occurs under differentdispersion categories and distances downwind. Inmore unstable conditions, the lateral spread tendsto be greater than in stable conditions.

Wind currents carry chemical clouds along theground with a rolling motion. This is caused by the

differences in wind velocity. Wind speeds increaserapidly from near zero at the ground to higherspeeds at higher elevations above the ground. Thedrag effect by the ground, together with theinterference of vegetation and other groundobjects, causes the base of an agent cloud to beretarded as the cloud stretches out in length. Whenclouds are released on the ground, the dragamounts to about 10 percent of the vertical growthover distance traveled over grass, plowed land, orwater. It amounts to about 20 percent over gentlyrolling terrain covered with bushes, growingcrops, or small patches of scattered timber. Inheavy woods, the drag effect is greatly increased.The vertical spread of the cloud is illustrated inFigure 1-4.

Wind speeds can vary at different heights. Thewind direction can also change with an increase inheight. This is known as wind shear. Because ofwind shear, a puff (or chemical cloud) may becomestretched in the downwind direction and maytravel in a direction different from that of thesurface wind. Additionally, a chemical cloudreleased in the air may be carried along faster thanit can diffuse downward. As a result, air near the

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ground on the forward edge of the cloud may beuncontaminated, while the air a few feet up may beheavily contaminated. This layering effectbecomes more pronounced and increasesproportionately with the distance of the forwardedge of the cloud from the source. Figure 1-5illustrates this. A small puff of agent cloudreleased from its source some time earlier has tiltedforward, while the bottom has been retarded due toslower winds caused by drag.

The vertical rise of a chemical cloud dependsupon weather variables, such as temperaturegradient, wind speed, and turbulence, and the

Vapors andWind, temperature, humidity, precipitation,

terrain contours, and surface cover influence thefield behavior of vapors and aerosols. Forexample, in a chemical attack on US forces (lstDivision) 26 February 1918 in the Ansauvillesection, extremely stable conditions, calm winds,

difference between the densities of the clouds andthe surrounding air. As mentioned earlier, thetemperature of both the cloud and the airinfluences their relative densities. Hotter gases areless dense and, therefore, lighter than cooler gasesand air. Therefore, they rise until they are mixedand somewhat diluted and attain the sametemperature and approximately the same densityas surrounding air.

The vapor cloud formed by an agent normallyemployed for persistent effect rises in a similarmanner, but vapor concentrations build up moregradually.

Aerosolsand heavy underbrush in the target areacontributed to the overall effectiveness of achemical attack. Several additional casualtiesresulted due to the increased chemical agentpersistency caused by the favorable weatherconditions. Favorable and unfavorable weather

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and terrain conditions for tactical employment ofa chemical aerosol or vapor cloud are summarizedin Table 1-4.

If a chemical cloud is to be placed directly onan occupied area, the best possible weatherconditions are calm winds with a strong, stabletemperature gradient. Under these conditions, thecloud diffuses over the target with minimumdilution and does not move away. Such conditionsare most apt to occur on a calm, clear night. If asmall amount of air movement is required tospread the cloud evenly over the target area, a lowwind speed and stable or neutral conditions aremost favorable. These conditions most often occuron a clear night, a cloudy night, or a cloudy day.

When the desired effect is for the chemicalcloud to travel, the most favorable conditions arestable or neutral conditions with a low to mediumwind speed of 3 to 7 knots. These conditions may bepresent on a clear night, a cloudy night, or a cloudyday. The presence of low to medium wind speedskeeps the cloud traveling over the area without toomuch diffusion, and the stable or neutralconditions keep the agent concentration high andthe cloud close to the ground.

Favorable terrain conditions for a chemicalcloud are smooth or gently rolling contours orwooded areas. Unfavorable conditions forchemical clouds (usually found on clear days) areextreme or marked turbulence, wind speeds above10 knots, an unstable dispersion category, rain,and rough terrain.

WindHigh wind speeds cause rapid dispersion of

vapors or aerosols, thereby decreasing effectivecoverage of the target area and time of exposure tothe agent. In high winds, larger quantities ofmunitions are required to ensure effectiveconcentrations. Agent clouds are most effectivewhen wind speeds are less than 4 knots and steadyin direction. The clouds move with the prevailingwind as altered by terrain and vegetation. Steady,low wind speeds of 3 to 7 knots enhance areacoverage unless an unstable condition exists. Withhigh winds, chemical agents cannot beeconomically employed to achieve casualties. Thechart at Figure 1-2 indicates the effect of wind onstability categories. Tables 1-1, 1-2, and 1-3

indicate the effects of wind and dispersioncategories upon dosage and area coverage.

Unstable conditions, as indicated in Figure 1-2and Tables 1-1, 1-2, and 1-3, are the least favorableconditions. Unstable conditions (such as manyrising and falling air currents and greatturbulence) quickly disperse chemical agents.Unstable is the least favorable condition forchemical agent use because it results in a lowerconcentration, thereby reducing the area affectedby the agent. Many more munitions are required toattain the commander’s objectives under unstableconditions than under stable or neutral conditions.

Stable conditions (such as low wind speedsand slight turbulence) produce the highestconcentrations. Chemical agents remain near theground and may travel for long distances beforebeing dissipated. Stable conditions encourage theagent cloud to remain intact, thus allowing it tocover extremely large areas without diffusion.However, the direction and extent of cloud travelunder stable conditions are not predictable if thereare no dependable local wind data. A very stablecondition is the most favorable condition forachieving a high concentration from a chemicalcloud being dispersed.

Neutral conditions are moderately favorable.With low wind speed and smooth terrain, largeareas may be effectively covered. The neutralcondition occurs at dawn and sunset and generallyis the most predictable. For this reason, a neutraldispersion category is often best from a militarystandpoint.

TemperatureThere will be increased vaporization with

higher temperatures. Also, the rate of evaporationof any remaining liquid agent from an explodingmunition can vary with temperature. Generally,the rate of evaporation increases as thetemperature increases. See FM 3-9/AFR 355-7 forspecific information on chemical agents, such astheir boiling and freezing points and vapordensity.

HumidityHumidity is the measure of the water vapor

content of the air. Hydrolysis is a process in whichcompounds reactchemical change.

with water resulting in aChemical agents with high

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hydrolysis rates are less effective under conditionsof high humidity.

Humidity has little effect on most chemicalagent clouds. Some agents (phosgene and lewisite)hydrolyze quite readily. Hydrolysis causes thesechemical agents to break down and change theirchemical characteristics. If the relative humidityexceeds 70 percent, phosgene and lewisite can notbe employed effectively except for a surprise time-on-target (TOT) attack because of rapidhydrolysis. Lewisite hydrolysis by-products arenot dangerous to the skin; however, they are toxicif taken internally because of the arsenic content.Riot control agent CS (see glossary) alsohydrolyzes, although slowly, in high humidities.High humidity combined with high temperaturesmay increase the effectiveness of some agentsbecause of body perspiration that will absorb theagents and allow for better transfer.

PrecipitationThe overall effect of precipitation is

unfavorable because it is extremely effective inwashing chemical vapors and aerosols from theair, vegetation, and material. Weather forecasts orobservations indicating the presence of orpotential for precipitation present an unfavorableenvironment for employment of chemical agents.

Terrain ContoursTerrain contours influence the flow of

chemical clouds the same as they influenceairflow. Chemical clouds tend to flow over lowrolling terrain and down valleys and settle inhollows and depressions and on low ground. Localwinds coming down valleys at night or up valleysduring the day may deflect the cloud or reverse itsflow. On the other hand, they may produceconditions favorable for chemical cloud travelwhen general area forecasts predict a calm.

A chemical cloud released in a narrow valleysubjected to a mountain breeze retains a highconcentration of agent as it flows down the valley.This is because of minimal lateral spread. Hence,high dosages are obtained in narrow valleys ordepressions. High dosages are difficult to obtainon crests or the sides of ridges or hills. After aheavy rain, the formation of local mountain orvalley winds is sharply reduced. In areas ofadjacent land and water, daytime breezes from the

water and nighttime breezes from the land controlchemical cloud travel.

Surface CoverGround covered with tall grass or brush

retards flow. Obstacles, such as buildings or trees,set up eddies that tend to break up the cloud andcause it to dissipate more rapidly. However, streetcanyons or spaces between buildings may havepockets of high concentrations. Flat country(during a neutral or inversion condition) or openwater promotes an even, steady cloud flow. Figure1-5 illustrates the horizontal and vertical spread ofa cloud over flat country.

The amount and type of vegetation in the areaof the chemical operation also influence the travelof a chemical cloud. Vegetation, as it relates tometeorology or diffusion, is called vegetativecanopy or just canopy. The effects of canopies areconsidered below.

Woods are considered to be trees in full leaf(coniferous or deciduous forests). The term“heavily wooded canopy” denotes jungles orforests with canopies of sufficient density to shademore than 90 percent of the ground surfacebeneath. For chemical operations, areascontaining scattered trees or clumps of bushes areconsidered to be open terrain although drag issomewhat increased. In wooded areas where treesare not in full leaf or where foliage has beendestroyed by previous attack so that sunlightstrikes the ground, the diffusion (stability)category will be similar to those in the open.

When bombs are dropped into a wooded area,some may be expected to burst in the treetops.Although the released aerosol and vapor settletoward the ground, some of the agent is lost,depending upon the thickness and height of thefoliage. The initial burst and pancake areas ofchemical clouds released within woods or junglesare smaller than those released in the open.However, concentrations within the initial cloudsare higher in wooded areas, sometimes three timesthat of bursts in the open. The magnitude ofconcentration from ground bursts depends uponthe density of undergrowth and trees.

Generally, when conditions in the open aremost favorable for the use of chemical agents,conditions also are favorable in heavily woodedareas if dispersion occurs below the canopy. Low

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wind speeds under the canopies spread agentclouds slowly in a downwind and downslopedirection. Areas of dense vegetation also increasethe potential surface area for the deposition ofchemical agents. If there are gullies and streambeds within the woods, clouds tend to follow thesefeatures. This flow may be halted or diverted byupslope winds.

Vegetation absorbs some agents. However, foran attack against troops poorly trained in NBC

defense (where lethal dosages may be obtained in30 seconds or less), the amount of agent absorbedby foliage will have little or no effect on the successof the attack. High concentrations of chemicalagents may destroy vegetation, since the leavesabsorb some of the agent. In some instances, theabsorbed agent may be released or desorbed whenthe vegetation is disturbed or crushed, creating asecondary toxic hazard.

LiquidsWeather, terrain contours, vegetation, soil,

and some other surfaces affect the rate ofevaporation. That, in turn, influences thepersistence of a chemical agent liquid and theconcentration of the vapor. Most weatherconditions do not affect the quantity of munitionsneeded for an effective initial liquidcontamination. Table 1-5 summarizes favorableand unfavorable weather and terrain conditionsfor the employment of a liquid chemical agent.

When a liquid agent is used to cause casualtiesthrough contact with the liquid in crossing oroccupying the area, its duration of effectiveness isgreatest when the soil temperature is just abovethe agent’s freezing point. This limits the rate ofevaporation of the liquid. Other favorableconditions are low wind speed, wooded areas, andno rain.

Conversely, unfavorable conditions are highsoil temperature, high wind speed, bare terrain,and heavy rain.

Favorable and unfavorable conditions forliquid agents for vapor concentration effects aremuch the same as those for chemical clouds. Inwoods, however, a high temperature with only avery light wind gives the highest vaporconcentrations.

WeatherDuration of the effectiveness of initial liquid

contamination may be affected by wind speed;stability, mixing height, and temperature; andprecipitation.

Wind SpeedWind direction

the upwind side of a

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is important in determiningtarget for release purposes but

has little impact on the duration of effectiveness,regardless of the method of release The vaporcreated by evaporation of the liquid agent,however, moves with the wind. Therefore, thevapor concentration is greatest on the downwindside of the contaminated area. Vapors are movedby the wind as discussed earlier in this chapter.

Evaporation due to wind speed depends on theamount of the liquid exposed to the wind (thesurface of the liquid) and the rate at which airpasses over the agent. Therefore, the duration ofeffectiveness is longer at the places of greaterliquid agent contamination and in places wherethe liquid agent is sheltered from the wind.

The rate of evaporation of agents employed forpersistent effect in a liquid state is proportional tothe wind speed. If the speed increases, evaporationincreases, thus shortening the duration ofeffectiveness of the contamination. Increasedevaporation, in turn, creates a larger vapor cloud.The vapor cloud, in turn, is dispersed by higherwinds. The creation and dispersion of vapor are acontinuous process, increasing or decreasing inproportion to wind speed.

Releasing agents for persistent effect by pointdispersal via bombs, shells, rockets, or land minesresults in an unevenly distributed contaminant.Heavier concentrations of the liquid are foundaround the point of burst. Lighter concentrationsresult farther from the bursting position. Thereprobably will be small areas between the points ofburst that are not contaminated, depending uponthe number of munitions used and the uniformityof dispersal.

Liquid agents released in the form of a sprayare fairly evenly distributed, exposing themaximum surface area of the contaminant to thewind. This results in a more rapid evaporation

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than when the liquid agent is unevenly dispersed downwind from the sprayed area.(as with bursting munitions). With spraying, the Some chemical agents have no significantduration of effectiveness decreases, and there is a vapor pressure, and, consequently, their rates ofcorresponding increase in the vapor concentration evaporation are not affected by wind speed. Also,

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some of these agents are extremely toxic, so even avery slight surface concentration represents amassive overkill dosage. When agents of thiscategory are released from spray munitions underlow wind speeds, they cover only a narrow zone.When released under higher wind speeds, theycover wider areas more effectively. Thus, whendownwind safety is not a limiting consideration,high wind speeds may be more desirable than lowwind speeds for these very persistent agents.

With agents that vaporize readily, high windspeeds may cause complete vaporization beforethe agent reaches the ground, creating only avapor hazard. The resulting vapor cloud isnonpersistent and dissipates quite rapidly due tothe high degree of mechanical turbulenceassociated with high wind speeds.

Turbulence has the same effect on agentsemployed for persistent effect, whether releasedfrom bombs, rockets, artillery shells, or landmines. Turbulence tends to reduce the duration ofeffectiveness in the liquid state by helping toincrease the rate of evaporation. Temperature,rather than turbulence, has the greater effect onthe duration of effectiveness of liquid agents.However, a contaminated area that has beensubjected to pronounced turbulence does notremain contaminated as long as one that has beensubjected to only slight turbulence with low windspeeds.

Turbulence also influences the spraying ofagents employed for persistent effect. High windsand air movements divert the drops from thetarget or spread them over a larger area. Steepmountain regions sometimes produce large-scaleeddies that prevent effective coverage of the target.Any vapor concentrations built up from sprayedareas are slight when the degree of turbulence ishigh.

Stability, Mixing Height,and Temperature

Unstable conditions are characterized bywarmer surfaces. The solar heating then causesevaporation to be more rapid.

Temperature, velocity, and turbulence alsoaffect the dispersion of spray. When stable(inversion) conditions prevail, there usually islittle or no thermal turbulence, wind speeds are

low, and the degree of mechanical turbulence isalso low. Often stable conditions exist continuallyonly near the ground. Above the top of the stablesurface layer, wind speed and turbulence areincreased. Wind direction here also may besubstantially different from the surface winddirection. A chemical spray released below the topof the inversion falls fairly quickly. The height ofthe top of an inversion varies throughout theperiod of the surface inversion existence, and itmay vary rapidly over large hills and mountains.

The mixing height is the capping inversion atthe top of the mixing layer and serves as a lid. Itprevents further upward vertical growth of achemical vapor. A mixing height can also existabove unstable or neutral surface stabilityconditions. In radiation inversions, whichcommonly form at night, the top of the surface-based (mixing) stable layer is very close to theearth’s surface shortly after the neutral conditionchanges to a stable condition (soon after sunset).As the surface stable layer intensifies, its top rises,reaching its maximum elevation between 0200 and0400 hours local time. Maximum elevation may be400 meters in a very intense stable layer. In themorning, solar radiation heats the surface andcauses a good mixing condition close to theground. The mixing height and turbulencecondition increase until they destroy the stablelayer. The mixing height can extend from theearth’s surface up to 2 kilometers in elevation on ahot summer day. On a calm, clear night, themixing height may extend only 50 to 100 metersabove the earth’s surface.

If a chemical agent is released above thesurface stable layer, most of the agent remainsaloft in the turbulence layer, and most of it willdissipate before settling low enough to be effective.For this reason, most spray missions are flown ateither sunrise or sunset to take advantage of aneutral temperature gradient. With this gradient,there is some vertical exchange of air, and thechemical spray, being relatively heavy, has anatural tendency to settle to the ground. The AirWeather Service or an assigned meteorologist canprovide information on the mixing height and theheight of the top of the surface stable layer.

Under unstable conditions, convectioncurrents often catch many very small droplets andcarry them upward above the level of release. As aresult, the spray takes longer to reach the ground,

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and much of it may dissipate before reaching thetarget area.

Temperature is one of the most importantfactors affecting the duration of effectiveness andvapor concentration of liquid agents. Agentsemployed for persistent effect acquire thetemperature of the ground and the air they contact.Their evaporation rates are proportional to thevapor pressure at any given temperature. Thetemperature of the ground surface in winter intemperate zones closely follows the airtemperature with a range of only 10 to 20 degreesbetween day and night. In the summer intemperate zones, the surface temperature may bemuch higher than that of the air in the daytimeand much cooler at night. Turbulence usuallyaccompanies a high ground temperature. Theresult is that although the vapor concentration inthe immediate area may be very high, it falls offrapidly a short distance away. Temperature ofvehicles, buildings, and other surfaces may bewarmer. This is because of internal heat sourcesand/or higher solar heating.

