ED 151 158

TITLEINSTITUTION

REPORT NOPUB DATENOTEAVAILABLE FROM

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The Greatest Storm on Earth:wHurricane.National Oceanic and Atmospheric Administration(DOC), Rockville, Md.NOAA/PA-7600877

-,

48p.; Photographs may not reproduce wellSuperintendent of Documents, U.S. Government PrintingOffice, Washington, D.C. 20402 (no price quoted)

EDRS PRICE 'MF-$0.83 HC-$2.06 Plus Postage.DESCRIPTORS Bibliographies; *Climatic Factors; Earth Science;

Economic Factors; Environmental Influences;*Ins4ructional Materials; *Meteorology; NaturalSciences; Science Education; *Secondary SchoolScience; Social Factors

IDENTIFIERS *Hurricanes

ABSTRACT .

This publication, produced by the National. Oceanicand Atmospheric Administration (NOAA), is an illustratednon-technical description of'the meteorology of hurricanes and theireffects on the land areas they hit. As an information source forstudents aid teachers alike, this publication also describes thedamage done in the past by hurricanes, the atmospheric processesbehind hurricanes, and the efforts of forecasters and hurricanehunters; A bibliography is also included. (MR)

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;.,,krft§Grepe..st.Storm onEarth. ttiOtaitift of Great Storms . . .

Atmospheric Selena A

A Storm is BornPortrait of a Hurricane

4

1 The Fatal Thrust Toward Land2 -11/e.struction in a Hurricane

-4 Forecasters and Huntee'mte

8 Stormfury10 Hurricane Bibliogiaphy

1214161821

thestorm

on earth

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Hurncanes manifest their brutestrength in many ways. At left. the landand sea beneath a passing hurricane. Atnght, a NOAA satellite image ofhurncane Ava, the classically organized1973 storm that is still the most violenthurricane of record for the eastern northPacific area.

5

4

"1:

There is nothing like them in theatmosphere. Even seen by sensorson spacecraft thousands of milesabOlie the earth, the uniqueness ofthese powerful, tightly coiled stormsis clear. They are not the largeststorm systems in our atmosphere, orthe most violent; but they combinethose qualities as no other phenorn.enon does, as if they were designedto be engines of death anddestruction.

In our hemisphere, they arecalled hurricanes, a term whichechoes colonial Spanish and Carib-beanIndian words for evil spirits andbig winds. The storms are product!.of the tropical ocean and atmosphere, powered by heat from thesea, steered by the easterly tradesand temperate westerlies, and theirown fierce energy. Around their tran-quil core, ..'nds blow with lethalvelocity, the ocean develops an inun-dating surge, and, as they moveashore, tornadoes may descend from-the advancing banns of thunder.'`clouds

Hyrricanes have a single belie.fitthey are a major source of rainfor th,ose continental corners whichfall ,beneath their tracks. Perhaps ,

there are other hidden benefits aswell. But the main consequence ofthe hurricane is tragedy.

In Asia, the price in life paidthe hurricane has had Biblical pro-portions. late as 1970, cyclonestorm tide's- along the coast of whatis now Bangladesh killed hundredsof thousands of persons.

Our hemisphere has not hadsuch spectacular losses, but the tollhas still been tragically high. InAugust 1893, a great storm wavedrowned more than a thousandpeople in Charleston, South Carolina.In October of that same year, nearly

two theusp-id more perished on theGulf Coage of Louisiana. The Galves,ton storm o'f- ipoo took more thansix thousand livese than 1,800perished along the Vuth shore ofFlorida's Lake Okeechobee in 1928when hurricane -driven watersbroached an earthen levee. Cuba lostmore than two thousand to a stormin 1932. Four hundred died in Flori-da in an intense hurricane inSeptember 1935the "Labor Day"hurricane that shares with 1969'sCamille the distinction of being themost severe of record.

Floods from 1974's hurricaneFifi caused one of the WesternHemisphere's worst natural disastersin history, with an estimated fivethousand persons dead in Honduras,and thousands more in Nicaragua,El Salvador, Guatemala, and Belize.

