unit 4 basic weather processes

4-1-S290-EPUnit 4 Basic Weather Processes

Unit 4Basic Weather

Processes


Objectives1. Describe the structure and composition of

the atmosphere.

2. Define weather and list its elements.

3. Describe the sun-earth radiation budget and the earth’s heat balance.


Objectives4. Describe factors affecting the temperature

of the earth’s surface and the lower atmosphere.

5. Describe the greenhouse effect and its influence on air temperature.

6. Describe temperature lag and the affect daily and seasonal temperature lags have on wildland fire behavior.


Our Atmosphere• Encircling the earth

is a blanket of gases bound to it by gravity.

• Similar to its oceans, the atmosphere is in constant motion.


Our “Thin” Atmosphere

The atmosphere extends hundreds of miles above the earth’s surface. However, compared to the diameter of the earth of nearly 8,000 miles, our

atmosphere is really quite thin.


Ninety-nine percent of its gases lie within 18

miles of the earth’s surface.

Because the upper portion of the

atmosphere gradually thins with increasing

altitude, it is impossible to say exactly where it

ends and interplanetary space begins.

18 miles


Our atmosphere can be divided into many layers based on its

change in temperature with altitude.

On average, temperature decreases with increasing altitude in the troposphere and

mesosphere, and increases with altitude in the stratosphere and

thermosphere.


The TroposphereThe lowest layer

of the atmosphere

varies in height from 9 to 12 miles

above sea level over the tropics, to about 6 miles above sea level over the polar

regions. The Tropics

Polar Region

Troposphere


The TropopauseThis boundary separates the

troposphere from the stratosphere, and marks the upper limit of nearly

all weather in our atmosphere.

Because nearly all weather occurs below

the tropopause, the underlying troposphere

is often referred to as the weathersphere.

Stratosphere

Tropopause

Troposphere(weathersphere)


Nearly three-quarters of all these atmospheric gases are

concentrated within the troposphere.

Composition of the Atmosphere

The earth’s atmosphere is principally composed of gases and water vapor.


Nitrogen occupies 78 percent and oxygen about 21 percent

of the total volume of dry gases in the troposphere.

Dry Gases

The remaining 1 percent of this volume includes argon, neon, helium, hydrogen, xenon and

carbon dioxide.


Water Vapor

• Is an extremely important element of the atmosphere.

• Forms clouds that produce precipitation.

• Stores and releases heat energy called latent heat that is used to power storms, such as thunderstorms and hurricanes.


Water Vapor

The concentration of thisinvisible gas varies greatly from

place to place, and from time to time.

Approximately half of all water vapor is found within the

lowest 3 miles of the atmosphere; in other words within the troposphere.


In tropical locations, water vapor may account for up to 4 percent of the atmospheric gases.In colder polar regions, its concentration may be a mere fraction of a percent.


Wildland Fire Environmental Factors• Wind, Stability, Temp, RH

• Fuel Moisture• Fuel Temperature• Fuel Characteristics

• Terrain• Aspect• Elevation


Of these three major components, weather

is the most variable over space and time.


Because of its variability,

weather can be difficult to predict,

particularly at scales of less than 50 miles

and greater than 24 hours.

0 50miles


What Is Weather?It is the short-term variations of the atmosphere.

These variations include:• Air pressure• Air temperature• Humidity• Wind• Clouds• Precipitation • Visibility

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It cannot be over emphasized:

A basic knowledge and awareness of

weather is essentialfor making criticalfire management

decisions.


According to the Standard Firefighting Orders in the NWCG Fireline HandbookAll firefighters should “keep informed on fire

weather conditions and forecasts.”

Watch Out Situations

• Unfamiliar with weather and local factors influencing wildland fire behavior

• Weather becoming hotter and drier• Wind increases and/or changes direction


To understandhow weather can influence

wildland fire behavior, we will begin with a discussion

of the weather element atmospheric pressure.


Atmospheric Pressure

This downward force or weight is the result of the pull of

gravity. PullOf

Gravity

Or simply air pressure, is defined as the amount of force

exerted by the weight of air molecules on a surface area.