From a defensive viewpoint, a dangeroussituation is likely to occur on a summer eveningwhen the ground temperature is still high and astable condition has started to set in. Under theseconditions, a heavy vapor cloud produced byevaporation could be dangerous downwind to adistance of 2,000 meters or more. With ordinaryconcentrations, however, danger from vapor issomewhat less.

Another important temperature factor toconsider is that people perspire freely and wearlightweight clothing in a warm climate. Thus, theyare more susceptible to the action of chemicalagents.

For effective tactical employment of bombs,shells, rockets, and land mines in releasing liquidchemical agents, the actual temperature of theagent itself is vitally important. Generally, liquidagents are not effective when used at temperaturesbelow their freezing points. However, liquid agentscan produce casualties when the frozen particlesthaw.

Humidity has little effect on how long liquidagents are effective. However, high relativehumidity, accompanied by high temperatures,induces body perspiration and, therefore,increases the effectiveness of these agents. Also,permeable protective clothing is less resistant

when sweat-soaked than when dry. Since sweatyskin is more susceptible to the action of vapor,lower vapor dosages produce casualties when thehumidity is high.

PrecipitationLight rains distribute persistent agents more

evenly over a large surface. Since more liquid isthen exposed to the air, the rate of evaporationmay increase and cause higher vaporconcentrations. Precipitation also accelerates thehydrolysis effect. Rains that are heavy or of longduration tend to wash away liquid chemicalagents. These agents may then collect in areaspreviously uncontaminated (such as stream bedsand depressions) and present an unplannedcontamination hazard.

The evaporation rate of a liquid agent reduceswhen the agent is covered with water but returns tonormal when the water is gone. Precipitation mayforce back to the surface some persistent agentsthat have lost their contact effectiveness bysoaking into the soil or other porous surfaces.These agents may again become contact hazards.

Snow acts as a blanket, covering the liquidcontaminant. It lowers the surface temperatureand slows evaporation so that only very low vaporconcentrations form. When the snow melts, thedanger of contamination reappears.

Terrain ContoursTerrain relief has little direct effect on a liquid

agent. However, a slope affects temperatures andwinds, and these influence the evaporation rates ofliquid agents. However, the slope or contour mayaffect the delivery means capable of mostefficiently delivering the agent on an area (forexample, reverse slopes are normally not good forartillery employment, and mountainous terrainmay restrict use of spray tanks).

VegetationWhen persistent agents are used in vegetated

areas, some of the contaminant clings to grass andleaves. This increases the surface agent exposed tothe air and, hence, the rate of evaporation.Personnel become most susceptible to liquidchemical agents in vegetated areas, because theyare more apt to come in contact with the agent by

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brushing against the foliage. Within shadedwoods, however, despite the greater surfacecovered by the liquid chemical agent (because ofthe vegetation), the reduction in surfacetemperature and wind speed increases theduration of effectiveness.

When bombs or shells burst in woods, usuallymost of the liquid falls near enough to the groundto be effective. An exception is bursts in virginforests with dense canopies that may extend to 50meters high.

A thick jungle or forest canopy usuallyprevents liquid agent spray from airplanes fromreaching the ground in quantities sufficient toproduce significant casualties. When stableconditions exist above the forest canopy, however,enough vapor penetrates the canopy to causecasualties.

SoilThe soil on which liquid agents are placed

influences the evaporation rate and the duration ofeffectiveness. Bare, hard ground favors short-termeffectiveness and high-vapor concentration. If thesurface is porous, such as sand, the liquid agentquickly soaks in; and the area no longer appears tobe contaminated.

The rate at which liquid agents evaporatefrom a sandy or porous surface is about 1/3 less

than the evaporation rate from nonabsorbentsurfaces. Extended contact with a contaminatedporous material is dangerous if unprotected.However, if there is no free liquid on the surface,the danger from brief contact is relatively small ifprotected. If a porous surface on which liquidcontamination falls has been wet by rain, thecontaminant does not soak in as readily, and thesurface is initially more dangerous to touch than itwould be if the liquid agent had soaked in. When amustard agent (HD) falls onto a wet surface, itstays in globules; and a thin, oily film spreads overthe surface, making contamination easier todetect.

Other SurfacesPersistence of liquids on painted surfaces of

vehicles is much shorter than on most terrain. Thisis due to a number of factors, including increasedsurface temperature, turbulence of airflow over thevehicles or other equipment, and greater spread ofdrops to give more surface area for evaporation.

Persistence varies greatly with surfacematerial. Absorption, adsorption, and resorptionalso vary with surface material. Rubber absorbsmost agents rapidly and desorbs slowly. Chemicalagent resistant coating (CARC) absorbs very littleagent.

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Smoke

CHAPTER 2

and Incendiaries

Smoke and incendiaries are combat long been employed as a means of concealingmultipliers. Their effective use on a target can battlefield targets. Additionally, incendiary fireprovide tactical advantages for offensive and damage causes casualties and materiel damagedefensive operations. For example, smoke has and can also impact psychologically.

SmokeChemical smokes and

obscurants can degrade theother aerosoleffectiveness of

sophisticated antitank guided missiles (ATGMs).The precision guidance systems of ATGMs aretypically electro-optical devices and generallyoperate in the near-, mid-, or far-infrared portionsof the electromagnetic spectrum, rather than in thevisible light band of the spectrum. The use ofsmoke in the target area can be a convincingcombat multiplier offensively and a dynamiccountermeasure defensively. Smoke should be ofprimary interest to all commanders and staffplanners because the proper use of smoke canprovide many operational advantages.

Smoke has four general uses on thebattlefield—obscuring, screening, deceiving, andidentify ing/signalling. Obscuring smoke is placedon an enemy to reduce vision both at, and out from,the position. Screening smoke is used in friendlyoperational areas or between friendly units andthe enemy. Deceiving smoke is used to mislead theenemy. Identifying/signalling smoke is a form ofcommunication that has multiple uses. Overall,the objective of smoke employment is to increasethe effectiveness of Army operations whilereducing the vulnerability of US forces.Specifically, smoke can be used to accomplish thefollowing:

Deny the enemy information.Reduce effectiveness of enemy target

acquisition.Disrupt enemy movement, operations,

command, and control.Create conditions to surprise the enemy.Deceive the enemy.

During offensive operations, smoke canscreen the attacker while an attack is carried out.

Some offensive applications include concealingmovement of military forces and equipment;screening locations of passages through barriers;and helping to secure water crossings,beachheads, or other amphibious operations.

For defensive operations, smoke can beeffectively used to blind enemy observation pointsto deprive the enemy of the opportunity to adjustfire, to isolate enemy elements to permitconcentration of fire and counterattack, and todegrade the performance of threat ATGMs.

There are generally two categories of smokeoperations on a battlefield-hasty and deliberatesmoke. Hasty smoke operations are conductedwith minimum prior planning, normally tocounter some enemy action or anticipated action ofimmediate concern to a commander. Hasty smokeis usually used on small areas, is of short duration,and is most often used by battalion or smallerunits. Deliberate smoke is planned in much greaterdetail. It is often employed over a large area for arelatively long period by brigades, divisions, orcorps. For further information on hasty anddeliberate smoke operations, refer to FM 3-50.

The following paragraphs on smoke operationcontain information on smoke characteristics,diffusion of smoke, weather effects, hasty anddeliberate smoke operations, and tacticalconsiderations.

CharacteristicsSmoke is an aerosol that owes its ability to

conceal or obscure to its composition of manysmall particles suspended in the air. Theseparticles scatter or absorb the light, thus reducingvisibility. When the density or amount of smoke

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material between the observer and the object to bescreened exceeds a certain minimum thresholdvalue, the object cannot be seen.

The effectiveness of smoke used to obscure orconceal depends primarily on characteristics suchas the number, size, and color of the smokeparticles. Dark or black smoke absorbs a largeproportion of the light rays striking individualsmoke particles. In bright sunlight, a largequantity of black smoke is required for effectiveobscuration because of the nonscatteringproperties of the particles. At night or under lowvisibility conditions, considerably less smoke isneeded.

Grayish or white smoke obscures by reflectingor scattering light rays, producing a glare. Duringbright daylight conditions, less white smoke thanblack smoke is required to obscure a target. Yearsof experience with smoke screen technology haveshown that white smoke is superior to black smokefor most applications. Available white smokeincludes white phosphorus (WP) and redphosphorus (RP) compounds, hexachloroethane(HC), and fog oil (SGF2). WP, RP, and HC arehydroscopic—they absorb water vapor from theatmosphere. This increases their diameters andmakes them more efficient reflectors andscatterers of light rays. Fog oils arenonhygroscopic and depend upon vaporization

techniques to produce extremely small diameterdroplets to scatter light rays. The reflecting andabsorbing qualities of smoke are illustrated inFigure 2-1.

Smoke, when placed between a target and aviewer, degrades the effectiveness of target-acquisition and aiming systems. The amount ofsmoke necessary to defeat aiming and acquisitionsystems is highly dependent upon the prevailingmeteorological conditions, terrain relief, availablenatural light, visibility, and the attenuationeffects of natural particles in the atmosphere.Other factors that must be considered includesmoke from battlefield fires and dust raised bymaneuvering vehicles and artillery fire.

The ability to detect and identify a targetconcealed by such a smoke screen is, in turn, afunction of target-to-background contrast.Additionally, the amount of available naturallight, the position of the sun with respect to thetarget, the reflectance of the smoke screen and thetarget, and the portion of the electromagneticspectrum to be attenuated below the thresholdcontrast for detection will impact on detecting andidentifying a target.

DiffusionThe diffusion of smoke particles into the

surface and planetary boundary layers of the

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atmosphere generally obeys physical laws.Diffusion is governed by wind speed, turbulence,stability of the atmosphere, and terrain. Thediffusion of smoke, as used on the battlefield,originates from four basic source configurations.These may be defined as continuous point sources,instantaneous point sources, continuous linesources, and area sources. A continuous pointsource may bethought of as a smoke release from asingle smoke generator or smoke pot. The burstingof a projectile containing WP is considered to be aninstantaneous source. A series of generators, setup crosswind, represent a line source. Munitionswhich scatter smoke-generating submunitions inan area are considered an area source.

Weather EffectsMeteorological conditions that have the most

effect on smoke screening and munitionsexpenditures (including the deployment of smokegenerators) include wind direction, relativehumidity, visibility, and atmospheric stability. Tobe effective, an obscuring screen must be placed inan advantageous position with respect to theprevailing wind direction. The target area to be

screened must be defined in terms of whether theprevailing wind direction is considered to be ahead or tail wind, a quartering wind, or a flankwind. Figure 2-2 illustrates these conditions. Itmust be remembered that flanking winds can befrom either the right or left side of the screeningarea and that there are four quartering-winddirections. Wind direction is critical fordetermining the adjustment or aim point forscreens deployed by artillery or mortars and alsofor the placement of generators if used to produceeither hasty or deliberate smoke.

As smoke is released into the atmosphere, it istransported and diffused downwind. The plume isdepleted quite rapidly by atmospheric turbulence.The obscuration power of the plume becomesmarginal at relatively short downwind distancesand must be replenished at each point where theattenuation of a line of sight approaches aminimum. The transport wind speed and directionfor a diffusing plume in the surface boundary layerof the atmosphere occurs at a height of about halfof the plume height. Usually, this would be aheight of about 10 meters. For smoke operations,then, speeds and directions should be obtained fora height of about 10 meters above the surface.

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The relative humidity of the atmosphere isimportant to the use of smoke on a battlefield. Aspreviously stated, WP, RP, and HC smokecompounds are hydroscopic—they absorbmoisture from the atmosphere. As relativehumidity increases, the amount of screeningmaterial available for target obscurationincreases. For example, the HC compound isconsidered to be only about 70-percent efficient;that is, for every 100 grams of HC in a munition,only 70 grams are available for screening. If therelative humidity yield factor is then added in, thescreening power of HC increases. This is shown inTable 2-1. Applicable technical references indicatethe amount of HC or WP contained in variousmunitions. For example, the 105-millimeter WP(M416) round contains 6 pounds of WP; the 155-millimeter HC (Ml16A1) round contains 5.45pounds of HC; and the 76-millimeter WP (M361A1)round contains 1.38 pounds of WP (453.6 gramsequals 1 pound).

Phosphorous compounds are considered to bebetter screening agents than HC. This is becauseWP and RP have large yield factors for variousrelative humidities. Yields for WP are also shownin Table 2-1. Upon ignition, WP burns at atemperature of about 800ºC to 850°C. As aconsequence, the smoke from a WP munitionpillars, creating an excellent vertical screen,especially with high relative humidities. However,only about 10 percent of the smoke generated fromWP munitions is available for screening near the

ground. This should be considered when planningsmoke missions.

Battlefield visibility can be practically definedas the distance at which a potential target can beseen and identified against any background.Reduction of visibility on a battlefield by anycause reduces the amount of smoke needed toobscure a target.

Turbulence, atmospheric instability, andwind speed can have an adverse effect upon smokeexpenditures. Unstable conditions are usuallyconsidered to be unfavorable for the use of smoke.Under calm or nearly calm conditions, the use ofsmoke is also sometimes unsatisfactory. Ingeneral, if the wind speed is less than 3 knots orgreater than 20 knots, smoke can be anunsatisfactory countermeasure on the battlefield.

OperationsSmoke operations are of two types: hasty and

deliberate.

Hasty SmokeHasty smoke generally is placed in the area to

be screened by artillery, smoke pots, or mortarprojectiles. Obscuring smoke usually is employedon enemy forces to degrade their vision bothwithin and beyond their location. Screeningsmoke is used in areas between friendly and enemyforces to degrade enemy ground and aerialobservation and to defeat or degrade enemyelectro-optical systems. Screening smoke also maybe employed to conceal friendly ground maneuver.Deception or decoy smoke is used in conjunctionwith other measures to deceive the enemyregarding friendly intentions. Decoy smoke can beused on several approaches to an objective todeceive the enemy as to the actual avenue of themain attack.

In the offense, hasty smoke may be used toestablish screens, enabling units to maneuverbehind or under screens and deny the enemyinformation about strength, position, activities,and movement. Ideally, a screen should be placedapproximately 500 to 800 meters short of theenemy to allow for maximum visibility formounted forces during the final assault. Hastyscreens on the flanks also can be used. Flankingscreens can be produced with mechanized

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generators. Hasty obscuring smoke also may be ideal for this type of hasty smoke.placed on enemy strongpoints. Figure 2-3 shows the positioning of an

On defense, hasty smoke may be used to obscuring hasty smoke cloud on enemy forces forimpede and disrupt enemy formations. It also may tail wind and head wind conditions. Figure 2-4be used beyond the forward line of own troops illustrates screening smoke for flank and(FLOT) to silhouette Threat targets as they emerge quartering winds ahead of an advancing force.through the smoke and are engaged. Smoke Figure 2-5 is an example of mechanized unitsscreens also may be used to conceal defensive generating a smoke screen for a counterattackingpositions and cover disengaging and moving force.forces. Mechanized smoke generator units are

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Deliberate SmokeLarge area smoke screens generally fall

within the realm of deliberate smoke in that theyare usually planned well in advance of theoperation. Large area screening or theestablishment of a smoke blanket or haze isgenerally carried out by the use of smokegenerators. Generators usually are positioned in aline source configuration at a right angle to theprevailing wind direction. Usually, if the terrainallows it, the generators are evenly spaced alongthe smoke line. Generators are ideal for screeningriver crossings if the prevailing wind direction isupstream, downstream, or a tail wind.

The employment of large smoke is probablymost effective if the screen is generated beforesunrise when stable conditions and light-to-moderate winds are most likely. Screens generatedin these conditions will remain close to the groundwith only moderate vertical diffusion. Screens alsoreduce incoming solar radiation reaching theground so that convective turbulence issuppressed, similar to overcast weather

conditions. Thus, smoke hazes and blankets canbe maintained and remain useful for longer timeperiods.

The use of large area smoke screens in anyarea depends upon the prevailing wind direction.Operators must be prepared to shift theirgenerators to preselected locations if the winddirection changes.

Tactical ConsiderationsIn addition to the importance of wind

direction, relative humidity, visibility, stability,and turbulence to the successful completion of asmoke mission, the effects of terrain and soilconditions should be considered. Terrain effectsdiscussed in Appendix C apply to smoke as well asNBC agents. A diffusing smoke plume also tendsto follow the terrain-influenced surface winds.Also, in forests and jungles smoke has a tendencyto be more evenly dispersed and to persist longerthan over more open terrain.

The condition of the soil influences theeffectiveness of artillery-delivered and mortar-

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delivered smoke but has very little direct effectupon screening or obscuring smoke. An impactingsmoke munition bursting in soft soil loseseffectiveness since part of the filling compound isdriven into the dirt. In some cases, totallyineffective screens result if smoke munitions aredelivered to a boggy or swampy target area.

A last point to consider involves winddirection effects upon smoke screens. Munitions

expenditures for a screen deployed in quarteringwind conditions must be increased by a factor ofabout 1.5 over a flank wind direction condition.For head and tail winds, expenditures are three tofour times those for flank winds. Thus, reductionin expenditures owing to visibility and relativehumidity effects may be negated by winddirections.

IncendiariesWeather conditions have little influence on

incendiary munitions themselves. Wind andprecipitation, however, may greatly influence thecombustibility of the target and its susceptibilityto fire spread. The purposes of incendiaries are tocause maximum fire damage on flammablematerials and objects and to illuminate. Initialaction of the incendiary munition may destroythese materials, or the spreading and continuingof fires started by the incendiary may destroythem. Incendiary materials used include gasolinegels, burning metals, incendiary mixes, and whitephosphorus.

To be effective, incendiary munitions shouldbe used against targets susceptible to fire or heatdamage. A considerable part of the target must beflammable, so the fire can spread. Fire walls andcleared lanes offer some resistance to the spread offires.