In the United States, the hurri-cane death toll has been greatlydiminished by timely warnings of ap-proaching storms. But damage tofixed property contipties t8 mount.Camille, in 1969, caused some' $1.42billion in property damage. Foodsfrom Agnes in 1972 cost anestimated $2.1 billion.

That is why such first -ordertechnical accomplishments as earlydetection and timely warning of ap-proaching hurricanes are not enoughfor sorrk! scientists. For them, thereis the persistent goal of somehowdisarming the greatest storm onearth.

In the United States, hurricinewarning and research are focused inNOAH, the National Oceanic and At.mosphenc Administration of theU.S. Department of Commerce. Anclthere, as the planet orbits towardthe summer solstice, scientists,technicians, pilots, and others bracefor another season of great storms.

1

theseasonofgreatstorms

Cumulus clouds, the atmosphere's basicweather -factories,- and building blocksfor hurricanes, climb high into the

o atmosphere under the energetic influenceof the tropical sun.

9

It is the northern summer. The illUsion of a moving sun caused by ourplanet's yearlong orbit brings thatstar's direct rays northward to theEquator, then toward the Tropic ofCancer. Behind this illusory solar _

track the sea and air grow warmer,and the polar air of winter beatsits seasonal retreat.

this northward shift of the sunbrings the season of tropical cy-clOnes to the Northern Hemisphere.Along ow. coasts and those of Asiait is time to look seaward. /

Over the western Pacific, thetropical cyclone season is never quiteover, but varies greatly in intensityEvery year, conditions east of thePhillippines send a score ofviolent storms howling toward Asia:but it is worst from JJnethrough October.

Southwest of Mexico, easternPacific hurricanes developduring spring and summer. Most ofthese will die at sea as they moveover colder ocean waters. Butthere are destructive exceptions whenstorms curve back toward Mexico.

Alorig our Atlantic and Gulfcoasts, the nominal hurricane seasonlasts from June throughNovember. Early in this season, thewestern Caribbean and Gulf ofMexico are the principal areas of ori-gin. In July and August, thiscenter begins an eastward shift; byearly September a few storms arebeing born as far, east as theCape Verde Islands off Africa's west

1 u

co_lc. Again after mid.September,most storms begin in the westernCaribbean and Gulf of Mexico.

In an average year, moreZhanone hurJred disturbances with hurricane potential are observed inthe Atlantic, Gulf, and Caribbean; butfeftrer than ten of these reach thetropical storm stage, and onlyabout six mature into hurricanes Onaverage, three of these hurricanesstrike the United States, wherethey kill'from about 50'tp. 100people somewhere between Texasand Maine anL4 Cause hundreds ofmillions of dollars' property damageIn a worse than rage year, thesame storms cause several hun-dred deaths, and property damagetotaling billions o. dollars;

For NOAA, the hurricane seasonmeans another hazard from the atrnophere, at a time whentornadoes, and floods, and severestorms are playing havoc elsewhereon the continent. Meteorologistswith NOAA's National Weather Serv-ice monitor the massive flow ofdata that might contain the earlyindications of a developing stornr ,

somewhere over the warm seacloud images from satellites, meteor-ological data from hundreds ofsurface stations and balloonprobes of the atmosphere,information from hurricane - huntinga:rcraft.

In NOAA's EnvironmentalResearch Laboratones, scientists wait.for nat,,re to furnish additional sped.mens of the great storms toprobe, analyzeeventually to modify,if they can be modified withbeneficial results.

11,11,

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Over the Atlantic, the buddingsummer brings other changes aswell. A sernipe,rmanent zone ofhigh pressurethe familiar "High" ofweather mapsreturns to a surfaceposition centered near Bermudaand the Azores. This clockwise spiralof descending air, or anticyclone,lies between the temperate andtropical bands of prevailing winds,and, during summer and earlyautumn, dominates the NorthAtlantic atmosphere.