Top of atmosphere


50% of Total Weight

90% of Total Weight

99% of Total Weight

99.9% of Total Weight Atmospheric pressure always decreases with

increasing altitude.

In this figure, note that50 percent of all gases are

concentrated within the lowest 18,000 feet (3 miles)

of the atmosphere.

Millibar is the most common pressure unit

used today.


Another common air pressure unit used in aviation and on

television and radio broadcasts is

inches of mercury.


Standard Atmospheric Pressure

At mean sea level, or the average height of the ocean surface, the average, or standard, value for

atmospheric pressure is 29.92 inches of mercury.

This value is equivalent to 1013.25 millibars.


Surface area

If we weigh a column of air with a cross section of 1 square inch, extending from sea level to the

top of the atmosphere, it would weigh nearly

14.7 pounds per square inch at its base.

This value also represents the standard atmospheric pressure.


Measuring Air Pressure

Barometer – It is the instrument used to measure air pressure.

More precisely, it is a calibrated weather instrument used to measure the weight of the

atmosphere on a surface area, normally one square inch in size.

1 square inch


Two CommonTypes of

Barometers

The mercurybarometer – for

precision.

The aneroid barometer – for convenience.

Calibrated Scale

PartialVacuum

Vacuum

Mercury Barometer Aneroid Barometer


On average, air pressure decreases approximately one inch of mercury every

1000 foot increase in elevation.


EXERCISE 1

Our Atmosphere


What Drives Our Weather?

The Sun – The principal source of light and heat energy

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On a much smaller scale,

heat also originates from

large fires, and other natural

and human related heat-releaseprocesses.


Solar and Terrestrial Radiation

Shortwave solarradiation

Longwave terrestrial radiation

Shortwave solar radiation travels

through the atmosphere and heats the earth’s

surface. This heat is then

transferred to the atmosphere as

longwave terrestrial

radiation through conduction and

convection.


Solar-Earth Radiation Budget


The Earth’s

Heat Balance

Any change in this

equation will cause the

earth to either

warm or cool.

Incoming Solar Radiation

Outgoing Terrestrial Radiation

Incoming Solar Radiation

Outgoing Earth Radiation= 4-35-S290-EP


Polar regions lose more heat than they gain.

Tropics gainmore heat than they lose.

Latitudinal Distribution of Heat


Factors Affecting the Temperature of the Earth’s Surface and Lower Atmosphere

• Solar angle and duration• Atmospheric moisture and air

pollutants• Surface properties of terrain and

vegetation


The Seasons in the Northern Hemisphere


Solar Angle and Duration

Solar angle, length of daylight, slope, aspect,

and shape of the countryside all affect the amount of solar radiation

striking a point on the earth’s surface.

Basically, the higher the solar angle and the longer the daylight, the greater

the solar heating.


Seasonal Change

In Solar Angle

At 40ºNLatitude


Atmospheric Moisture and Air PollutantsClouds, water vapor

and air pollutants absorb, reflect and

scatter both solar and terrestrial radiation.

Their presence and amount significantly

influence the temperature of

the earth’s surface and its atmosphere.


Heat Loss At Night

Cloudy nights tend to bewarmer than clear nightsbecause of the insulating

effect of cloud cover.

Clear nights tend to be cooler than cloudy nightsbecause terrestrial heat isallowed to escape freely

to space.


• Influence the amount of heat absorbed and reflected by the terrain and vegetation.

• Effect on surface air temperature can be quite dramatic.

Surface Properties ofTerrain and Vegetation


Surface Properties of Terrain and Vegetation

• Color and texture• Transparency• Conductivity• Specific heat• Evaporation• Condensation


Earth’s Power Plant The difference in

surface air temperature can be quite large because of these properties, such as the 30 degree difference between a shoreline and a rocky cliff just 20 miles apart.


Color and Texture Rough textured,

irregular and dark colored materials

are good absorbers of solar radiation.

Whereas smooth, uniform and light colored materials

such as snow, water and sandy

soils are good reflectors

of solar radiation.


AlbedoRefers to the ability of a substance to reflect light and heat energy.

Rough textured and dark colored materials, such as tree bark, a rocky cliff, granite, a newly plowed field, a forest canopy, and the surface of a lake at high sun angle, all have a low albedo.