Winds assist in the effectiveness ofincendiaries, increase the rate of combustion, andcan spread fires downwind more rapidly. Actually,each large fire can create a wind system of its own.This wind system results from the tremendousheat generated and the resulting vertical windcurrents. Incoming winds can feed more air to thefire. This increases the rate of combustion, which,in turn, can increase the wind. In extreme cases,this wind is called a fire storm and sometimesexceeds 60 knots.

Smoke, sparks, and flames fly in the directionof the wind. Incendiary strikes (at successivetargets) should be planned to begin with thefarthest downwind target and proceed upwind.This will prevent aiming points from becomingobscured by smoke traveling downwind of initialfires. Additionally, the position of friendly forcesor facilities that must not be damaged must be

considered (in relation to the wind direction) whenplanning incendiary strikes.

Temperature, temperature gradient, andclouds have little if any effect on incendiaries.Humidity also has little effect upon incendiarymunitions but may affect combustible material.Wood, vegetation, and similar material absorbsome moisture from the air over a period. If relativehumidities have been high for some time, as in thetropics, it may be more difficult to achievecombustion from incendiary action.

Rain or snowfall, even when light, can rendergrass and brush quite incombustible and make acontinuing fire unlikely. Heavy timbers are notaffected unless they have been exposed to longperiods of precipitation. Combustible materialsexposed to rain may be susceptible to fire damage,such as in mass incendiary attacks. In theseattacks, the heat of combustion may be sufficientto dry combustible materials in the target area.

In regions of high humidities, such as thetropics, mass incendiary attacks generatetremendous amounts of heat, causing verticalwind currents. This rising air can causethunderstorms, counteracting the effects of theincendiaries.

It is difficult to extinguish burning metalswith water; a spray actually speeds the burning.Water surrounding the area of burning metalsprevents fire spread. Water extinguishes burningphosphorus, but unconsumed particles will burnagain when dry.

Three elements of terrain affect the efficientuse of incendiaries. These are soil, vegetation, andtopography. The type of soil affects the impactingof the munition; combustibility of the vegetationaffects the efficiency of the incendiary; andtopography influences wind speed and direction.

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CHAPTER 3

Biological Agents and Nuclear Detonations

In a general war, US forces may be faced by anenemy capable of employing nuclear or biologicalweapons. The effects of weather and terrain on

BiologicalIn a general war, US forces may be faced by an

enemy capable of producing and employingbiological agents. These include disease-causingmicroorganisms (pathogens) and toxins. Toxinsare biologically derived chemical substances thathave desirable characteristics for use as biologicalwarfare agents. Toxins may be natural orsynthetic.

Biological agents will most likely bedisseminated as an aerosol. Therefore, a basicknowledge of their field behavior is essential forestimating friendly vulnerability. These agentsdiffer from chemical agents in some aspects offield behavior. Pathogens decay as a result offactors such as weathering. They also require timeto invade a body and multiply enough to overcomethe body’s defenses. This is known as theincubation period. This period may vary fromhours to months, depending on the type ofpathogen.

The following paragraphs discuss biologicalagent dissemination, weather effects, and terraininfluences, and they briefly summarize theinfluence of these on biological agent fieldbehavior.

DisseminationPathogens are most likely to be disseminated

as aerosols. Toxins, on the other hand, may bedisseminated as either aerosols or large liquiddrops. An aerosol is composed of particlescontaining pathogens or toxins. The force of thewind moves it along. At the same time, the aerosolspreads by turbulent diffusion.

Biological agents that die rapidly are said tohave a high decay rate. High wind speeds (10 to 20knots) carry these agents over more extensive

biological agent aerosols and on nuclear weaponsfollow.

Agentsareas during the agent survival period. Multiplewind shifts occur at low wind speeds. These shiftsmay cause more lateral spread and downwinddiffusion than higher speeds. Optimum effectdepends on the nature of the agent andatmospheric conditions. Highly virulent(malignant) agents with low decay rates canspread over large areas (by low or high windspeeds) and still present a casualty threat.Virulent agents with higher decay rates employedunder the same atmospheric conditions are muchless effective.

Weather EffectsAir stability, temperature, relative humidity,

pollutants, cloud coverage, and precipitation havean effect on biological agents.

Air StabilityAtmospheric stability influences a biological

cloud in much the same way it affects a chemicalcloud. However, biological agents may be moreeffective in lower concentrations than chemicalagents. This is because of their high potency. Astable atmosphere results in the greatest cloudconcentration and area coverage of biologicalagents. Under unstable and neutral stabilityconditions, more atmospheric mixing occurs. Thisleads to a cloud of lower concentration, but theconcentration is sufficient to inflict significantcasualties. The coverage area under unstablestability conditions is also reduced.

TemperatureAir temperature in the surface boundary layer

is related to the amount of sunlight the ground has

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received. Normal atmospheric temperatures havelittle direct effect on the microorganisms of abiological aerosol. Indirectly, however, anincrease in the evaporation rate of the aerosoldroplets normally follows a temperature increase.There is evidence that survival of most pathogensdecreases most sharply in the range of -20°Cto -40°C and above 49°C. High temperatures killmost bacteria and most viral and rickettsialagents. However, these temperatures will seldom ifever be encountered under natural conditions.Subfreezing temperatures tend to quick-freeze theaerosol after its release, thus decreasing the rate ofdecay. Exposure to ultraviolet light—one form ofthe sun’s radiation—increases the decay rate ofmicroorganisms. Ultraviolet light, therefore, has adestructive effect upon the biological aerosol. Mosttoxins are more stable than pathogens and are lesssusceptible to the influence of temperature.

Relative HumidityThe relative humidity level favoring

employment of a biological agent aerosol dependsupon whether the aerosol is distributed wet or dry.For a wet aerosol, a high relative humidity retardsevaporation of the tiny droplets containing themicroorganisms. This decreases the decay rate ofwet agents, as drying results in the death of thesemicroorganisms. On the other hand, a low relativehumidity is favorable for the employment of dryagents. When the humidity is high, the additionalmoisture in the air may increase the decay rate ofthe microorganisms of the dry aerosol. This isbecause moisture speeds up the life cycle of themicroorganisms. Most toxins are more stable thanpathogens and are less susceptible to the influenceof relative humidity.

PollutantsAtmospheric pollutant gases can also affect

the survival of pathogens. Pollutant gases havebeen found to decrease the survival of manypathogens. These gases include nitrogen dioxide,sulfur dioxide, ozone, and carbon monoxide. Thiscould be a significant factor in the battlefield overwhich the air is often polluted.

Cloud CoverageCloud coverage in an area influences the

amount of solar radiation received by the aerosol.Thus, clouds decrease the amount of destructiveultraviolet light the microorganisms receive.Cloud coverage also influences factors such asground temperature and relative humidity, asdiscussed in Chapter 1.

PrecipitationPrecipitation may wash suspended particles

from the air. This washout may be significant in aheavy rainstorm but minimal at other times. Highrelative humidities associated with mists, drizzles,and very light rains are also an important factor,These may be either favorable or unfavorable,depending upon the type of agent. The lowtemperatures associated with ice, snow, and otherwinter precipitation prolong the life of mostbiological agents.

Terrain InfluencesSoil, vegetation, and rough terrain influence a

biological agent aerosol.

Soil and VegetationSoil influences a biological agent aerosol as

related to temperature and atmospheric stability.Appendix C discusses the interrelationshipbetween soil and these weather elements.

Vegetation reduces the number of aerosolparticles. Impact of the suspended particles upontrees and grass causes some particles to settle, andthis settling reduces agent concentration.However, vegetative cover reduces exposure toultraviolet light, increases relative humidities,and may reduce temperatures (while fostering aneutral temperature gradient). All these factorsfavor the survival of wet aerosols.

Rough TerrainRough terrain creates wind turbulence, and

turbulence influences the vertical diffusion ofaerosol. This turbulence reduces agenteffectiveness and area coverage. Terrain affectsthe path of the aerosolsurface concentration.

and the distribution of

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Nuclear DetonationsWhen a nuclear explosion occurs, blast

radiation and heat or thermal effects will occur.The influence of weather and terrain on theseeffects will be discussed in this section. When anuclear weapon detonates at low altitudes, afireball results from the sudden release of immensequantities of energy. The initial temperature of thefireball ranges into millions of degrees, and theinitial pressure ranges to millions of atmospheres.Most of the energy from a nuclear weapondetonation appears in the target area in the form ofthree distinct effects. These are nuclear radiation,blast, and thermal radiation.

Nuclear Radiation. Neutron and gammaradiation from the weapon detonation producescasualties and, in many cases, material damage aswell. Ionized regions, which may interfere ‘with thepropagation of electromagnetic waves associatedwith communication systems and radars, resultwhen the atmosphere absorbs nuclear radiation.

Blast. A blast wave with accompanying drageffects travels outward from the burst.

Thermal Radiation. Intense thermalradiation emits from the fireball, causing heatingand combustion of objects in the surrounding area.

In the detonation of a typical fission-typenuclear weapon, the percentage of the total energyappearing as nuclear radiation, blast, or thermalradiation depends on the altitude at which theburst takes place (subsurface, surface, or air) andon the physical design of the weapon. For burstswithin a few kilometers above the earth’s surface,slightly more than 50 percent of the energy mayappear as blast, approximately 35 percent asthermal energy, and approximately 15 percent asnuclear radiation.

Certain weather conditions will influence theeffects of nuclear weapons. Likewise, differenttypes of terrain will also influence the effects ofnuclear weapons. In addition to theseconsiderations, the type of operation can have adirect bearing on weather and terrain effects onnuclear weapons use.

Nuclear RadiationWhen a nuclear explosion occurs, one usual

result is the well-known mushroom-shaped cloud.This cloud may extend tens of thousands ofmeters, and in the case of a surface burst or

shallow subsurface burst, it is a tremendousvertically developed aerosol cloud bearingradioactive material. The effect of wind speed anddirection at various altitudes is of particularinterest. These factors are of great importance inpredicting the location(s) of the fallout that mayresult from a nuclear explosion.

The effects of weather and terrain apply toboth the initial and residual effects of nuclearexplosions, although this section will primarilyaddress the residual aspects. For moreinformation on the effects of weather on bothinitial and residual effects, refer to FM 3-3.

PrecipitationPrecipitation scavenging can cause the

removal of radioactive particles from theatmosphere. This is known as rainout. Because ofthe uncertainties associated with weatherpredictions, the locations that could receiverainout cannot be accurately predicted. Rainoutmay occur in the vicinity of ground zero or thecontamination could be carried aloft for tens ofkilometers before deposition. The threat of rainoutespecially exists from a surface or subsurfaceburst. Vast quantities of radioactive debris will becarried aloft and be deposited downwind.However, rainout may cause the fallout area toincrease or decrease and also cause hot spotswithin the fallout area.

For airbursts, rainout can increase theresidual contamination hazard. Normally, theonly residual hazard from an airburst is a smallneutron induced contamination area around GZ.However, rainout will cause additionalcontaminated areas in unexpected locations.

Yields of 10 kilotons or less present thegreatest potential for rainout, and yields of 60kilotons or more offer the least. Additionally,yields between 10 kilotons and 60 kilotons mayproduce rainout if the nuclear clouds remain at orbelow rain cloud height.

Rain on an area contaminated by a surfaceburst changes the pattern of radioactiveintensities by washing off higher elevations,buildings, equipment, and vegetation. Thisreduces intensities in some areas and possiblyincreases intensities in drainage systems; on lowground; and in flat, poorly drained areas.

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Wind Speed and DirectionWind speed and direction at various altitudes

are two factors that determine the shape, size,location, and intensities of the fallout pattern onthe ground because contaminated dirt and debrisdeposit downwind. The principles and techniquesof fallout prediction from winds-aloft data are inFM 3-3. Surface winds also play an important rolein the final location of fallout particles. Just assnow falls on pavements or frozen surfaces andsurface winds pile it in drifts, so, too, can localwinds cause localization of fallout material increvices and ditches and against curbs and ledges.This effect is not locally predictable, but personnelmust be aware of the probability of these highlyintense accumulations of radioactive materialoccurring and their natural locations.

Clouds and Air DensityClouds and air density have no significant

effects on fallout patterns.

Terrain ContoursDitches, gullies, small hills, and ridges offer

some protection against the gamma radiationemanating from the contaminated area. Terraincontours also cause local wind systems to develop.These wind systems will affect the finaldisposition of fallout on the ground, creating bothhot spots and areas ofpattern.

Heavy FoliageHeavy foliage can

low-intensity within the

stop some of the falloutfrom reaching the ground. This may reduce theintensity on the ground.

SoilSoil surface materials (soil) at the burst site

determine particle size (large or small). Theparticle size helps determine when and where mostof the fallout will reach the ground, the largerparticles settling first. Composition of the soil nearground zero will materially affect the size anddecay rate of the pattern of residual radiationinduced by neutrons from the weapon.

Type of OperationTemperature and terrain can also influence

the effects of nuclear radiation on tacticaloperations. The effects of cold weather, desert,jungle, mountain, and urban operations onnuclear defense planning follow.

Cold Weather OperationsWeather conditions limit the number of

passable roadways. Radiological contaminationon roadways may further restrict resupply andtroop movement. Seasonal high winds in the arcticmay present a problem in radiologicalcontamination predictions. These winds mayreduce dose rates at ground zero. At the same time,they extend the area coverage and create aproblem for survey/monitoring teams. Hot spotsor areas of concentrated accumulation ofradiological contamination may also occur inareas of heavy snow and snow drifts.

Desert OperationsDesert operations present many varying

problems. Desert daytime temperatures can varybetween 90°F to 125°F (32°C to 52°C). Thesetemperatures create an unstable temperaturegradient. However, with nightfall, the desert coolsrapidly and a stable temperature gradient results.A possibility of night attacks must be consideredin all planning.

Nuclear defense planning in a desert isgenerally much the same as in other areas, with afew exceptions. Lack of vegetation and permanentfixtures, such as forests and buildings, makes itnecessary to plan for and construct fortifications.Construction may be difficult because ofinconsistencies of the sand. However, sand, incombination with sandbags, gives additionalprotection from radiation exposure. Blowingwinds and sand make widespread radiologicalsurvey patterns likely. The varying terrain maymake radiological survey monitoring verydifficult.

Jungle OperationsRadiation hazards also may be reduced

because some of the falling particles are retainedby the jungle canopy. Subsequent rains, however,

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will wash these particles toconcentrate them in waterRadiation hot spots will result.

Mountain Operations

the ground andcollection areas.

In the mountains, the deposit of radiologicalcontamination will be very erratic because ofrapidly changing wind patterns. Hot spots mayoccur far from the point of detonation, and low-intensity areas may occur very near it. Limitedmobility makes radiological surveys on the grounddifficult, and the difficulty of maintaining aconstant flight altitude makes air surveys highlyinaccurate.

Urban OperationsBuildings provide a measure of protection

against radiological contamination. Taking thisinto consideration, troops who must move in orthrough a suspected contaminated urban areashould travel through buildings, sewers, andtunnels to reduce contamination risk. However,they should consider the dangers of collapsebecause of blast. They should also considerhazards of debris and fire storms resulting fromruptured and ignited gas or gasoline lines.

BlastMost of the materiel damage and a

considerable number of the casualties caused byan airburst result from the blast wave. For thisreason, it is desirable to consider the phenomenaassociated with the passage of a blast wavethrough air.

The expansion of the intensely hot gases atextremely high pressures within the fireballcauses a blast wave to form in the air, movingoutward at high velocities. The maincharacteristic of the blast wave is the abrupt rise inpressure above ambient conditions. Thisdifference in pressure with respect to the normalatmospheric pressure is called the overpressure.

Initially, the velocity of the shock front ismany times the speed of sound. However, as thefront progresses outward, it slows down andmoves with the speed of sound.

The magnitude of the air blast parameters isdependent on the yield of the weapon, height ofburst, and the distance from ground zero.

The blast wave may last from tenths of asecond to seconds, depending on the yield and thedistance from the burst. Weather, surfaceconditions, topography, and the type of operationbeing conducted all affect the blast wave.

WeatherRain and fog may lessen the blast wave

because energy dissipates in heating andevaporating the moisture in the atmosphere.

Surface ConditionsThe reflecting nature of the surface over which

a weapon is detonated can significantly influencethe distance to which blast effects extend.Generally, reflecting surfaces, such as thin layersof ice, snow, and water, increase the distance towhich overpressures extend.

TopographyMost data concerning blast effects are based

on flat or gently rolling terrain. There is no quickand simple method for calculating changes hilly ormountainous terrain produce on blast pressures.In general, pressures are greater on the forwardslopes of steep hills and are diminished on reverseslopes when compared with pressures at the samedistance on flat terrain. Blast shielding is nothighly dependent on line-of-sight considerationsbecause the blast waves will bend or diffractaround obstacles. The influence of small hills orfolds in the ground is considered negligible fortarget analysis. Hills may decrease dynamicpressures and offer some local protection fromflying debris.

Type of OperationTemperature and terrain can also influence

the effect of blast on tactical operations. Theeffects of cold weather and jungles or forests onoperations follow.

Cold Weather OperationsAt subzero temperatures, the radius of damage

to material targets can increase as much as 20percent. These targets include such items as tanks,APCs, artillery, and military vehicles. Anincreased dynamic pressure can result from a

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precursor wave over heat-absorbing surfaces.However, tundra, irregular terrain features, andbroken ice caps break up the pressure wave.

Blast effects can drastically interfere withtroop movement by breaking up ice covers andcausing quick thaws. These effects can causeavalanches in mountainous areas. In flat lands,the blast may disturb the permafrost to such anextent as to restrict or disrupt movement.