North of the high-pressuresystem, prevailing westerlies floweastward in a deep layer extendingfrom the surface to altitudes of40,000 feet (12,000 meters) or more.Near the center of the BermudaHigh, winds are variable. But to thesouth, the surface flow of air is pre-dominantly to the west theeasterly tradewinds. In the northernsummer, these easterlies maydeepen until they reach from thesurface to the stratosphere, and cov-er vast areas of the tropics; orthey may break up into smallvortices (a whirlpool is a vortex)which drift wvstward into the Caribbean or Gulf of Mexico. It is in thistradewind current that most Atlantichurricanes are born.

The characteristic sinking of airwithin the anticyclone produceslayers in the deep current of theeasterly trades. As air sinks to If:velsof greater atmospheric pressure, itis heated by compression,producing at lower altitudes what iscalled a temperature inversionacondition in which air, instead of

14

F

cooling with altitude, cools only tothe inversion altitude, then grry..swarmer, before it begins to cc ilaaarn at greater heights. Beneath theinversion layer air sweeps for hun-dreds of miles over the surfaceof the sea, receiving a charge ofmoisture to altitudes of several thou-sand feet from evaporated oceanwater.

Convection, or vertical motion, inthis lower, heated layer, and inter-mittent intrusions of moist air,weaken the inversion, which gracLiallydissipates as the low-level aircontinues its trajectory above thewarm sea. This vertical penetration ofthe inversion permits water vaporto be lifted to Look; altitudes,where it condenses into clouds andraindrops. As water vapor con-denses, it releases heat energy intothe atmosphere. This latent heatenergy is gradually transported tohigher levels, changing the verticalcharacter to the tradewind belt.Sometimes the easterly flow is suf-ficiently disturbed by such processesthat rain areas become concentrated, and the concentrationintensifies into a storm.

The Polar Trough. When theBermuda High is weak and south ofits normal position, and the easterlytrades are shallow (below about15,000 feet, or 4,500 meters),an elongated area of low pressurea troughembedded in thetemperate westerlies may pushsouthward into the tropics. When t'southern end of the intrudingtrough slows its eastward move-ment, or is trapped in the easterlytrades, a hurricane may develop:or the trough may separate to be-come another weather-makertheeasterly wave.

The Easterly Wave. Once ernbedded in the deep easterly current,this westward-drifting region oflow-pressure tends to organize low-level circulation into alternate areasof converging and diverging airflow.Where air is diverging (highpressure), weather is fine. But whereconvergence occurs (low pressure),the depth of the moist laye: increases, and convection producesheavy cumulus and cumulonimbusclouds which build to heights6above30,000 feet (9,000 meters).

Many of the waves in theeasterlies have been traced back towestern Africa, where they are mostevident in the middle and lowlevels of the atmosphere as theycross Africa's Atlantic coast. Thesesystems may travel thousands ofmiles with little change in form. Butwhere the waves are destabilizedby intense convection or by so.neexternal forcefor example, high.level winds that promote greaterorganization of the circulation in thewavethey may curl inward. The

vertical circulation accelerates, and avortex develops that sometimesreaches hurricane intensities.

Easterly waves aie present oversome part of the Caribbean almostdaily ' -m June through Septem-be- ,e months of highest incidenceof ril -icanes. The waves are alsopre, ) a lesser extent in May,October, and November.

Migrants from the Doldrums. Thetropical easterly tradewind belt ofthe Northern Hemisphere has amirror image in a similar belt ofwinds south of the Equator. A high.ly simplified view of theatmosphere would show these trade-wind belts separated by an equatorialregion of low pressure, popularlycalled the doldrumswhat meteorologists call the IntertropicalConvergence Zone, or ITCZ.'

The ITCZ varies considerablymore than this simple view indicatesSometimes it is sc weak as to be

155

FEB 1 MAR 1 APR I MAY JUN JUL 1 AUG SEP I OCT NOV I DEC JAN

7.'