Smooth and light colored materials, such as a field of snow, sandy soil, and the surface of a lake at low sun angle, all have a high albedo.


TransparencyAffects the distribution of light and heat through a substance.

Water allows solar radiation to travel to a much greater depth than soil and rock.

Soil will concentrate heat within the top layer.


Conductivity

The transfer of heat between molecules in contact with one another.


Conductors and InsulatorsMaterials that allow for the

efficient transfer of heat energy, such as metal and granite, are

referred to as conductors.

Materials that are poor conductors of heat energy, such as dry air, wood and water, are

referred to as insulators.

metalgranite

sandstone

wet snowice at 0oC

mudwatersoil

wooddry air


Specific Heat

The specific heat of a substance refers to its capacity to ABSORB, STORE and RELEASE heat energy.

The greater the specific heat capacity of a material, the longer it will take for it to GAIN and LOSE heat energy.

The specific heat capacity of allmaterials is compared to that of the most common element on earth – WATER.


The Specific Heat

Capacityof

CommonMaterials

waterice at 0oC

iron

goldcopper

brick

woodglasspaperdry air

cementrocks

dry soil

For comparison, the specific heat of wateris five times that of rock.

Thus, water has the capacity to store more heat energy for longer compared to that ofrocks.


Tropical and Coastal Regions

Forest andWoodland Regions

Plains and Agricultural Regions

Mountain andPlateau Regions

Semi-Arid andDesert Regions

Polar and HighAltitude Regions

Regional Variation in Specific Heat

High Moderate to High

Low Very Low Low to Moderate

Moderate


Evaporation and condensationhave a large effect of the

heating and cooling of nearly all objects,

especially the atmosphere.


Evaporation

It is the process where liquid changes to vapor or gaseous state.

During this process, heat energy is removed from the environment.

Thus, evaporation is a cooling process.

Heat is removed from the environment

vapor


CondensationIt is the process

where water vapor changes to liquid.

During this process, heat is

added to the environment.

Thus, condensation

is a warming process.

Heat is added to the

environment

liquid state(water droplets)

vaporstate

steam cloudsteam cloud


Evaporation and Condensation In the Environment

Heat added to the atmosphere during this process is also essential for the growth of clouds and thunderstorms.

Besides removing heat from the environment, moisture is also added to

the atmosphere.


Greenhouse Effect

The ability of the atmosphere to retain infrared radiation (heat energy)

through absorption by greenhouse gases, such as water vapor, carbon dioxide,

methane and nitrous oxide.


Without these critical greenhouse gases, the earth’s radiant heat would escape to space without going into

heating the atmosphere.

On the other hand, should the atmospherepossess too much of these greenhouse

gases, the earth would become unbearably hot.

4-60-S290-EPUnit 4 Basic Weather Processes 4-60-S290-EP


Temperature LagThe warmest and coldest times of the day and

year rarely coincide with the times of maximum and minimum incoming solar

radiation (insolation).

This difference in time between the maximum temperature and maximum insolation, and

minimum temperature and minimum insolation is known as the

temperature lag.


The highest temperatures

occur on average 3 to 5 weeks

after maximum solar intensity

(highest solar angle) around June 21st.

Seasonal Temperature Lag

Tem

pera

ture

Lag

Tem

pera

ture

Lag

The lowest temperatures occur

on average 3 to 5 weeks after minimum solar intensity (lowest solar angle) around

December 21st.


While the highest daily temperatures

occurs roughly 2 to 4 hours after solar

noon.

During the summer season, the lowest daily temperatures occur on average 1 to 2 hours after

sunrise.

Daily Temperature Lag


EXERCISE 2

Factors Affecting Air Temperature


Review Objectives1. Describe the structure and composition of

the atmosphere. 2. Define weather and list its elements.3. Describe the sun-earth radiation budget and

the earth’s heat balance.


Review Objectives4. Describe factors affecting the temperature

of the earth’s surface and the lower atmosphere.

5. Describe the greenhouse effect and its influence on air temperature.

6. Describe temperature lag and the affect daily and seasonal temperature lags have on wildland fire behavior.

unit 4 basic weather processes

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