Jungle or Forest OperationsInitial effects of nuclear detonations are not

significantly influenced by the dense vegetation.However, the blast wave will probably causeextensive tree blowdown and missile effects.Forests, in general, do not significantly affect theoverpressure but do degrade the dynamic pressureof an air blast wave.

Thermal RadiationThermal radiation results from the heat and

light produced by the nuclear explosion. During anuclear explosion, the immediate release of anenormous quantity of energy in a very small spaceresults in an initial fireball temperature thatranges into millions of degrees. For a given type ofweapon, the total amount of thermal energyavailable is directly proportional to the yield.

Within the atmosphere, the principalcharacteristics of thermal radiation are that it—

Travels at the speed of light.Travels in straight lines.Can be scattered.Can be reflected.Can be easily absorbed.

The thermal effects will be influenced byweather, terrain, height of burst, and type ofoperation.

• • • • •

WeatherAny condition that significantly affects the

visibility or the transparency of the air affects thetransmission of thermal radiation. Clouds, smoke(including artificial), fog, snow, or rain absorb andscatter thermal energy. Depending on theconcentration, they can stop as much as 90 percentof the thermal energy. On the other hand, clouds

above the burst may reflect additional thermalradiation onto the target that would haveotherwise traveled harmlessly into the sky.

TerrainLarge hill masses, forests, or jungles, or any

opaque object between the fireball and the targetmay provide some protection to a target element.Trucks, buildings, or even another individual mayprotect an individual from thermal radiation.Foxholes provide good protection. However,personnel protected from direct line-of-sightradiation from the fireball may still receivethermal injury because of reflection frombuildings or other objects. Good reflectingsurfaces, such as water, snow, or desert sand, mayreflect heat onto the target and intensify thethermal radiation effect. Even the backs and sidesof open foxholes will reflect thermal energy. Thereflective capability of foxhole materials variesfrom 8 percent for wet black soil to 93 percent forsnow. Because of atmospheric scattering andreflections, thermal casualties may result at agreater range than casualties from other effects.

Height of BurstThe amount of thermal radiation that a

surface target receives from a nuclear burst of agiven yield will vary with the height of burst. Themaximum thermal effect at the target will usuallybe produced by an airburst. A surface burstproduces about half the amount of the thermalradiation that would be produced by an airburstbecause of the interaction of the fireball with thesurface. Thermal radiation from a subsurfaceburst where the fireball is not visible isinsignificant.

Type of OperationTemperature and terrain can also influence

the effect of thermal radiation on tacticaloperations. The effects of cold weather andmountains on thermal radiation follow.

Cold Weather OperationsThe high reflectivity of ice and snow may

increase the minimum safe distance as much as 50percent for unwarned troops and even warned,exposed troops. Reflectivity may also increase the

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number of personnel whose vision is affected bythe brilliant flash, or light dazzle, especially atnight. The pale colors normally used to covermaterial in a cold weather environment give anadvantage. Their low absorption properties maymake personnel less vulnerable to thermal effects.Cold temperatures also reduce thermal effects onmaterials. Snow, ice, and even frost coverings oncombustible materials greatly reduce the tendencyof the materials to catch fire. However, thermal

effects will dry out exposed tundra areas, andgrass fires may result.

Mountain OperationsThe clear mountain air extends the range of

casualty-producing thermal effects. Within thisrange, however, the added clothing required by thecool temperatures at high altitudes reducescasualties from these effects.

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APPENDIX A

Air Weather Service

The US Air Force Air Weather Service (AWS)provides operational weather service support, asdescribed in AR 115-10 and AFR 105-3. Asupporting Air Force weather unit will beassigned to all corps and divisions and to separatebrigades, regiments, and groups on request.Assignment is subject to the following:

When requested in peacetime in accordancewith AR 115-12 and in wartime as stated incontingency, mobilization, and war plans.

When it is jointly agreed that remote weatherservice will be inadequate.

When consistent with jointly agreed tacticaldoctrine and operational support concepts.

Planning and executing a successfuloperation require timely and accurate weatherinformation. To ensure prompt receipt of weatherinformation and to ensure that both themeteorologist and the NBC officer understandwhat is required, close and continuouscoordination is essential. The NBC officer shouldestablish (through the intelligence officer) andmaintain direct contact with the AWS detachmentor staff weather officer (SWO) before, during, andafter operations.

SWOs can provide the following services:Weather observations. Under field conditions,

the SWO will not be able to establish a denseobservational network. There will usually beareas of concern for which no observations areavailable. Therefore Army personnel should alsobe prepared to provide supplemental observationsto the AWS weather unit. The SWO also shouldhave access to observations and upper airsoundings from artillery meteorological (arty met)units to further supplement weather collectionefforts.

Forecasting services. These services, which canvary considerably, are provided according to localarrangements with the SWO.

Climatological data (weather history). Thisplanning information can be obtained from theSWO. Those units without an SWO should obtainit from the USAF Environmental Technical

Applications Center, Scott AFB, IL 62225 (see DAPamphlet 115-1).

The dissemination of weather informationwithin Army units is the responsibility of theintelligence officer. The AWS detachment or staffweather officer can supply information directly tothe using agency, or the information can be routedthrough the intelligence officer for disseminationto staff and lower units. The intelligence officerdetermines the method to be used.

AWS operational support products aredefined in terms of long-range planning (usuallybeyond 48 hours), mission planning (usually 24 to48 hours), and execution support (usually 0 to 24hours). For forecast periods in excess of five days,climatological analyses normally are provided.(NOTE: The forecast reliability decreases and theforecast provided becomes less specific as theforecast period increases. Significant changes ormodifications may occur after the forecast isissued. The requestor must then inform the AWSfacilities of the criteria for significant changes.

The following observation and forecastparameters and elements are normally available;however, additional products may be provided,depending on Army stated requirements and theAWS’s ability to satisfy those requirements:

Sky conditions, including amounts (tenths inCONUS and eighths overseas), type (according tostandard classification), and cloud base height (infeet).

Precipitation and/or obstructions to visibility,including intensity, type, and times of beginningand ending (in coordinated universal time andzone Z).

Surface visibilities in statute miles andfractions.

Surface wind direction and speed.Surface temperature.Temperatures and winds at desired standard

levels above the surface.Humidity.

Prior to planning an operation, the targetanalyst should collect AWS climatic studies, other

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climatological data, and/or forecasts for theoperational area valid for the time of theoperation. To obtain maximum weather forecastassistance, the NBC officer must provide the AWSfacility complete requirements as far in advanceas possible. The request should include thefollowing:

Time period for the forecast and desired deliverytime.

Target or area to be covered by the forecast.Clearly identify an area by map coordinates,aerial photograph grid numbers, or establishedgeographic boundaries.

Special elements or conditions to be covered.Criteria for changes (amendments) in the

forecast if desired.If possible, supplementary forecast

information should be obtained from the AWSfacility prior to the release of agents whenobservations indicate the original forecast to besignificantly in error; when the release time isappreciably delayed; or when, for any reason, theforecast requires updating.

The target analyst should also evaluateforecasts received. The analyst should use adetailed reconnaissance map, an aerialphotograph, or a mosaic or study of the terrainand vegetation in and around these areas andthose that might affect the behavior of the agentsto be released.

After the operation, the NBC officer shouldpass to the intelligence officer, SWO, or AWSfacility information on adequacy of support andany problems encountered. This information aidsAWS forecasters in better tailoring future support.

Using agencies receive weather informationin five general types of reports—weatherforecasts, current weather observation reports,weather summaries, climatic summaries, andclimatic studies.

A weather forecast is a prediction of weatherconditions at a point, along a route, or within anarea for a specified period. The accuracy andreliability of weather forecasts depend uponfactors such as characteristics of the forecast area,age of the data available, reliability of weathercommunications facilities, length of the forecastperiod, state of meteorological science, andexperience of the forecaster. The reliability andspecificity of forecasts generally decreases as theforecast period increases. Also, the forecast

becomes less specific as the forecast periodincreases.

Routine weather forecasts for use by troopunits should be in plain language and should be asaccurate as possible. Forecasts are Air WeatherService operational support products. Theseforecasts are defined in terms of long-rangeplanning (usually beyond 48 hours), missionplanning (usually 24 to 48 hours), and executionsupport (usually O to 24 hours). Figures A-1 and A-2 provide an example of a sample forecastcontaining information elements that could beprovide by Air Weather Service or Fleet WeatherService and supporting artillery meteorologicalsources.

Current weather observation reports are oral,written, or graphic representations of existingweather conditions or specific weather elements.These reports are used in the operation of aircraft;in the employment of nuclear weapons, chemicalagents, and smoke; and in other activities.

A weather summary describes the weatheralong a route or within an area during a specifiedrecent period. Weather summaries are used inanalyzing the effects of weather on recentoperations. These summaries are also used inestimating the effects of weather on futureoperations.

Climatic summaries tabulate averages,extremes, and frequencies of weather elements orphenomena. These cover a specified period–-ayear, season, or month—and a given point, alonga route, or within an area.

Climatic studies are analyses andinterpretations of climatic summaries. Corps andhigher headquarters usually prepare thesestudies. At the request of the intelligence officer,the supporting AWS unit prepares or obtainsclimatic studies on specific problems for givenareas.

Care must be taken to understand themeanings of the technical terms used in thismanual. Some of these terms have a stricttechnical definition that may be different from thedefinitions many laymen understand.

Field behavior of NBC agents and smokedepends upon weather variables, which are wind,temperature, vertical temperature gradients,cloud cover, humidity, and precipitation. Localtopography, vegetation, and soil affects thesevariables. The cumulative effect of these variables

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governs the required quantity and optimum typeof chemical agent and smoke best suited toachieve operational objectives. Since weathergoverns the transport of chemical agents andsmoke clouds, it is a primary factor in determiningthe effectiveness of a specific agent and the extentof the hazard area.

You must understand the basic principlesgoverning weather and have access to accurateforecasts to be able to use chemical agents

effectively or to defend against their use by theenemy. You must be capable of using the dataprovided in weather forecasts and predictions inpreparation of plans and estimates. Appendix Cdiscusses weather elements and primary weatherfactors in further detail for you to work with yourforecaster on how best to employ chemical agents,smoke, and other obscurants, or defend againstNBC agent use.

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APPENDIX B

Units of Measure

This appendix lists the units of measure and their abbreviations commonly used inmeteorology. It also contains factors for converting from one unit of measure to another.

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APPENDIX C

W e a t h e r

Field behavior of NBC agents, smoke, and Weather also determines the effectiveness ofother obscurants depends upon weather variables, agents and possible downwind hazards.which are wind, temperature, vertical temperature This appendix discusses weather elementsgradients, humidity, and precipitation. The and primary weather factors in further detail forinfluence of each variable depends upon synoptic you to work with your forecaster on how best toor general weather conditions. Local topography, employ chemical agents, smoke, and othervegetation, and soil affect these variables. obscurants or to defend against NBC agent use.

ElementsThis section will outline several basic weather

elements that must be understood. The weatherelements discussed will include the atmosphere,wind speed and direction, atmospheric turbulence,air and surface temperatures, humidity, dew point,clouds, precipitation, and atmospheric stability.In this section, each element will be discussed toprovide the needed background information forthe NBC staff officer.

AtmosphereThe sun is the fundamental source of energy

for the earth and its atmosphere; its influence isfelt in the radiant energy that is the basic source ofheat to the atmosphere. The spherical shape of theearth causes the unequal absorption of this energyby the earth’s surface and the atmosphere. Theunequal heating results in a strong polewardtransport of heat from the equator. Without thetransport of heat by the atmosphere and theoceans, temperatures would be much colder at thepoles and much warmer in equatorial regions.

A revolution of the earth around the sun takesone year or 365-1/4 solar days. Every fourth year is366 days long, hence leap year. The revolution ofthe earth about the sun is associated with fourseasonal changes. If the plane of the earth’s orbitwere in the plane of the equator, there would beonly a small seasonal change. Figure C-1 shows anexplanation of the four seasons. A season is one ofthe four quarters into which the year is divided.Figure C-1 shows that the earth wobbles (is

inclined) at an angle of approximately 23-1/2degrees north and south from the equator. Thiswobble and revolution (tilt and movement) aroundthe sun are responsible for the four seasons. Thewinter solstice (Tropic of Capricorn) occurs whenthe sun, with respect to the earth, is farthest south.Conversely, the summer solstice (Tropic of Cancer)occurs when the sun is farthest north. The twopoints midway between the solstices occur on theequator two times each year when the sun crossesthe equator. Day and night everywhere are equalin length. These are known as the spring andautumnal equinoxes (about 21 March and 23September). A year is divided into 12 months (orone-twelfth of a year). A month equals four weeksor 30 days. A season is composed of 91-1/4 days.

The atmosphere is the envelope of air thatsurrounds the earth and is bound to it by theearth’s gravity. The atmosphere extends from thesolid and liquid surfaces of the earth to anindefinite height. It may be subdivided verticallyinto a number of layers. The most commonsubdivision divides the atmosphere into atroposphere from the surface to about 10 to 20kilometers, a stratosphere which extends to about80 kilometers, and the ionosphere above thatheight. Each of these layers may be furthersubdivided.

In tropical latitudes, the troposphere extendsfrom the surface to a height of 15 to 20 kilometers.In polar regions, it may be as low as 10 kilometers.The troposphere also contains about three-quarters of the atmospheric mass. It also containsnearly all of the atmospheric water vapor. Most

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weather events are associated with thetroposphere. The troposphere or "region ofchange" can be characterized by decreasingtemperature with height, increasing wind speedwith height, and considerable vertical windmotion. Weather changes in the troposphere are afunction of the seasonal changes and the polewardtransfer of heat. The troposphere may besubdivided into three vertical zones. These zonesare the surface boundary layer, the planetaryboundary layer, and the free atmosphere. Thesurface boundary layer extends from the air-earthinterface to a height of 50 to 100 meters. Theplanetary boundary layer extends to heights of 500to 1,500 meters over fairly level terrain and may beas thick as 3,000 meters over mountainous regions.Above the planetary layer is the free atmosphere.Figure C-2 illustrates the layered structure of theatmosphere.

The surface boundary layer and the planetaryboundary layer—also known as the friction zoneor Ekman layer—are of primary concern for NBCoperations. Nearly all releases of chemical agentsand smoke will be near the surface or within thebounds of the surface and planetary layers.

The following paragraphs discuss many of thecharacteristics of the surface and planetaryboundary layers with respect to NBC and smokeoperations.

Wind Speed and DirectionWind is air in motion with respect to the

surface of the earth. Vertical components ofatmospheric motion are relatively small near thesurface. Therefore, the term “wind” is used almostexclusively to define the horizontal speed anddirection. NBC agents, smokes, and otherobscurants, either deliberately or inadvertentlyreleased into the atmosphere, will traveldownwind, that is, they will move in the generalwind direction. Therefore, you must understandhow the air moves in a given situation to be able topredict the dispersion behavior of agents. Youneed to understand how topography can affect themean airflow near the surface. You shouldunderstand how obstacles such as buildings, hills,trees, and other vegetation generate gusts,updrafts, and downdrafts downwind of theobstructions. Also, you need some basic knowledgeof the mean airflow within forests and othervegetative canopies.

The large-scale circulation of the atmosphereis driven by solar heating and radiation coolingand is affected by factors such as topography.Above the surface and planetary layers, large-scale air movements are determined by the generalheat balance of the earth. Air is heated from belowin the equatorial regions. It rises, loses heat nearthe poles, and sinks. Figure C-3 shows the general

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circulation of the atmosphere on a rotating earth.In the temperate latitudes, observations show thatthe surface winds tend to blow from southwest tonortheast. In polar latitudes, surface windsgenerally blow from northeast to southwest. Theregion between the polar and tropical air masses isthe spawning ground for major mid-latitudestorms. These storms generally move from west toeast. They assist in transporting cold polar airsouthward and force the warm tropical air to riseand move northward.

It is important to know how wind direction isrecorded and reported. Wind direction is thecompass direction from which the wind blows. Thenormal flow of air is not steady. The direction willfluctuate about its mean value, randomlydeviating from the prevailing direction. Thesefluctuations are usually larger in light windconditions (5 knots or less) than with higher windspeeds. The basic principles governing windchange and how to interpret wind directions areimportant for the NBC staff officer to understand.

The US Air Force Air Weather Servicemeteorological forecasts for NBC operations givethe direction from which the wind blows to thenearest 10 degrees measured from true north.Winds-aloft reports and aviation forecasts ofupper winds also give the direction from which thewind blows to the nearest 10 degrees.

Field artillery ballistic meteorological (met)messages follow the form outlined in FM 6-40 andFM 6-15. Both field manuals give wind directionsin increments of artillery mils. One degree ofcompass direction is equal to approximately 17.8mils, or 360 degrees equal 6,400 mils. A ballisticmet message lists direction to the nearest 100 mils,or about 5.5 degrees. For the recently adoptedStandard Biological and Chemical MeteorologicalMessage Quadripartite StandardizationAgreement (QSTAG 388 and STANAG 2103),wind directions are reported to the nearest 10 milsmeasured from true north. In radioactive falloutand computer meteorological messages, directionis also in tens of mils. Line O of these messagesrefers to a surface value that may not berepresentative. For the ballistic met message orthe computer message, line 1 gives the averagewind for the lowest 200 meters of the atmosphere.Line 1 of the fallout message is for the lowest 2,000meters of the atmosphere. An example of a falloutwind message is in Table C-1.

Figure C-4 shows the relationships governingthe two basic methods of reporting winddirections. The figure also includes the genericmethod. This consists of the four major and four

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intermediate points of the compass, that is, north,northeast, east, southeast, south, southwest, west,and northwest.

Surface wind speeds and directions at anyspecific locale are caused by many factors andforces operating in the atmosphere. Wind speedand direction can be considered to be localphenomena and must be measured directly orobserved to be useful for NBC operations anddispersion predictions.