6

nearly undetectable, and almostcompletely free of significant weath-er. At other times it can be anintensely active zone a hundred milesor more wide, with cloud topsrising to the stratosphere andweather, as violent as that in thecontinental squall lines which sweepour own great plainS.

The ITCZ also follows the sun.

16

In February the ITCZ moves from aposition near the Equator to itsextreme limit near about twelvedegrees north latitude in August;however, its day-to-day surfaceposition varies greatly.

This interhemispheric borderlandhas a flow consisting mainly ofeddies (individual currents in a mov-ing fluid) that drift westward, andits intense activity is thought to beassociated with the movement ofthese ed 'es into the tropics.

While the ITCZ is in the

southern range of its annual migration (it'is never south of theEquator) the effect of the earth's rotation is small; but a., it shiftsnorthward, the influence of the ro-tating globethe Coriolis force isgreat enough to permit a circulationto develop that can evolve into thetight, violent eddy of a tropicalcy )ne.

Hurricanes in Other Oceans. Whatis true for hurricanes in the NorthAtlantic is generally true for the

TROPIC OFCANCER

EQUATOR

TROPIC OFCAPRICORN

hurricanes of the eastem North Paci-fic, and similar storms like thecyclones of the Indian Ocean, andthe western Pacific's typhoon.The storms tend to be bom overwarm water, spun from disturbancesin the equatorial tradewind beltsThere is little tropical cyclone activitysouth of the equator, except forthe ocean area west of Australia dur-ing the southern summer.

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HIGH

4

Increased cc.rivection, and the pumping action of

high altiiude winds. may cause the easterly v,,,e

to deepen further becoming strong from the sur

face to 15 000 feet or incie Atmospheric pressure

at the surface drops an area of ;OM pressurebecomes an isolated dopresqon aid the tradewinds beg n to turn in on themselves forming a

weak cyclonic miculation abut the cente, of low

pressure This circulatioo may intensify, pressure

may continue to dror, the winds spiralling around

the cuter of low pressure may acceleratedisturbance may become a severe tropical cyclone

4111MINCEMUM1111111111i1M11=M111011

13

C.

Intensified convection may be accompamed by a

trough of low pressure in the trade wind belt This

poorly developed easterly wave is weak at the

saface stronger nea' 15 009 feet weak at highaltitude Thurdrstorms may develop behind the

Nave as the air is raised and shunted northward,

ahead of the wave air is subsiding weather is

fine A wave of this intensity may travel for

thousands of miles without 4, eciable change

B.

As the inversion layer weakens to the west and

as the lower layers become increasingly warm and

moist convection intensifies Clouds form, and

begin to rise to greater altitudes

A.

The characteristic vertical structure of the east

erly trade wind belt, in MEd a relatively shallowlayer cd moist air is overlain by a deeper layer of

subsidi,s2 wym dry air 1' e two layers are separated

by a temperature 'inver_lon Moisture from the

evaporates into the lower atmosphere but can

not rise above the inversion However the moisture

charge received by the lower layers contains the

latent heat energy that once released by ccndensa

tion and convection to high levels can drive a

tropical disturbance

PRESSURE INCREASING

19

astormisborn

Birth of hurricane Carmen in August1974 is watched by sensors aboard aSynchronous Meteorological Satellite, inan orbit that keeps it always about22,000 miles (33,000 kilometers) overthe same point on the equator Theembryonic Carmen begins as a poorlyorganized disturbance in the easterlywave (1), Then seems to weaken (2) asthe young storm crosses a persistentupperlevel trough. But Carmen survivesto become a tropical storm headedfor the Antilles (3), and a fullscale hurricane (4) that stnkes into Yucatan. Coloredcir, -s are centered on the storm's approximte center of rotation.

21

,,

Hurricane formation was oncebelieved to be something likethe generation of thunderstorms,where surface heating causes air torise, water vapor to condense, andthunderclouds to grow. But, in hur-ricanes, there was an additionaldementthe inexplicable drop in at-mospheric pressure that organizedthese elements and spun theminto a cyclonic spiral.

This theory left much to bedesired. Few hurricanes mature inthe doldrums, where this kind ofactivity is usually found, and thereare too many seemingly idealconvective situations there and toofew hurncanes. And what of thedrop in atmospheric pressure?

Even storms as neatlyconstructed and carefully observedas hurricanes present scientists

22

with an infinitude of unknowns.There is no full understanding ofwhat triggers a hurricane, or how adisturbance in the easterly tradesor a disturbance along the ITCZ istransformed into a mature storm.

This is what seems to happen.Some starter mechanismthe in-truding polar trough, easterlywave an eddy from an active ITCZstimulates an area of verticalair motion. The initial disturbancecreates a region into which low-level air from the surrounding areabegins to flow, accelerating theconvection already occurringinside the disturbance. As water va-por in the ascending moistcolumns condenses (releasinglarge amounts of heat energy todrive the wind system), the verticalcirculation acquires greaterorganization; and the horizontalform of the disturbance becomesthe familiar cyclonic spiral, in which

HIGH ALTITUDE WINDS

(in the Northern Hemisphere)the movement of low- and mid-level air is counter-clockwise. Nowthe storm is an embryohurricane.

High-altitude winds help thedeveloping hurricane's massivevertical transport exhaust air intothe upper reaches of the tropo-sphere, the lower level of theatmosphere where most verticalmixing and weather occur. Thesewinds pump ascending air outof the cyclonic system into theclockwise circulation of a high-altitude anticyclone, carrying airwell away from the distur-bance before it can sink to thesurface again.

Thus, a large-scale verticalcirculation is set up in which low-level air spirals up the cyclonic coreof the disturbance, and is ex-

As the disturbance becomes bet_organized and more intense, the familiarshape of the humcane appears Atmatunty, the storm is nounshed by airconverging at lower levels in a greatspiral toward the center of low pressure,where convection (vertical motion) andthe pumping actirn of high-altitude windsthrust it upward into a larger, anticycloniccirculation. Vertical scale is greatlyexaggerated.

hausted at -high altitudes. Thispumping actionand the heat re-leased by the ascending, water-bearing airexplains the suddendrop in atmospheric pressure at thesurface. The drop produces thehurricane's uniquely steep pressuregradient, along whose contourswinds are accelerated to hurricanespeeds.

It is generally believed thatthe interaction of low-leveland high-altitude wind systems atscales larger than the hurricane'sdetermines the intensity thestorm will attain. Scientists also be-lieve planetary wind systems,displaced northward, set up an es-sential large-scale flow whichsupports the budding storm, andthat the development of hurricanesis often preceded by high-levelwarming and low-ievel inflow, insome balance that is not fullyunderstood. There is still much tobe learned. For example, re-searchers have begun to ponderthe connection between thun-derclouds over the hotplate ofCentral Afri '-a and disturbances inthe easterly Eradewind belt, andhow these are affected by themassive natural cloud seeding ofSaharan dust.

23

10

24

portofahurricane

4

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Given that the hurricane, as anengine, is inefficient and hard to startand sustain, once set in motion,once mature, it is an awesomenatural event indeed.

The young storm stands uponthe'sea as a whirlwind of awfulviolence. Its hurricaneforce windscover thousands of square' miles,and gale force windswinds of 33to 55 knots* (16 to 28 meters perlecOnd),over areas ten times larger.Along the contours of its spiral runbands of dense clouds from whiChtorrential rains fall. Tt ese spiralrainbands ascend in decks ofcumulus and cumulonimbus cloudsto the high upper atmosphere, wherecondensing water vapor is swept offas ice crystal wisps of cirrus cloudsby highaltitude winds. Lightningglows in the rainbands, and thiscloudy terrain is whipped by

ottorbulence.In the lower few thousand feet,

air flows in toward the center of thecyclone, and is whirled upwardthrough ascending columns of airnear the center. Above 40,000 feet(12,000 meters), this cyclonic patternis replaced by an anticycloniccirculationthe high-level pumpwhich is* the exhaust system of thehurricane engine.