Atmospheric TurbulenceDispersion of NBC agents and screening

smokes in the surface and planetary layers can bedirectly attributed to atmospheric turbulence. Theinteraction of weather systems on all scales at anylocale results in a three-dimensional wind thatvaries continuously with time. These continuousfluctuations are defined as turbulence.Atmospheric turbulence results from four factors.These are (1) the mechanical or drag effects ofobjects such as vegetation, hills, and man-madestructures protruding into the airstream; (2) thevertical rate of increase of wind speed plus theturning of the wind with height; (3) the verticaltemperature structure of the atmosphere; and (4)the relative moisture content.

If the ground is rough, mechanical turbulence(Figure C-5) results, since the air passing over itrises and falls with the terrain relief or flowsaround obstacles, generating both vertical andhorizontal turbulence. This turbulence is greater

with high wind speeds because of the increase ofdrag. It also decreases with height.

The wind increases in speed and shifts indirection from the surface to the top of theplanetary layer. This change of speed anddirection with height is known as wind shear.

Atmospheric turbulence also depends uponthe vertical temperature gradient. If air is carriedupward, air pressure decreases, its volumeincreases, and there will be a correspondingdecrease of temperature. If there is no exchange ofheat between the ascending parcel and itssurrounding environment, the process is labeledadiabatic.

There are four basic vertical temperaturegradient conditions. These are adiabatic,superadiabatic, inversion, and isothermal.

Adiabatic conditions are an idealized state forthe earth’s atmosphere. The adiabatic lapse ratefor such an atmosphere is a temperature decreasewith height of 9.8°C per 1,000 meters.

If an ascending parcel of air arrives at somespecified height warmer than its environment,then it will continue to rise. An ascending parcelthat rises and is cooler than the surrounding airwhen the lifting process ceases will sink back to itsoriginal level. These two processes are known asthermal or static instability and stability,respectively.

If the air next to the ground during thedaylight hours is heated by contact with thesurface and by conduction until it is warmer thanthe air above, a heat-energy gradient exists

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upward through the atmosphere. The warmer airtends to rise (see Figure C-6). Thus, there is a netupward flow of heat. This is known as convectiveturbulence. The larger the heat gradient, thegreater the rate of convective mixing. Hence, theturbulent transfer is greater.

At night, mechanical turbulence is thedominating feature of the mean wind flow. Atnight, heat is extracted from the air andtransferred to the ground. The cooling of the airnext to the surface results in a temperatureinversion, that is, an increase of temperature withheight. The net result is an absence of buoyantmotion or convective turbulence. Under inversionconditions, the mean wind flow shows a tendencyto approach nonturbulent flow conditions.

On occasion, a layer of air will exhibit notemperature change with height. This is known asan isothermal condition. During the daylighthours, particularly in the first 100 meters or so ofthe atmosphere, lapse rates greater than adiabaticwill exist. These are termed as beingsuperadiabatic.

Knowledge of the vertical distribution oftemperature is particularly important to NBCoperations. The height above the ground to whichan inversion exists is an important factor fordetermining the concentration or dosage ofchemical agents. In addition, surface-basedinversion heights or the height of elevatedinversions plays an important role with respect to

aerial spray releases or other elevated releases ofagents. Dissemination above an inversion layercan result in little if any agent reaching thesurface and/or a particular target area.

Atmospheric turbulence is the prime factorcontrolling the dispersion of NBC agents orscreening smokes released into the atmosphere.Extreme turbulence dilutes an expanding puff orplume quite rapidly. Effective downwinddistances are thus reduced drastically. You mustremember that the most effective use of NBCagents and/or smoke will be in periods whenturbulence is low and wind speeds are moderate.

Air and Surface TemperaturesAir temperature is the ambient temperature

measured at about 1.5 meters above the surface.Ground temperature for NBC operations is thetemperature of the surface(s) on which the agentscome to rest. Ground temperatures may be manydegrees warmer or cooler than the air temperature.The difference depends on the amount ofradiational heating or cooling at the surface.Surface temperatures play a major role in howlong liquid contamination on the surface iseffective and how concentrated the vapor is abovethe liquid. Air temperature varies the rate ofevaporation of liquid droplets in the atmosphere.Forecasts for NBC operations, therefore, should

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include predicted temperatures to the nearest 3°Cfor air, soil, or water surfaces.

HumidityIn general, humidity is a generic term

indicating a measure of the water vapor content ofthe atmosphere. Popularly, it is interpreted to bethe same as relative humidity. Relative humidityexpresses the percentage of water vapor actuallycontained in the air as compared to how much itwould contain if saturated at the sametemperature and pressure.

The surface and planetary boundary layerscontain about half of all the atmospheric watervapor. Water vapor is one of the most importantparts of the atmosphere. The amount in the airvaries widely because of the great variety ofsources of evaporation and “sinks” (condensationsites) that contribute to the hydrologic cycle.Water vapor is not only the raw material for cloudsand rain; it also affects the transport of heatenergy.

In warm air, high relative humidities indicatea large water vapor content. When airtemperatures are low, high relative humidities donot indicate large water vapor contents, since coldair cannot hold as much water vapor as warm air.

For example, blister agents are more effectivewhen both temperature and relative humidity arehigh. When the water vapor concentration ishigher, people perspire more freely and skinbecomes more sensitive to the effects of blisteragents. Humidity is also an importantconsideration for use of smoke obscurants. Thechemical smokes—hexachloroethane (HC), whitephosphorus (WP), and red phosphorus (RP)—areaerosols that absorb water (hydroscopic) vaporfrom the atmosphere. As smoke particles absorbwater vapor, this increases the screening power ofthe obscuring aerosols. For example, WP smokescreen effectiveness increases by at least 1/3 if therelative humidity increases from 30 to 50 percent.

Dew PointDew point is the temperature to which air

must be cooled at constant pressure for it tobecome saturated. The dew point is convenient asan approximate measure of the water vaporpresent in the air. For example, if the airtemperature is 16°C and the dew point is 10°C, theatmosphere would become saturated if the

temperature fell to 10°C. As the temperatureapproaches the dew point, condensation occurs.Dew is one form condensation can take. When thedifference between the temperature and the dewpoint is 2°C or less, fog will likely form. Forecastsfor NBC operations should also include the dewpoint.

CloudsCloudiness is another variable that can

influence the weather near the ground. If the sky isovercast, the amount of incoming solar radiationreaching the ground is greatly reduced. Effects onsurface air temperatures and lapse rates also varywith the degree of cloudiness.

Classification of clouds is based on their form,appearance, and height. The following is aclassification of clouds according to their heightabove ground.Low clouds . . . . . . . . . . . . . . . . . . . .2,000 metersMiddle clouds . . . .2,000 meters to 6,000 metersHigh clouds . . . . . . . .6,000 meters and higherClouds of vertical . . . . . . . . . . . . . . . . . . . 500 metersdevelopment to highest

cloud levelCloud coverage is the portion of the sky

covered by clouds. Coverage is observed andforecast in eighths except in the United Stateswhere it is reported in tenths. For most purposes,the descriptions in Table C-2 will suffice.

The thickness of a cloud layer is estimatedvisually. Thin clouds are those through which theoutline of the sun or moon can be seen. Thickclouds obscure the sun and look especially darkwhen the sun is behind them.

Persistent overcast low clouds usuallyindicate a neutral condition and small diurnal(daily) variations in weather factors near theground.

Broken low clouds generally indicate a weakto moderately unstable (lapse) condition duringthe day and a weak to moderately stable(inversion) condition at night.

Thick, overcast middle clouds generallyproduce the same neutral conditions as overcastlow clouds.

Broken middle clouds usually permit amoderate lapse (unstable) condition during the

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day and a moderate inversion (stable) condition atnight.

High clouds, whether overcast or broken, tendto indicate a moderately unstable (lapse)condition during the day and a moderately stable(inversion) condition at night. High clouds areusually of low density and have a limited impacton incoming solar radiation.

Scattered clouds of all types and heightsgenerally indicate a moderate to strong lapse(unstable) condition during the day and amoderate inversion (stable) condition at night.

A clear sky indicates a strong lapse (unstable)condition for most of the day and a stronginversion (stable) condition for most of the night ifthe surface wind is light or calm.

Weather clouds have no direct effect onchemical vapor or aerosol clouds, but they alterthe temperature and stability categories.

Clouds have significant vertical developmentwhen their bases form at anywhere from a fewhundred to 3,000 meters and extend upward fromtheir bases to as high as 20,000 meters. Verticaldevelopment clouds originate from lapse(unstable) conditions beneath them. Under thistype of cloud—regardless of its thickness or skycoverage—the temperature gradient may varyconsiderably. In general, vertically developedclouds over operational areas indicate that achemical operation must contend with anunfavorable temperature gradient andturbulence. Additionally, when clouds havesignificant development there is strong likelihoodof rain showers.

NOTE:The AWS routinely reports cloud heights infeet and wind speeds in knots. However,cloud heights may also be reported in metersby other nations. However, kilometers perhour or miles per hour may also be used.

PrecipitationRain and snow influence NBC operations and

must be considered. Precipitation results whencloud droplets or ice crystals grow large enough tofall. It is usually accompanied by neutral orunstable conditions.

Heavy rains or snows reduce the effectivenessof agent clouds by diluting or washing chemicalvapors or aerosols from the air, vegetation, andmaterial. Air Weather Service synoptic weatherforecasts predict the types, amounts, intensities,and general beginning and ending times ofprecipitation.

In tropical regions, afternoon showers tend tooccur at about the same time each day. Thistendency may also be influenced by seasonalfactors, such as the monsoons. Such regularity isnot common in temperate regions.

Precipitation causes some chemical agents tohydrolyze or to break down into less harmfulcompounds. Therefore, the amount ofprecipitation and the hydrolysis rate of somechemicals must be taken into account whenconsidering the use of some chemical agents.

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Atmospheric Stability

The criterion for describing the state of theatmosphere consists of dividing stability intothree categories-stable (inversion), neutral, andunstable (lapse). Stable conditions are usually anighttime condition that is assumed to exist froman hour before sunset to an hour after sunrise. Thedaylight hours from an hour after sunrise to anhour before sunset are presumed to be unstable.Neutral conditions are normally associated withhigh wind speeds greater than 15 knots or

overcast sky conditions for all wind speeds, day ornight. The atmosphere also tends toward neutralconditions at sunrise and sunset.

Determination of stability categories is basedon the relationships between surface wind speed,incoming solar radiation, vertical transfer of heatin the surface and planetary boundary layers,cloud cover, and the time of day.

On a battlefield, NBC operations may beconducted without the benefit of forecasts ormeasured surface winds. As a contingency forunavailable wind information, the Beaufort wind

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scale may be used to estimate wind speeds and is landscape, such as tree leaves and limbs. Table C-given in Table C-3. The scale provides an estimate 3 is easy to use, even by the inexperienced. It isof mean wind speed, an associated range about the based on a description of wind conditions and themean, and a description of the observed wind general specifications for use on land.effect upon easily recognized features of a

F a c t o r sThis section outlines factors that must be

considered for the analysis of NBC operations.Assessing some of these factors will be difficultunder combat conditions because of theprobability of very limited observations andintelligence concerning the battlefield and thetarget area.

The primary factors affecting a forecast arethe synoptic (general weather) situation,climatology, topography, vegetation, and soil. Inthis section, each factor is discussed in relation toits influence on forecasting wind, temperature,relative humidity, and the stability category. All ofthese must be known in planning NBC and smokeoperations. The forecaster needs hourlyinformation about these factors. This person usesthis information for analysis, applyingestablished principles.

Some of these principles are describedbasically in this manual, but they encompass alarge part of the whole field of meteorology. Only atrained meteorologist can be expected tounderstand and fully exploit these principlesproperly. However, under combat conditions,anyone making a weather analysis must try tocollect information on the following items:

Current general weather (synoptic) situation inthe target area.

Current upper air soundings close to orrepresentative of the target analysis. Thisinformation normally comes from artillerymeteorology sections. These sections at divisionand higher echelons transmit upper air data to thenearest AWS detachment.

Current surface winds in the target area.Topography in and around the target area.Times of sunrise and sunset.Vegetation.Types of soil at the target.

The first three factors are constantlychanging. The last four factors are relativelyconstant.

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Synoptic SituationThe synoptic situation is the general weather

situation over an extensive area. Normally,knowledge of the synoptic situation comes fromobservations taken at or near the same time toafford an accurate overall picture of weatherconditions. You usually receive this information ina synoptic situation map. (See an example inFigure C-7.)

In tropical areas and often in summer in themid-latitudes, the synoptic situation changes solittle or so slowly from day to day, and the sky is sooften clear, that the diurnal (daily) weathervariations account for much of the weather andare useful in forecasting. However, in the colderseasons of the mid-latitudes, marked day-by-daychanges in synoptic situations and weather are

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the rule. These changes tend to obscure any dailypattern to weather elements.

Estimating changes in the general weathersituation is feasible by using only localobservations. It is possible to draw conclusionsabout changing conditions with only a generalknowledge of weather prediction. For example, ifthe weather has been dominated by high pressure(clear skies with light winds), a trend (in thenorthern hemisphere) to falling pressure; risingwinds from the northeast, east, and southeast; andhigh clouds lowering to middle or low overcast willindicate a storm or disturbance (low pressuresystem) is approaching and steady rain or snowmay be expected in a few hours. A frontal passagefollowed by clearing skies and much colder air overthe region is generally preceded by low overcastskies, precipitation, southerly winds, low pressure,a wind shift to the west or northwest, fallingtemperature, and rising pressure.

Wind speed, temperature, vertical temperaturegradient, and percent of relative humiditynormally follow typical day-night (diurnal)patterns as presented in Figure C-8. In Figure C-8each vertical line represents two hours of time—between midnight to noon or noon to midnight.Low-level wind speeds experience diurnalvariations (Figure C-8, A), There is a well-definedmaximum wind speed in the afternoon at about thetime of maximum lapse. The minimum windspeed, which is not so well-defined, occurssometime during the night and continues untilabout sunrise, when the cycle begins again. Actualwind speeds, even when a diurnal variation isevident, vary with the synoptic situation,topography, and vegetation.

There is no diurnal change in wind directionexcept in valleys and in coastal areas where valleywinds or land and sea breezes may influence winddirection and/or speed at certain hours.

Wind speed and direction may change withheight. A contaminated cloud (released aloft or atthe surface) will be acted upon by the winds in eachlayer as it passes through that layer. To predict thelocation and extent of the downwind hazard area,the effects of the wind must be considered withineach layer as the material travels to the surface.The final hazard area characteristics (size,concentration, and cloud continuity) aredetermined by the collective action of each layerthrough which the cloud passes. (See Figure C-9 for

an exaggerated example of this effect.)Variations in wind speed and direction

determine the lateral spread of a chemical agentcloud. The greatest lateral spread will occur underlight wind conditions, because the wind directionis more likely to fluctuate at low wind speeds thanhigh wind speeds. Lateral spread may approach50 percent of the downwind distance the chemicalcloud travels. With steady winds, lateral spreadmay be only about 15 percent of the distancetraveled. Under average conditions, lateral spreadis about 20 percent of the distance traveled.

Vertical temperature gradients are functionsof the synoptic situation, cloudiness, and windspeed. With clear skies and light winds, stronginversion (at night) and lapse (during the day)

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conditions exist in the surface and planetaryboundary layers.

Overcast skies indicate only a weak inversionat daybreak, with midday temperature gradientsreaching values of only 1/5 to 1/4 as great as withclear skies. At night a gradient may form that willbe only 1/10 that formed under clear skies.

With partly cloudy skies, temperaturegradients are between the just-discussed 1/10extremes. The temperature gradient is unaffectedby cloudiness up to 3/10. With sky cover of 4/10 to6/10, temperature gradients decrease by amountsof 1/10 to 1/5. With sky cover of 7/10 to 9/10,temperature gradients reduce by 1/5 to 1/2 of thevalues for clear skies.

There is a large diurnal variation in the typeand degree of temperature gradient (Figure C-8,C). The variation is least in winter and decreaseswith increased cloudiness. In winter with overcastskies, the temperature gradient is substantiallyneutral throughout the day and night. When theground is covered with snow or ice, an inversion ismore probable than a neutral condition, and alapse condition is rare regardless of clouds andtime of day. The latter is especially true in polarregions.

As the wind speed increases, the strength ofthe inversion decreases. Ground inversions do notexist in winds over about 20 knots. Strong windspeeds tend to erase vertical temperaturegradients at the surface. Strong lapse rates(unstable conditions) can result in variable andgusty winds.

Over the sea, diurnal variation in temperaturegradients near the surface is almost absent. Thisis because of the sameness of the sea surfacetemperature at a given location.

In mid-latitudes, neither very stable nor veryunstable conditions exist with precipitation. Thismay not be true in tropical or polar regions. Withthe formation of ground fog, a neutral or weakinversion condition is most probable in the lowerlayers.

The stability of the air layer above the surfaceboundary layer does not always indicate thetemperature gradient in the lower layer. It is notpossible to predict lapse conditions in the lowerlayer from observation or forecast strong lapseconditions aloft without considering the factors ofwind,wind

clouds, and ground temperature.tends to stir up any stable air

A strongnear the

ground and thus destroys or prevents formation ofan inversion. A light wind favors the formation ofinversions. Upper air soundings or radiosondesare thus very useful in forecasting weather.

The intelligence section furnishes thereported surface air temperatures. There is apronounced diurnal trend of this temperature. Theminimum temperature occurs just before dawnand the maximum in the early afternoon. Thedaily range in temperature increases in the lowestlayer; thus, the greatest diurnal variation occursat the surface of the earth.The diurnal range of temperature may be as greatas 55°C over the desert or as small as 1°C overwater; but the actual range depends uponcloudiness, vegetation, and the composition of theearth’s surface.