At lower levels, where thehurricane is most intense, winds onthe rim of the storm follow a widepattern, like the slower currents onthe rim of a whirlpool; like thosecurrents,, these winds accelerate asthey appioach the central vortex.This inner band is the eyewall, wherethe storm's worst,winds are felt, andwhere moist air entenng at thesurface is chimneyed upward,releasing heat to drive the storm. Inmost hurricanes, these winds exceed90 knots (50 meters per second)nearly twice that in extreme cases.

Makimum winds run still higher intyphoons.

Hurricane winds are produced,as all winds are, by diffe;ences inatmospheric pressure, or density. Thepressure gradientthe rate ofpressure change with distance4produced in hurricanes is thesharpest in the atmosphere,excepting only the pressurechange believed to exist across theynarrow funnel of,a tornado. st ^4

Atmosperic pressure is popularlyexpressed as the height of a columnof mercury that can be supporteckbythe weight of the overlying air at agiven time.** In North America, bar°metric measurements; at sea levelseldom go below 29 inches ofmercury (982 millibars), and in thetropics it is generally close to 30inches (1,016 millibars) undernormal conditions. Hurricanes dropthe bottom out of those normalcategories. The Labor Day hurricanethat struck the Florida Keys in 1935had a central pressure of only 26.35inches (892 millibars). And thechange is swift: pressure may dropan inch (33 millibars) an hour, witha pressure gradient change of atenth of yn inch (3 millibars) permile.

At the center of the storm is aunique atmospheric entity, and apersistent metaphor for order in themidst of chaosthe eye of thehurricane. It is encountered suddenly.From the heated tower of maximum.winds and thunderclouds, one burstsinto the eye, where winds diminish to

'A knot is one nautical mile per hour, anautical mile is about 1 15 statute miles"Weather maps show atmosphenC pressurein millibars units equal to a thousandth of abar. the bar is a unit of pressure equal to295.3 inches of mercury in the Englishsystem, and to one million iiynes per squarecentimeter in the metnc system

J

.: ..

something less than 15 knots (eightmeters per second). Penetrating the ..opposite wall, one is abruptly in the

--worst of winds again..A mature hurricane orchestrates

more than a million cub miles ofatmosphere. Over the deep ocean,waves generated by hurricaneindscan reach heights of 50 feet (15meters) or more. der the stormcenter the ocean su ce is drawn ''':upward like water in a giant straw,forming a mound a foot or sohigher than'the surrounding, oceansurface. This mound translatesint4 coastal surges of 20 feet (six

. meters) or more. Besides this surge,massive swells pulse otit through theupper layers of the seaPacific,surfers often rick he oceanic

- memory . distant typhoons.Hurricane Eloise, which struck

the Florida panRendle in September1975, taught scientists somethingnew about the influence.of passinghurricanes on the marineenvironment. Expendablebathytherrnographs dropped from/ '.NOAA research aircraft ,ahead of, in, ,and in the wake of the stormshowed that the ocean was disturbe!.,to depths of huncifedsnf feet by apassing . hurricane, and "remem-bered': hurricane passage with inter-nal waves that persisted for weeksafter the storm had gone. The samestorm also demonstrated that apassing hurricane can b. felt deep inthe sea-floor sediments.

While a hurricane lives, thetransaction of energy within itscirculation is immense. The ..

condensation heat energy released.'by a hurricane in one day can bethe equivalent of energy released byfusion of four hundred 20-megatonhydrogen bombs. One day's releasedenergy, converted to electricity, couldsupply the United States' electricalneeds for about six months.

26 -

O

Portrait of a hurricane, as seen'bysatellite, radar, and illustrator. Cut-away view of storm is greatly exag-gerated in vertical dimension; actualhurricanes are less than 50,000 'set(15,000 kilometers) high and maycover a diameter,of several hundredmiles.