With increasing cloudiness, outwardradiation from the earth is blocked by the cloudsand temperature ranges are reduced. The lowerand denser the clouds, the greater their effect onmodifying surface temperatures. Also, thepercentage of sky covered by any particular typeof cloud is a consideration. If clouds are high andscattered, the outward radiation is only about 4percent less than that with clear skies; but ifclouds are low and the sky is overcast, theoutgoing radiation is approximately 90 percentless than with clear skies.

There is a diurnal trend of relative humidity inthe surface layer of the atmosphere. (See FigureC-8, D.) The magnitude of the relative humidity isdetermined by the temperature and the absolutehumidity. Maximum relative humidity occurs atthe time of minimum temperature, during theearly morning hours. Minimum relative humidityoccurs in the afternoon, at the time of maximumtemperature, if the absolute humidity remainsunchanged.

If a pronounced change in weather is takingplace, weather change influences the relativehumidity more than does the temperature. Thus, ifit is raining or there is fog, the relative humiditywill be near 100 percent even though it isafternoon when relative humidity is normally at aminimum.

ClimatologyClimatology

climate. Climateis the science dealing withis a historical average of the

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weather for a place or area over a given period.Current weather is a phase—a single element—inthis average.

Personnel can use climatological summariesto determine expected weather conditions withincertain probability limits to aid long-rangeplanning. However, they should use currentweather information to predict short-rangeconditions. Climatic and special data summariesare available for locations with weather records.The summaries can provide diurnal variations,frequencies, and correlations between elements.This information should be used as early aspossible in the planning stage.

Climatological wind values at a givenlocation are normally available for most cities andairfields but may not be available for smallsettlements or unpopulated areas. For these, youmust estimate expected conditions, using data fornearby areas. Wind data can be obtained forspecific hours of the day to show diurnal trends, orthey can be averaged to show mean conditions.

An indication of the probability of differentstability categories may be forecast from theclimatological values of sky cover and wind for thearea. Particular attention must be paid to thedifference between day and night conditions,because the diurnal trend of temperature gradientunder the two conditions varies widely, eventhough the climatological average of sky coverand wind may not.

the hill, displaying the normal friction eddies(Figure C-10, A.)

Air crossing the crest of a hill tends to eddy(Figure C-10, B). The sharper the hillcrest and thesteeper the drop on the leeward side, the morepronounced the eddy (Figure C-10, C). Heavilycontoured terrain or mountainous regions tend tohave sharp eddies and pronounced updrafts in theair above them. Mountain ranges may deflect thesurface airflow for an appreciable distance. Steephills split the wind so that there is an eddyingaround the hill as well as over it. In such cases, ifthe leeward side is very steep, providing shelter forthe air in the valley, airflow in the valley tends tobe very slight, speeds are low, and large-scale airmovements are unlikely (Figure C-10, D).

TopographyTopography (terrain conditions) influences

the wind, temperature gradient, temperature, andhumidity.

A topographic (local) wind is usuallydescribed in terms of what would occur if theeffects of the overall synoptic situation wereeliminated. These winds may be thermally orgravitationally induced. Topography influencesthe wind by thermal effects and by physicallydiverting and altering the normal flow of wind.Figure C-10 makes use of arrows and fishhooks toshow flow of the wind. Arrows and fishhooks areused in other figures to depict wind direction andmotion. Air tends to follow the line of a valley.Also, air moving up a hillside does not leavethe ground at the hilltop but tends to remainon the ground and travel down the other side of

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Obstacles, such as buildings, large rockyoutcrops, and groves of small trees, also causeeddies. Such eddies may influence the wind for adownwind distance 15 to 20 times the height of theobstacle producing the eddy. Beyond thisdistance, these obstacles have little influence(Figure C-11).

Air moving over a forest is turbulent wherethe uneven treetops disturb the air flowing overthem (Figure C-12, A). Large eddies form wherethere is a sufficient opening or a well-defined treeline that permits the air to descend to the groundagain (Figure C-12, B). The same turbulence mayoccur if the free air blows at right angles to aforested gully, although air in such gullies or inriverbeds is normally well protected from eddiesand turbulence (Figure C-12, C).

Thermally induced topographic winds resultfrom definite temperature differences. Examplesof such winds are valley-mountain winds, slopewinds, and land-sea breezes. Sea breezes andvalley winds result from daytime heating of theearth’s surface. Land and mountain breezes resultfrom nighttime cooling of the ground.

Valley winds and slope winds usually arepresent at all latitudes where there are hills andmountains, regardless of weather conditions (seeFigure C-13). However, they develop best withclear skies and weak winds aloft. In the tropics,where seasonal changes are small,develop best during the dry season.

such winds

Valley winds develop best in large, broad, U-shaped valleys with gently sloping floors to highridges, rather than in steep V-shaped valleys.

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Up-valley winds develop whenground radiation, rises and moves

air, heated byup the valley—

under a layer of cooler air. This cooler air occupiesapproximately the upper third of the valley airmass. Up-valley winds begin one to four hoursafter sunrise, earlier in small valleys, and often notin large valleys until late morning. Maximumdevelopment is reached in the early afternoon insmall or side valleys and at midafternoon in largevalleys. Valley winds reach their maximum speedat about one-third of the ridge height. Minimumspeed of valley winds occurs at the boundary of thevalley system, immediately below the ridge top.

Up-valley winds have greater vertical depththan down-valley winds. Up-valley winds may fillan entire valley to the height of the surroundingridges. These winds continue until about an hourbefore sunset.

Down-valley winds, resulting from cooler airflowing down along the cooling ground surface, setin one to three hours after sunset. With favorableconditions of light winds and clear skies, thesewinds persist until one to two hours after sunrise.They are shallower than up-valley winds, beingonly about 1/3 to 2/3 as deep. Their horizontaldevelopment is limited by the immediate

surrounding hills.The extent and intensity of topographic up-

valley and down-valley winds are controlled by theshape of the valley, height of the ridges, sky cover,and gradient winds. Strong surface winds canobliterate topographic valley winds. Valley windsusually have slow air movements but maysometimes combine with another wind system,such as the sea breeze along a coastal valley, tocause a stronger wind. If the mouth of the valley isnarrow, the down-valley wind accelerates and thiseffect may extend outward for several kilometers.The wind speed may increase to 30 knots or moreand is difficult to predict.

Slope winds are thermally induced by hill andmountain sides. Upslope winds set in during theday and downslope winds at night (Figure C-14).On clear days, upslope winds begin 15 to 45minutes after sunrise and stop about sunset,reaching a maximum speed around noon. It is verydifficult to predict upslope winds.

Downslope winds usually begin 15 to 45minutes after sunset and tend to persistthroughout the night until shortly after sunrise. Inthe northern hemisphere, south-facing slopes havethe most fully developed slope winds, both in

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vertical development and velocity. North-facingslopes have the least-developed slope winds. Themaximum depth of slope winds is about 200meters, which may be attained on large mountainslopes under conditions of clear skies and lightgradient winds.

Land and sea breezes occur almost daily intropical and mid-latitude regions on the coasts ofall islands and continents. They occur because theland cools and heats more rapidly than theadjacent water. (See Figure C-15.) Breezes developquickly during the dry season on clear days withlight gradient winds, during any season, and evenduring bad weather. With little or no surface wind(0 to 4 knots), the sea breeze sets in about two hoursafter sunrise and increases to its maximum inmidafternoon. The sea breeze stops one to twohours before sunset. The land breeze sets in shortlyafter sunset.

An off-land surface wind of 4 to 9 knots delaysthe onset of the sea breeze until late morning ormidday. If the surface wind is off-land at 9 to 12knots, the sea breeze will not set in untilmidafternoon. Under these conditions, the seabreeze starts several kilometers out to sea. It

slowly progresses shoreward and arrives on landas a gusty, sharp wind shift. Surface winds of 13knots or greater neutralize all but the strongest seabreezes.

With a strong surface wind from the seatoward the land, no real land and sea breezes exist.The effect of the land and sea breezes is then aforce added to the surface wind. The sea breeze isdiscernible as a daytime strengthening of thewind, and the land breeze is observed as anighttime weakening of the wind.

Sea breezes are deeper and stronger than landbreezes. They usually begin about midmorningand begin to subside toward evening. Sea breezesestablish at approximate right angles to the coast.Sea breezes increase in speed from their onset,with maximum speeds being reached in theafternoon, usually about one hour after themaximum land temperature is reached. Maximumspeeds exist just above the surface and ordinarilyare not more than 13 knots.

Sea breezes usually extend aloft from 200 to500 meters, but they have been observed at over700 meters. On extremely large islands, themovement of air in a sea breeze is quite appreciable

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at 15 to 20 kilometers from shore. Inlandpenetration of sea breezes depends on topography;and under favorable conditions such as valleysrunning inland, they may penetrate 35 to 50kilometers. Sea breezes are nonturbulent whenthey first reach the shore but become more andmore turbulent in passing over land.

A land breeze is considerably weaker andmore shallow than a sea breeze. It usuallyinfluences a layer less than 200 meters thick. Itstarts about an hour after sunset and ends shortlyafter sunrise. On flat terrain, land breezes rarelyexceed 3 knots, but, in combination with down-valley winds, they may attain considerable speed.Because of passage over ground, land breezes areinclined to be turbulent in nature; however, thisturbulence is dampened by inversion (stable)conditions normally occurring during this period.Land breezes usually extend only 6 to 8 kilometersout to sea.

At sea, the stability of the surface boundarylayer depends upon the relative temperatures of airand water. Warm air moving over cold waterresults in stable conditions, and cold air movingover warm water creates instability. In shallowwater, such as in a lagoon, there is an appreciablewarming of the water by the sun; lapse (unstable)conditions may exist in the lower levels but not tothe extent of causing large convection currents.

On land, the main influence of topography isslope effects on stability. Since south-facing slopesreceive the greatest amount of incoming solarradiation in the northern hemisphere, theydevelop lapse conditions stronger and morepersistent than those on north-facing slopes. Inthe southern hemisphere, the north-facing slopesattain the strongest lapse conditions. Stable(inversion) conditions are always associated withlight, nocturnal downslope or down-valley winds,and unstable (lapse) conditions accompanyupslope or strong daytime up-valley winds.

Topography influences temperature byaffecting the amount of incoming solar radiationthat a parcel of land receives. This is determinedprimarily by the angle of the slope, direction of itsexposure to the sun, latitude, and the season.

South-facing slopes receive the greatest directincoming solar radiation in the northernhemisphere and reach maximum temperatures.For example, at a latitude of 45 degrees, a north-facing slope of 30 degrees receives no direct solar

radiation from November through February.Temperatures are correspondingly lower than onsouth-facing slopes, which are exposed to solarradiation during these periods. Figure C-16 showsthe solar loading situation for the northernhemisphere. The opposite situation occurs in thesouthern hemisphere.

Two sources of thermal energy affect airtemperature. These sources are incoming solarradiation and heat being radiated outward fromthe earth. Incoming solar radiation peaks on adaily and a seasonal basis. The daily peak occursone to two hours after noon when the sun is at itshighest. The seasonal peak occurs during thesummer solstice when the sun reaches its zenith inthe summer sky. Incoming solar radiation alsoshows a seasonal minimum, at the winter solstice,but does not truly have a daily minimum. Instead,from about sunset to about sunrise, the value ofthe incoming solar radiation is zero. Maximumand minimum air temperature lags are illustratedin Figure C-17.

Relative humidity increases when influencedby nearby watery areas, such as swamps or lakes.Also, when winds move up a high mountain slope,the relative humidity tends to increase, as shownby frequent mountaintop clouds.

Vegetation and WindLeaves and branches produce drag on wind

blowing through and across the vegetation. Thedenser the vegetation, the greater the decrease inwind speed. Also, the taller the vegetation, thegreater the depth of the friction layer. Finally, themore uneven the top of the vegetation, the greaterthe turbulence induced in the wind flowing overthe vegetation.

In scattered tall shrubs and scrub forests, thesparse vegetation produces only a moderate drageffect; thus, the wind speed is only slightlyinfluenced. The predominant effect is a deflectionof the airflow and changes in wind direction.When the vegetation is thick enough to beclassified as medium-dense, the wind speed anddirection at any given instant will differ fromlocation to location.

In coniferous (evergreen) forests, high windspeeds at the surface are rare and wind direction isextremely variable. This also is true in medium-dense deciduous (seasonal, leafy) forests in full

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leaf. Tropical jungles affect the mean wind floweven more adversely than do temperate latitudeforests. The thick, luxuriant growth reducessurface wind speeds to extremely low values, andwind directions become meandering.

Vegetation and TemperatureVegetation influences the temperature

gradient by keeping much of the solar energy frompenetrating to the ground. It has a blanketing orinsulating effect, so that strong lapse or stronginversion conditions are not probable in heavilyvegetated areas. The temperature gradient withinor below vegetation approaches a neutralcondition as the density of the vegetationincreases.

In tall shrubs or scrub forests, shading affectsthe air adjacent to the ground, so that extremesurface temperatures are not reached and stronginversion or lapse conditions are relativelyinfrequent. In coniferous forests, moderate lapseand inversion conditions can and do occur.However, they are not as frequent or as marked asin the open. Temperature gradients in the forest

range from one-third to one-half of the valuesattained in the open. The tendency is towardneutral temperature gradients.

Vegetation influences temperature by raisingthe radiating level from the earth’s surface to thetop of the vegetation, which acts as the coolingand heating surface. Maximum and minimumvalues are, therefore, reached at or nearvegetation top, while the temperatures below thislevel are modified. Since vegetation has aninsulating effect, maximum temperatures are notas high and minimum temperatures are not as lowas those in the open. The normal diurnal trend oftemperatures continues, but the vegetation causesa time lag in reaching maximum and minimumvalues. The degree of these effects is determinedby the density of the vegetation.

In grass, the lag in reaching maximum andminimum temperatures is small, andtemperatures are not appreciably modified. In tallshrubs and scrub forests, also, the lag is slight; buttemperatures at 1.5 meters are considerablymoderated.

In coniferous and deciduous forests, the timelag between forest temperatures and temperatures

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in the open is approximately two hours. Foresttemperatures are usually 5 to 6 degrees lower atmidday than those at corresponding levels in theopen. Temperatures in a forest at night are higherthan nighttime temperatures in the open.

In a tropical jungle, the lag in reachingmaximum temperatures is about one hour. Sincethe diurnal range of jungle temperature is so smallin tropical climates (8°C), the difference betweenjungle and open temperatures is only 10 to 3°C.

There is a pronounced and regular trend ofrelative humidities in forests and jungles.Minimum relative humidities occur withmaximum temperatures, and maximum relativehumidities occur with minimum temperatures.Relative humidities within a forest averagebetween 60 and 90 percent. During the night andforenoon, the relative humidity within a forest isabout 5 percent higher than in the open; but in theafternoon, until about sunset, the relativehumidity in the forest may be from 15 to 20 percenthigher than in the open.

In canopied jungles on tropical islands or nearwindward tropical coasts, the relative humidityranges from 65 to 95 percent and is almost alwayswithin 5 percent of the relative humidity in theopen.

SoilSoil influences ground burst munitions and

ground contamination. Variations in soil typesusually do not materially influence winds. The

primary effect of soil on temperature gradient isdue to the great range of temperatures attained atthe soil surface. Compared to other naturalsurfaces, bare rock attains the highest daytimesurface temperature and the lowest nighttimetemperature. Bare soil, with the exception of sand,acts like rocks, but to a lesser extent.

On a snow-covered surface, snow is thecritical factor in determining the temperaturegradient. Inversions (stability) on snow-coveredsurfaces are strongest about sunrise; but duringclear, cold, and calm weather, the temperatureinversion is not always completely destroyedduring the day. In polar regions, winter inversionsmay persist for days, or even weeks, because of thelow sun or nearly complete darkness.

Soil and vegetation surface temperatures areimportant because they are a factor indetermining the rate of evaporation of agents usedfor persistent effect. The primary factors indetermining rate of evaporation of these agentsare surface temperature and the rate at which thesubstance flows out to the radiating surface. Thisrate varies with the nature and texture of thesurface. Heavy clay soil does not absorb as readilyas porous sandy soil. The extent of absorptionfurther depends upon the relative depth of theabsorbing topsoils and subsoils, type of soil, andmoisture content.

The soil also affects relative humidity valuesas it alters the temperature. Evaporation from wetsoils tends to raise the relative humidity.

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G l o s s a r y

absolute humidity (also called vapor concen-tration and vapor density) - in a system ofmoist air, the ratio of the mass of water vaporpresent to the volume occupied by the mixture,that is, the density of the water vaporcomponent. It is not commonly used bymeteorologists. See relative humidity.

absorption - the process of an agent being takeninto the vegetation, skin, materiel, or soil.Important for only a few agents.

active front - the boundary between two dif-ferent air masses, or a portion thereof, whichproduces appreciable cloudiness andprecipitation and is usually accompanied bysignificant shifts in wind direction.

adiabatic lapse rate - the rate of decrease oftemperature with height of a parcel of dry airlifted upward through the atmosphere with noaddition or deletion of heat.

adiabatic process - a thermodynamic change ofstate of a system in which there is no transfer ofheat or mass across the boundaries of thesystem. In an adiabatic process, compressionalways results in warming, expansion incooling. In meteorology the adiabatic processoften is also taken to be a reversible process. Formany purposes, changes of state in the freeatmosphere over periods of two days or less areassumed to be adiabatic.

adsorption - adding a thin layer to vegetation(usually aerosol). Important in densevegetation.

advection fog - a type of fog caused by thepassage of moist air horizontally over arelatively colder surface and the consequentcooling of that air to below its dew point.

adverse weather - weather in which militaryoperations are generally restricted or impeded.

aerology - the study of the air and of theatmosphere. Used in the US Navy until early1957. The same as meteorology; however, thisusage tended to be more administrative thanscientific.

aerosol - a colloidal system in which thedispersed phase is composed of either solid or

liquid particles and in which the dispersalmedium is some gas, usually air. There is noclear-cut upper limit to the size of the particlescomprising the dispersed phase in an aerosol,but as in all other colloidal systems, it is rathercommonly set at 1 micron. Haze, most smokes,and some fogs may thus be considered aerosols.

air - the mixture of gases comprising the earth’satmosphere. Since the composition of theatmosphere is slightly variable with respect tocertain components, the term "pure air" has noprecise meaning, but is commonly used to implyfreedom from nongaseous suspensoids (dust,hydrometeors) and also freedom from suchgaseous contaminants as industrial effluents.

air drainage - general term for gravity-induced,downslope flow of relatively cold air. Windsthus produced are called gravity winds.

air mass - a widespread body of air, the propertiesof which can be identified as (a) having beenestablished while that air was situated over aparticular region of the earth’s surface (air-mass source region) and (b) undergoing specificmodifications while in transit away from thesource region. An air mass is often defined as awidespread body of air that is approximatelyhomogeneous in its horizontal extent,particularly with reference to temperature andmoisture distribution; in addition, the verticaltemperature and moisture variations areapproximately the same over its horizontalextent.

air mass classification - a system used toidentify and to characterize the different airmasses according to a basic scheme. A numberof systems have been proposed, but theBergeron classification has been the mostwidely accepted. In this system, air masses aredesignated first according to the thermalproperties of their source regions: tropical (T);polar (P); and less frequently, arctic or antarctic(A). For characterizing the moisturedistribution, air masses are distinguished as tocontinental (c) and maritime (m) source regions.Further classification according to whether the

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air is cold (k) or warm (w) relative to the surfaceover which it is moving indicates the low-levelstability conditions of the air, the type ofmodification from below, and is also related tothe weather occurring within the air mass. Thisoutline of classification yields the followingidentifiers for air masses:

cTk continental-tropical-coldcTw continental-tropical-warmmTk maritime-tropical-coldmTw maritime-tropical-warmcPk continental-polar-coldcPw continental-polar-warmcAk continental-arctic-coldcAw continental-arctic-warmmAw maritime-arctic-warmmPk maritime-polar-coldmPw maritime-polar-warmmAk maritime-arctic-cold

air mass source region - an extensive area ofthe earth’s surface over which bodies of airfrequently remain for a sufficient time toacquire characteristic temperature andmoisture properties imparted by that surface.Air so modified becomes identifiable as adistinct air mass. See air mass.

air parcel - an imaginary body of air to whichmay be assigned any or all of the basic dynamicand thermodynamic properties of atmosphericair. A parcel is large enough to contain a verygreat number of molecules, but small enough sothat the properties assigned to it areapproximately uniform within it and so that itsmotions with respect to the surroundingatmosphere do not induce markedcompensatory movements. It cannot be givenprecise numerical definition, but a cubic foot ofair might fit well into most contexts where airparcels are discussed, particularly those relatedto static stability.

albedo - the fraction of light or the amount ofelectromagnetic radiation reflected by a body tothe amount incident upon it, commonlyexpressed as a percentage. The albedo isdistinguished from the reflectivity, which refersto one specific wavelength (monochromaticradiation). (As the moon, a planet, a cloud, theground, or a field of snow reflects light.)

altocumulus (abbreviated Ac) - a principalmedium-level cloud type, white and/or gray incolor, which occurs as a layer or patch with a

waved aspect, the elements of which appear aslaminae, rounded masses, or rolls. Theseelements usually are sharply outlined, but theymay become partly fibrous or diffuse; they mayor may not be merged; they generally haveshadowed parts; and, by convection, whenobserved at an angle of more than 30° above thehorizon, subtend an angle between 1° and 5°.

altostratus (abbreviated As) - a principalmedium-level cloud type in the form of a gray orbluish (never white) sheet or layer of striated,fibrous, or uniform appearance. Altostratusvery often totally covers the sky, and may, infact, cover an area of several thousand squaremiles. The layer has parts thin enough to revealthe position of the sun; and if gaps and riftsappear, they are irregularly shaped and spaced.

anabatic wind - an upslope wind; usuallyapplied only when the wind is blowing up a hillor mountain as a result of local surface heatingand apart from the effects of the larger scalecirculation; the opposite of katabatic wind. Themost common type anabatic is the valley wind.

APC - armored personnel carrier.arctic front - the semipermanent, semicontinuous

front between the deep, cold arctic air and theshallower, basically less cold polar air ofnorthern latitudes; generally comparable to theantarctic front of the southern hemisphere.

arty met - artillery meteorological.ATGM - antitank guided missile.AWS - Air Weather Service.bora - a cold, often dry, northeasterly wind which

blows, sometimes in violent gusts, down frommountains on the eastern shore of the Adriatic.It also applies to cold, squally, downslope windsin other parts of the world.

CARC - chemical agent resistant coating.CDM - chemical downwind message.Celsius (abbreviated C) - (formerly referred to as

centigrade) thermometric scale with 100 degreesbetween freezing and boiling, OC for freezingand 100°C for boiling.

centigrade - see Celsius.chinook - the name given to the descending,

warm, dry wind on the eastern side of the RockyMountains. The chinook generally blows fromthe southwest, but its direction may be modifiedby topography. When it sets in after a spell ofintense cold, the temperature may rise by 20°Fto 40°F in 15 minutes due to replacement of a

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cold air mass with a much warmer air mass inminutes.

cirrocumulus (abbreviated Cc) - a principalhigh-level cloud type appearing as a thin, whitepatch of cloud without shadows, composed ofvery small droplets in the form of grains orripples. The elements may be merged orseparate, and more or less regularly arranged;they subtend an angle of less than 10 whenobserved at an angle of more than 30° above thehorizon. Holes or rifts often occur in a sheet ofcirrocumulus.

cirrostratus (abbreviated Cs) - a principalhigh-level cloud type appearing as a whitishveil, usually fibrous but sometimes smooth,which may totally cover the sky and whichoften produces halo phenomena, either partiallyor completely. Sometimes a banded aspect mayappear, but the intervals between the bands arefilled with thinner cloud veil. The edge of the veilof cirrostratus may be straight and clean-cut,but more often it is irregular and fringed withcirrus. Some of the ice crystals that comprise thecloud are large enough to fall and therebyproduce a fibrous aspect. Cirrostratusoccasionally may be so thin and transparent asto render it almost indiscernible, especiallythrough haze or at night. At such times, theexistence of a halo may be the only revealingfeature, such as producing a halo around themoon.

cirrus (abbreviated Ci) - a principal high-levelcloud type composed of detached cirriformelements in the form of white, delicatefilaments, of white (or mostly white) patches, orof narrow bands. These clouds have a fibrousaspect and/or a silky sheen. Many of the icecrystal particles of cirrus are sufficiently largeto acquire an appreciable speed of fall; therefore,the cloud elements have a considerable verticalextent. Wind shear and variations in particlesize usually cause these fibrous trails to beslanted or irregularly curved. For this reason,cirrus does not usually tend, as do other clouds,to appear horizontal when near the horizon.Because cirrus elements are too narrow, they donot produce a complete circular halo.

climate - the long-term manifestations ofweather. The climate of a specified area isrepresented by the statistical summary of itsweather conditions during a period long enough

to ensure that representative values areobtained (generally 30 years).

climatic study - analysis and interpretation ofclimatic summary data in light of probableimpacts on operations, plans, construction, andthe like.

climatic summary - tabular data for averages,extremes, and frequencies of weather elementsor phenomena for a year, season, month, orother period at a specific location or area.

climatology - the science that deals withclimates and investigates their phenomena andcauses.

cloud - a collection of very small water droplets orice crystals or both, with its base above theearth’s surface.

colloidal system (also called colloidal disper-sion, colloidal suspension) - an intimatemixture of two substances one of which, calledthe dispersed phase (or colloid), is uniformlydistributed in a finely divided state throughoutthe second substance, called the dispersionmedium (or dispersing medium). The dispersionmedium may be a gas, a liquid, or a solid, andthe dispersed phase may also be any of these,with the exception that one does not speak of acolloidal system of one gas in another. A systemof liquid or solid particles colloidally dispersedin a gas is called an aerosol. A system of solidsubstances or water-insoluble liquidscolloidally dispersed in liquid water is called ahydrosol.

coniferous forests - concentrations of evergreentrees normally found on slopes and mountains;for chemical behavior purposes, the same asmedium-dense deciduous forests or woods(when in full foliage only).

CONUS – continental United States.convection - in general, mass motions within a

fluid resulting in transport and mixing of theproperties of that fluid. Convection, along withconduction and radiation, is a principal meansof energy transfer. Distinction is made betweenfree convection (or gravitational convection)—motion caused only by density differences withthe fluid-and forced convection—motioninduced by mechanical forces such as deflectionby a large-scale surface irregularity, turbulentflow caused by friction at the boundary of afluid, or motion caused by any applied externalforce.

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coriolis force - a force exerted on a parcel of air(or any moving body) due to the rotation of theearth. This force causes a deflection of the bodyto the right in the northern hemisphere and tothe left in the southern hemisphere.

cumulonimbus (abbreviated Cb) - a principalcloud type, exceptionally dense and verticallydeveloped, occurring either as isolated clouds oras a line or wall of clouds with separated upperportions. These clouds appear as mountains orhuge towers, at least a part of the upper portionsof which are usually smooth, fibrous, or striated,and almost flattened. This part often spreadsout in an anvil shape (incus) or vast plume.Under the base of a cumulonimbus, which oftenis very dark, there frequently exists virga,precipitation, and low, ragged clouds, eithermerged with it or not. Its precipitation is oftenheavy and always of a showery nature. Theusual occurrence of lightning and thunderwithin or from this cloud leads to its commonnames, thundercloud, thunderhead (usuallyrefers only to the upper portion of the cloud), andthunderstorm.

cumulus (abbreviated Cu) - a principal cloudtype in the form of individual, detachedelements which are generally dense, low-levelwith vertical development and possess sharpnonfibrous outlines. These elements developvertically, appearing as rising mounds, domes,or towers, the upper parts of which oftenresemble a cauliflower. The sunlit parts of theseclouds are mostly brilliant white; their bases arerelatively dark and nearly horizontal. Near thehorizon, the vertical development of cumulusoften causes the individual clouds to appear tobe merged. If precipitation occurs, it is usually ofa showery nature.

current weather report - information onexisting weather conditions or specific weatherelement; may be oral, written, or graphicrepresentations.

cyclone - a system of winds rotating around acenter of low atmospheric pressure. A cyclonerotates counterclockwise in the northernhemisphere and clockwise in the southernhemisphere (opposite to that of an anticyclone).Modern meteorology restricts the use of the termcyclone to the cyclonic-scale circulations. But, itis still applied popularly to the more or lessviolent, small-scale circulations such as

tornadoes, waterspouts, and dust devils (whichmay in fact exhibit anticyclonic rotation), andeven, very loosely, to any strong wind. Becausecyclonic circulation and relative lowatmospheric pressure usually coexist (in thenorthern hemisphere), in common practice theterms cyclone and low are used interchangeably.Also, because cyclones nearly always areaccompanied by inclement (sometimesdestructive) weather, they are frequentlyreferred to simply as storms.

deciduous forests - concentrations of seasonal,leafy trees; for chemical behavior calculations,the same as coniferous forests or woods when infull foliage only.

deliberate smoke - smoke operations which areplanned with much detail for implementationover large areas or relatively long time periods.

dew - water condensed onto grass and otherobjects near the ground. Dew forms whentemperatures fall below the dew point of thesurface air due to radiational cooling during thenight but are still above freezing. Hoarfrost (orwhite frost) forms if the dew point is belowfreezing. If the temperature falls below freezingafter the dew has formed, the frozen dew isknown as white dew or jackfrost.

dew point (or dew point temperature) -temperature to which a given parcel of air mustbe cooled at constant pressure and constantwater vapor content for saturation to occur.When this temperature is reached, water iscondensed onto grass and other objectscontacting the cooled air. When the dew point isbelow 32°F (0°C), it is sometimes called the frostpoint. The dew point may be defined as thetemperature at which the saturation vaporpressure of the parcel is equal to the actualvapor pressure of the contained water vapor.

diffusion - exchange of airborne media betweenregions in space in an apparently randommotion of a small scale.

diurnal - repeated or recurring daily. Having adaily cycle of completed actions in 24 hours andrecurring every 24 hours. Thus, most referenceis made to diurnal tasks, cycles, tides, or sunriseto sunset.

dose - amount of agent taken into or absorbed bythe body.

dose rate - how fast a dose is absorbed or takeninto the body.

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drizzle - very small, numerous, and uniformlydispersed water drops, mist, or sprinkle. Unlikefog droplets, drizzle falls to the ground. It issometimes accompanied by low visibility andfog.

dry-bulb humidity - dryness of the free airas measured by use of two thermometers. One isdry-bulb and the other is wet-bulb. Thedifference between the two readings is thehumidity for surrounding air. (See hygrometer,psychrometer.)

dry-bulb temperature - temperature of the freeair as measured with a dry thermometer on asling psychrometer over a grassy surface at aheight of approximately 6 feet (1.8 meters).

easterlies - any persistent wind from the east(usually applied to broad currents or belts ofeasterly winds). The easterly belts are referredto as the equatorial easterlies, the tropicaleasterlies, and the polar easterlies.

electromagnetic spectrum - the entire range ofwavelengths of all known electromagneticradiations extending from gamma rays throughvisible light, infrared, and radio waves. It isdivided into 26 alphabetically designatedbands.

fire storm - an atmospheric wind system causedby a large fire (as after the bombing of a city).The intense burning creates vertical windcurrents, which induces a strong wind to bringin more air to feed the fire. Incoming wind speedcan exceed 60 knots in extreme cases.

foehn - name for a warm dry wind blowing downthe side of a mountain in northern and centralEurope; (same as chinook-type warm dry windthat descends eastern slopes of RockyMountains).

fog oil - petroleum based oil specially blended toproduce a dense, efficient screening smokewhen vaporized and recondensed atatmospheric temperatures. Officially, fog oil isstandard grade fuel number 2 (SGF2).

front - in meteorology, generally, the interface orboundary between two air masses of differentdensity. Since the temperature distribution isthe most important regulator of atmosphericdensity, this front almost invariably separatesair masses of different temperatures. Frontsreceive their names from the movement of theair masses involved. A cold front is the leadingedge of an advancing mass of cold air. A warm

front is the trailing edge of a retreating mass ofcold air. When an air mass boundary is neitheradvancing nor retreating along the surface, thefront is called a stationary front. An occludedfront occurs when a cold front overtakes a warmfront at the surface and a temperature contrastexists between the advancing and retreatingcold air masses.

frost - a cover of minute ice crystals on objectsthat are exposed to the air. Some of these are treebranches, plant stems, leaves, wires, poles,vehicles, rooftops, or aircraft skin. Frost is thesame process by which dew is formed exceptthat the temperature of the frosted object isbelow freezing. Frost can be light or heavy.

FWS - Fleet Weather Service.geostrophic - relates to or arises from the

deflective force exerted on the atmosphere due tothe rotation of the earth.

geostrophic wind - a wind whose direction andspeed are determined by a balance of thehorizontal pressure gradient force and the forcedue to the earth’s rotation to the left in thenorthern hemisphere and to the right in thesouthern hemisphere.

geostrophic wind level (also called gradientwind level) - the lowest level at which the windbecomes geostrophic. In practice, thegeostrophic wind level is between 1.2 kilometers(3,928 feet) and 1.6 kilometers (5,238 feet). Thiswind level probably marks the upper limit offrictional influence of the earth’s surface. Thegeostrophic wind level may be considered to bethe top of the planetary boundary layer, that is,the base of the free atmosphere.

glaze - a smooth coating of ice formed on objectsdue to the freezing of rain.

gradient wind - any horizontal wind velocitytangent to the contour line of a constantpressure surface (or to the isobar of ageopotential surface) at or above 2,500 feet (762meters).

gravity wind - see air drainage and katabaticwind.

greenhouse effect - the heating effect exerted bythe atmosphere upon the earth because theatmosphere (mainly, its water vapor) absorbsand reemits infrared radiation. In detail, theshorter wavelengths of solar radiation aretransmitted rather freely through theatmosphere to be absorbed at the earth’s

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surface. The earth then reemits this as long-wave (infrared) terrestrial radiation, a portionof which is absorbed by the atmosphere andagain emitted. Some of this is emitteddownward back to the earth’s surface(counterradiation). It is essential, inunderstanding the concept of the greenhouseeffect, to note that the important additionalwarming is due to the counterradiation from theatmosphere. The glass panes of a greenhousefunction the same way as the atmosphere doesto maintain high greenhouse temperatures andhence the name.

hasty smoke - smoke operations conducted witha minimum of prior planning usually to counterenemy action or anticipated action ofimmediate concern to a commander.