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27

thefatal

thrusttowardland

44

Hurricane waves besiege a seawall as astorm makes landfall along our Atlanticcoast. This thrust toward land is suicidalfor the storm, which dies soon after ithas extracted its toll in life and pro 'vwhere it comes ashore.

9 9

From birth, the hurricane lives in anenvironment that constantly tries tokill itand ultimately succeeds Thehurricane tends to survive whale it is

over warm water. But its movementis controlled by the forces whicheventually destroy it, forces whichdrive the storm ashore or overcolder water beyond the tropicswhere it will fill and die. This thrustaway from the tropics is theclockwise curve which takestyphoons across the coastlines ofJapan and into the Asian mainland,and Atlantic hurricanes into theeastern United States.

Even before a hurricane forms,the embryonic storm has forwardmotion, generally driven by the east-erly flow in which it is embedded.

30

As long as this easterly drift isslowless than about 20 knots, or10 meters per second- -the younghurricane may intensify. More rapidforward motion generally inhibits in-tensification in the storm's earlystages. Entering the trnperatelatitudes some storms may movealong at better than 50 knots (25meters per second), but such :ast-moving storms soon weaken.

At middle latitudes. the endusually comes swiftly. Colder airpenetrates the cyclonic vortex, thewarm core cools, and acts as athermal brake en furtherintensification. Water below 80degrees Fahrenheit (27 degrees

Celsius) does not contribute muchenergy to a hurricanL. Even thoughsome large hurricanes may travel fordays over cold North Atlantic water,all storms are doomed once theyleave the warm tropical waters whichsustain them. The farther theyventure into higher latitudes, the lessfuel they receive from the sea; thislack of fuel finally kills the storms.

Over land, hurricanes break uprapidly. Cut off froin their oceanicsource of energy, and with theadded effects of frictional drag, theircirculation rapidly weakens andbecomes more disorganized.Torrential hurricane rains, however,may continue even after the windsare much diminished. In the south-

.....611112.

--

Trends and exceptions in the great storms'thrusts toward land. Shaded areas showgeneral patterns of westward drift andrecurvature. Act Jai storm tracks describe afew of the countless variations possible

eastern United States, about a fourthof the annual rainfall comes fromdissipating hurricanes, and the Asianmainland and Japan suffer typhoonsto get water from the sky.

Hurricanes are cften resurrectedinto extratropical cyclones at nigherlatitudes, or 'ombine with existingtemperatezone disturbances. Manystorms moving up our Atlantic coastare in the throes of this transfor-mation when they strike NewEngland, and large continental Lowsare often invigorated by theremnants of storms born over thetropical sea.

31

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ifictiovr Az,

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destructionin

hurricane

Wind and water do the humcane'sdestructive work. The mighty storm tideand powerful, debris.laden winds create

. such scenes of havoc as these.

33

Hurricanes are the unstable,"unreli-able creatures of a moment in ourplanet's natural history. But their brieflife ashore can leave scars that never

'quite heal. In the mid1970's, thehand of 1969's Camille could still beseen along the Mississippi GulfCoast. Most of a hurricane'sdestructive work is done by thegeneral rise in the height of the seacalled storm surge, wind, and flood-producing rains.

Hurricane winds can be the leastdestructive of these, although thereare important exceptions like 1971'sCelia, whose high winds did most ofthe storm's destructive work. Thesewinds are a force to be reckonedwith by coastal communities decid-ing how strong their structuresshould be.' For example, normalatmospheric pressure at sea level isabout two thousand pounds persquare foot (19,000 kilograms persquare meter). As winds increase,pressure against objects is added ata disproportionate rate. Pressuremounts with the square of windvelocity, so that a tenfold increase inwind speed increases pressure one-hundred-fold. Thus, 20-knot (12meter-per-second) wind increasesatmospheric pressure by about twopounds per square foot (10kilograms per square meter); a windof 200 knots (110 meters persecond) increases atmospheric pres-sure by more than 225 pounds persquare foot (1,100 kilograms persquare meter). For some structures,this added force is enough to causefailure. Tall structures like radiotowers can be worried to destructionby gusting hurricaneforce winds.Winds also carry a barrage of debristhat can be quite dangerous.