HC - hexachloroethane.head wind - in this manual, wind blowing away

from the objective and directly toward your site.heavily wooded - for chemical behavior

purposes, jungles or forests with canopies thatshade more than 90 percent of the groundsurface beneath.

hoarfrost (commonly called frost, whitefrost, crystalline frost, or hoar) - a depositof interlocking ice crystals formed by directsublimation on objects, usually those of smalldiameter freely exposed to the air, such as treebranches, plant stems and leaf edges, wires, andpoles. Also, frost may form on the skin of anaircraft when a cold aircraft flies into warm andmoist air or when it passes through air that issupersaturated with water vapor. Hoarfrost isformed similarly to the way dew is formedexcept that the temperature of the frosted objectmust be below freezing. Frost forms when airwith a dew point below freezing is brought tosaturation by cooling. In addition to itsformation on freely exposed objects (air hoar),hoarfrost also forms inside unheated buildingsand vehicles, in caves, in crevasses (crevassehoar), on snow surfaces (surface hoar), and inair spaces within snow, especially below a snowcrust (depth hoar). Hoarfrost is more fluffy andfeathery than rime, which in turn is lighter thanglaze. Hoarfrost is designated light or heavy(frost) depending upon the amount anduniformity of deposition. See also dew and dewpoint.

humidity - a moderate degree of wetness,

especially of the atmosphere; dampness.hurricane - a severe tropical cyclone in the North

Atlantic Ocean, Caribbean Sea, Gulf of Mexico,or in the eastern North Pacific off the west coastof Mexico with winds of 75 miles per hour orgreater accompanied by rain, lightning, andthunder that sometimes moves into temperatelatitudes. Variant names given to the same typeof storm in other areas of the world includetyphoon (eastern Asia), cyclone (India), winywiny (Australia), and baguio (China Sea).

hydrolysis - process of an agent reacting withwater. It does not materially affect the agentcloud in tactical use, because the rate ofhydrolysis is too slow.

hygrometer - consists of two similar ther-mometers with the bulb of one being kept wet.This is so that the cooling that results from theevaporation makes the wet bulb register a lowertemperature than the dry one. The differencebetween the readings constitutes a measure ofthe dryness (humidity) of the atmosphere.

hygroscopic - readily takes up and retains water,such as water in clay.

infrared radiation - thermal electromagneticradiation lying outside the visible spectrum atthe red end with wavelengths longer than thoseof visible light.

inversion - an increase of air temperature withincrease in altitude (the ground being colderthan the surrounding air). When an inversionexists, there are no convection currents andwind speeds are below 5 knots. The atmosphereis stable and normally is considered the mostfavorable state for ground release of chemicalagents.

ionosphere - the part of the earth’s atmospherebeginning at an altitude of about 50 kilometers(30 miles) and extending outward 500kilometers (300 miles) or more.

isobar - a line drawn on a map or chartconnecting places of equal or constant pressure.In meteorology, it most often refers to a linedrawn through all points of equal atmosphericpressure along a given reference surface, suchas a constant height surface (notably mean-sea-level on surface charts); the vertical plane of asynoptic cross section, or a map of the airunaffected by surface heating or cooling. Thepattern of isobars has always been a mainfeature of surface chart analysis. Until recently

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it was standard procedure to draw isobars at 3-millibar intervals. However, the advent ofconstant pressure charts for upper-air analysishas brought about the use of 4-millibar intervalsto simplify the conversion from surface isobarsto 1,000-millibar contour lines.

isothermal - of equal or constant temperaturewith respect to space, volume, or pressure.

joules (abbreviated J) - international systemunit of energy, equal to the work done when thepoint of application of a force of 1 newton isdisplaced 1 meter in the direction of the force.

katabatic wind - any wind blowing down anincline; the opposite of anabatic wind. If thewind is warm, it is called a foehn; if cold, it maybe a fall wind (such as the bora) or a gravitywind (such as a mountain wind).

kg - kilogram(s).kmph - kilometer(s) per hour.lapse - a marked decrease in air temperature with

increasing altitude because the ground iswarmer than the surrounding air. Thiscondition usually occurs when skies are clearand between 1100 and 1600 hours, local time.Strong convection currents exist during lapseconditions. For chemical operations, the state isdefined as unstable. This condition is normallyconsidered the most unfavorable for the releaseof chemical agents.

lapse rate - the rate of change in atmospherictemperature with increase of height. Thevariable normally is temperature unlessspecified otherwise. This is a vertical directionof travel (up or down) and the temperature mayrise or fall suddenly.

leeward - the side the wind is blowing awayfrom. Used most often in this manual inreference to a slope facing away from the wind.

M - meter(s).mechanical turbulence - irregular motion of air

resulting from surface roughness and windspeed.

met message - gives wind direction in differentincrements. May be artillery, ballistic, fallout,computer, or NATO in origin.

meteorology - the science that deals with thestudy of the atmosphere (or weather) and itsphenomena, especially with weather andweather forecasting.

mg - milligram.micrometeorology - the portion of meteorology

dealing with the observation and explanation ofsmall-scale weather and weather forecasting fora local area up to several kilometers in diameter.

min - minute(s).mistral - a northwesterly or northerly wind

which blows offshore along- the north coast ofthe Mediterranean from Ebra to Genoa. It ischaracterized by its frequency, strength, anddry coldness.

mixing height - the height to which atmosphericpollutants can be distributed by convectivemixing in unstable conditions.

mph - miles per hour.NBC - nuclear, biological, and chemical.neutral - when the temperature of the ground is

approximately the same as the temperature ofthe lower air up to 4 meters above it. Thiscondition has light to moderate winds andslight turbulence, and is considered average forthe release of chemical agents.

nimbostratus (abbreviated Ns) - a low-level,principal cloud type, gray colored and oftendark, rendered diffuse by more or lesscontinuously falling rain, snow, or sleet of theordinary varieties and not accompanied bylightning, thunder, or hail. In most cases theprecipitation reaches the ground, but notnecessarily. Nimbostratus is composed ofsuspended water droplets, sometimessupercooled, and of falling raindrops and/orsnow and ice crystals or flakes. It occupies alayer of large horizontal and vertical extent.The great density and thickness (usually manythousands of feet) of this cloud preventobservation of the sun. This, plus the absence ofsmall droplets in its lower portion, givesnimbostratus the appearance of dim anduniform lighting from within. It also followsthat nimbostratus has no well-defined base, butrather a deep zone of visibility weakness. A falsebase may frequently appear at the level wheresnow melts into rain.

pibal - see pilot-balloon observation.pillaring - rapid rising of smoke clouds due to

heat generated by burning munitions and/orexisting convection currents.

pilot balloon - a small unmanned balloon whoseascent is followed by a theodolite (instrument)to obtain data for the computation of the speedand direction of winds in the upper air.

pilot-balloon observation (abbreviated

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pibal) - a method of winds-aloft observation,that is, the determination of wind speeds anddirections in the atmosphere above a station.This is done by reading the elevation andazimuth angles of a theodolite (instrument)while visually tracking a pilot balloon. Theascension rate of the balloon is approximatelydetermined by careful inflation to a given totallift. After release from the ground, periodicreadings (usually at one-minute intervals) ofelevation and azimuth angles of the balloon arerecorded. These data are transferred to a winds-aloft plotting board, and wind speed anddirection at selected levels are calculated bytrigonometric methods.

planetary boundary layer (also called thefriction layer or atmospheric boundarylayer) - that layer of the atmosphere from theearth’s surface to the geostrophic wind level.

precipitation - any or all of the forms of waterparticles, whether liquid or solid, that fall fromthe atmosphere and reach the ground. It is amajor class of hydrometeor, but it isdistinguished from cloud, fog, dew, rime, andfrost in that it must fall. It is distinguished fromcloud and virga in that it must reach the ground.

pressure gradient (also, in meteorology,called barometric gradient) - the rate ofdecrease (gradient) of pressure in space at afixed time. The term is sometimes used to denotesimply the magnitude of the gradient of thepressure field.

psychrometer - a hygrometer (which consists oftwo similar bulb thermometers with one bulbkept wet and the other one dry). Thepsychrometer has a sling attached to themounted bulb thermometers. A handle is on thefree end of the sling. The operator can whirl thebulb thermometers in a circular pattern andspeed up the evaporation of water from the wetbulb thermometer. The difference between thetwo readings constitutes the measure of thedryness of the air—or humidity of theatmosphere—at your location.

QSTAG - quadripartite standardizationagreement.

radiation inversion - a stable condition causedby heat released from the earth. See alsoinversion.

radiosonde - a miniature radio carried aloft byan unmanned balloon to broadcast the pressure,

temperature, and relative humidity of the upperair and to automatically transmit thatinformation to the ground.

raob - an abbreviation for radiosondeobservation.

rate of dosage - see dose rate.rawin - a winds-aloft observation made by

balloon and radio methods (rawinsondeobservation).

relative humidity (popularly calledhumidity) - the ratio of the actual amount ofwater vapor present in the air to the saturationpoint at the same temperature. Thecorresponding ratios of specific humidity or ofmixing ratio give approximations of sufficientaccuracy for many purposes in meteorology.The relative humidity is usually expressed inpercent and can be computed frompsychometric (wet bulb-dry bulb temperature)data.

rime - a rough, white icy covering deposited ontrees, or other exposed objects, somewhatresembling white frost, but formed only fromfog- or vapor-bearing air.

RP - red phosphorus.screening length - distance from the source of a

smoke screen to the point downwind where thebackground begins to become recognizable.

sferics - a phonetic contraction of the word“atmospherics.”

SGF2 - standard grade fuel number 2.smoke - a particulate of solid or liquid particles

dispersed into the air on the battlefield todegrade enemy ground and aerial observation.Smoke has many uses—screening smoke,signaling smoke, smoke curtain, smoke haze,and smoke deception. Thus it is an artificialaerosol.

solstice - one of the two points on the sun’sapparent annual path where it is displacedfarthest north or south from the earth’s equator.In the northern hemisphere, the summersolstice is reached about 22 June. In thesouthern hemisphere, the winter solstice isreached about 22 December.

STANAG - standardization agreement.storm - any disturbed state of the atmosphere,

especially as affecting the earth’s surface, andstrongly implying destructive or unpleasantweather.

stratocumulus (abbreviated Sc) - a principal

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low-level cloud type, predominantly stratiform,in the form of a gray and/or whitish layer orpatch, which nearly always has dark parts andis nonfibrous.

stratosphere - the region of the upperatmosphere characterized by little or notemperature change with altitude. Thestratosphere extends from the tropopause (13kilometers) to approximately 80 kilometers.

stratus (abbreviated St) - a principal low-levelcloud type in the form of a gray layer with arather uniform base. Stratus does not usuallyproduce precipitation, but when it does occur, itis in the form of minute particles, such asdrizzle, ice crystals, or snow grains. Stratusoften occurs in the form of ragged patches orcloud fragments in which case rapidtransformation is a common characteristic.When the sun is seen through the cloud, itsoutline is clearly discernible, and it may beaccompanied by corona phenomena. In theimmediate area of the solar disk, stratus mayappear very white. Away from the sun, and attimes when the cloud is sufficiently thick toobscure it, stratus gives off a weak, uniformluminance.

sublimation - the transition of a substance fromthe solid phase directly to the vapor state, or viceversa, without passing through theintermediate liquid phase.

superadiabatic lapse rate - an environmentallapse rate greater than the adiabatic lapse ratesuch that potential temperature decreases withheight.

surface boundary layer - the portion of theatmosphere lying next to the surface of the earthand extending up to between 50 and 100 meters.

SWO - staff weather officer.synoptic - in general, pertaining to or affording

an overall view. In meteorology, this term hasbecome somewhat specialized in referring to theuse of meteorological data obtainedsimultaneously over a wide area for presentinga comprehensive and nearly instantaneouspicture of the state of the atmosphere. Thus, to ameteorologist, synoptic takes the additionalconnotation of simultaneity.

thermal turbulence - irregular motion of aircaused by convection currents rising fromheated surfaces. See also mechanicalturbulence.

tornado (sometimes called cyclone ortwister) - a violently rotating column of air,pendant from a cumulonimbus cloud, andnearly always observable as a funnel cloud ortuba. On a local scale, it is the most destructiveof all atmospheric phenomena. Its vortex,commonly several hundred yards in diameter,whirls usually counterclockwise with windspeeds of 100 to more than 300 miles per hour(161 to 483 kmph). Its general direction of travelis governed by the motion of its parent cloud.

TOT - time on target.toxicity - relating to a poison or toxin; poisonous.toxin - a colloidal poisonous substance that is a

specific product of the metabolic activities of aliving organism and is notably toxic whenintroduced into living tissue.

tropopause - the zone of transition between thetroposphere and the stratosphere(approximately 13 kilometers). The tropopausenormally occurs at an altitude of between 25,000and 45,000 feet in polar and temperate zones. Itoccurs at 55,000 feet in the tropics.

troposphere - the lower levels of the atmosphereextending from the earth’s surface up to thetropopause. It is characterized by convective airmovements and a large vertical temperaturechange.

turbulence - irregular motion of air. See alsomechanical turbulence and thermal turbulence.

unstable condition - see lapse.venturi effect - constricting a passageway, so

that the air (or fluid) moving through theconstriction is greatly accelerated.

virga - rain or snow that is dissipated in fallingand does not reach the ground, commonlyappearing in trails descending from a cloudlayer.

virulent agents - agents that produce rapid,severe, and malignant results in victims.

volatile - pertaining to a readily vaporizableliquid that evaporates at a relatively lowambient temperature.

weather - the state of the atmosphere, mainlywith respect to its effects upon life and humanactivities. As distinguished from climate,weather consists of the short-term (minutes tomonths) variations of the atmosphere.Popularly, weather is thought of in terms oftemperature, humidity, precipitation,cloudiness, brightness, visibility, and wind.

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weather forecast - a prediction of weatherconditions expected at a place, within an area,or along a route for a specified time or during aspecified period.

weather summary - a description of weatheralong a route or within an area during a specificperiod; used in analyzing the effects of weatheron recent operations and estimating effects onfuture operations.

wind - air in motion, usually parallel to theearth’s surface.

wind direction - the compass point, degree, orroils (see Figure C-4) from which the wind blows.

wind shear - a change of wind speed, direction,and magnitude.

wind velocity - the horizontal direction andspeed of air motion.

windward - the side receiving the wind’s force.Used most often in this manual in reference to aslope facing into the wind.

woods - for chemical behavior purposes, trees infull leaf (coniferous or medium-dense deciduousforests). See, also, heavily wooded, coniferous,and deciduous.

WP - white phosphorus.

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References

All references with information on the subjects listed in thispublication may not be listed here. New material is constantly beingpublished and present references may become obsolete. Consult theapplicable directory of publications and instructional materialcatalogues to keep updated.

Required Publications

Required publications are sources that users must read in order tounderstand or to comply with this publication.

Field Manual (FM)

100-5 Operations

Joint Chiefs of Staff (JCS) Publication1 Dictionary of Military and Associated Terms

Related Publications

Related publications are sources of additional information. Usersdo not have to read them to understand this publication.Air Force Regulation (AFR)105-3 Meteorological Support for US Army

Army Regulations (ARs)

115-10 Meteorological Support for US Army115-12 US Army Requirements for Weather Service Support316-25 Dictionary of US Army terms310-50 Authorized Abbreviations and Brevity CodesDepartment of Army Pamphlet (DA Pam)115-1 Requests for Climatology to Army Activities

Field Manuals (FMs)3-3 NBC Contamination Avoidance3-4 NBC Protection3-5 NBC Decontamination3-9 Military Chemistry and Chemical Compounds3-10-1 Chemical Weapons Employment3-10-2 Chemical Target Analysis3-50 Deliberate Smoke Operations

References-1

FM 3-6

3-100 NBC Operations3-101 Chemical Staffs and Units6-15 Field Artillery Meteorology6-40 Field Artillery Cannon Gunnery34-81 Weather Support for Army Tactical Operations

Technical Manual (TM)

3-216 Technical Aspects of Biological Defense

Naval Warfare Publications (NWPs)12-1 Tactical Threat to Naval Air Forces12-2 Tactical Threat to Naval Surface Forces22 Doctrine for Amphibious Operations22-1 The Amphibious Task Force Plan22-2 Supporting Arms in Amphibious Operations28 Nuclear Warfare Operations28-2 The Nuclear Explosion Environment28-3 Vulnerability and Survivability of Navy Systems3 6 Armed Forces Doctrine for Chemical Warfare and Biological Defense

(to be renumbered NWP 18)36-2 Employment of Chemical Agents (to be renumbered NWP 18-1)36-4 Employment of Chemical Agents (to be renumbered NWP 18-2)

Allied Tactical Publications (ATP)ATP-1 Vol 1 Allied Maritime Tactical Instructions and ProceduresATP-25 Nuclear Fall-Out Forecasting and Warning OrganizationATP-31 NATO Above Water Warfare ManualATP-45 Reporting Nuclear Detonations, Biological and Chemical Attacks

Instructions

OPNAVINST S3400.10C Offensive Chemical Warfare and Chemical,Biological, and Radiological Defense

NAVOCEANCOMINST 3140.1 DON Meteorological and OceanographicSupport Manual

SECNAVINST 3403.1 Riot Control Agents and Chemical Herbicidesin War (rules governing use)

References-2

FM 3-6

Index-1

FM 3-6

Index-2

FM 3-6

Index-3

FM 3-6

Index-4

FM 3-6FMFM 7-11-H

AFM 105-73 NOVEMBER 1986

By Order of the Secretaries of the Army and Air Force:

Official: JOHN A. WICKHAM, JR.General, United States Army

Chief of Staff

R. L. DILWORTHBrigadier General, United States Army

The Adjutant GeneralLARRY D. WELCH

General, United States Air ForceChief of Staff

NORMAND G. LEZYColonel, United States Air Force

Director of Administration

PAUL X. KELLYGeneral, United States Marine Corps

Commandant, Marine Corps

JOHN PHILLIPSLieutenant General

United States Marine CorpsDeputy Chief of Staff for

Plans, Policies, and Operations

DISTRIBUTION:

Active Army, USAR, and ARNG: To be distributed in accordance with DA Form 12-11A, Require-ments for Field Behavior of NBC Agents (Qty rqr block no. 3830) and DA Form 12-34B, Require-ments for Field Behavior of CBR Agents (Qty rqr block no. 89).

U.S. Air Force: Distribution F.

U.S. Marine Corps: MARCORPS CODE: TGE.

* U.S. GOVERNMENT PRINTING OFFICE : 1994 0 - 154-444

PIN: 060871-000