34

. All the wind damage does notnecessarily come from the hurricane.As the storm moves shoreward, inter-actions with other weather systemscan produce tornadoes, which workaround the fringes of the hurricane.Although hurricanespawnedtomadoes are not the most violentform of these whirlwinds, they haveadded to the toll we pay thehurricane.

Floods from hurricane rainfallare quite destructive. A typical hur-ricane brings 6 to 12 inches (150to 300 miilimeters) of rainfall to thearea it crosses, and some havebrought much more. The resultingfloods have caused great damageand loss of life, especially inmountainous areas, where heavyrains mean flash floods. The mostwidespread flooding in United Stateshistory (through 1976) wascaused by the remnants ofhurricane Agnes in 1972. Rains fromthe dying hurncane broughtdisastrous floods to the entire At-lantic tier of states, causing 118deaths and some $2.1 billion inproperty damage.

The hurricane's worst killercomes from the sea, in the form ofstorm surge, which claims nine ofevery ten victims in a hurricane.

As the storm crosses thecontinental shelf and moves close tothe coast, mean water level mayincrease 15 feet (5 meters) or more.The advancing storm surgecombines with the normal astro-nomical tide to create the hurricanestorm tide. In addition, wind wavesfive 10 feet high are superimposedon the storm tide. This buildup ofwater level can cause severe floodingin coastal areas, particularly when thestorm surge coincides with normalhigh tides. Because much of theUnited States' densely populated.

t. ,

coastline alongalong the Atlantic and Gulfcoasts lies less than 10 feet (3meters) above mean sea level, thedanger from storm surges is great.

Wave and current action associ-ated with the surge also causesextensive damage. Water weighssome 1,700 pounds per cubic yard(1,000 kilograms per cubic meter);extended pounding by frequentwaves can demolish any structuresnot specifically designed to with-stand such forces.

Currents set up along the coastby the gradient in storm surgeheights and_wind combine with the

4.

waves to severely erode beaches andcoastal' highways. Many buildingswithstand hurricane winds until, theirfoundations uncle- )fined by erosion.they are weakeneu acid fail. Stormtides, waves, and currents in con-fined harbors severely damage ships,marinas, and pleasure boats. Inestuarine and bayou areas, intrusionsof alt water endanger the publichealthand create bizarre effects likethe saltcrazed snakes fleeing Lou-isiana's flooded bayous.

33

.orecasterandhunters

Stalking a storm (shown in the satellitephotograph), rough nding meteorolr ists,may fly right to the hurricane's eye nordetailed information .

37

The day is past when a hurricanecould develop to maturity far out tosea and go unreported until its thrusttoward land. Earth - orbiting satellitesoperated by NOAA keep the earth'satmosphere under virtuallycontinuous surveillance, night andclay. Long before a storm hasevolved even to the pdint of ruf-fling the easterly wave, scientists atNOAA's National Hurricane Center*(a National Weather Service ForecastOffice) in Miami, Florida, have begunto watch the disturbance. In thesatellite data coming in from polar-orbiting and geostationary spacecraft,and in reports from ships andaircraft, they look for subtle cluesthat mark the development of hur-ricanescumulus clouds covered bythe cirrostratus deck of a highly or-ganized convective system; showersthat become steady rains; droppingatmospheric pressure; intensificationof the tradewinds, or a westerly windcomponent there.

Then, if this hint of adisturbance blooms into a tropicalinorm, a time.honoyed convention is/applied: it receives a name. Thisipractice began in the 1940's, whenwartirne weathermen plotted themovement of stormsinformallynamed for wives and sweetheartsacross vast theaters of operations It

has been Weather Service policysince 1953. The system uses analphabetical series of names -- Anita,

More tropical cyclone advisones are issued byNOAA's Humcane Warning Offices inHonolulu and San Francisco than by MornThese Weather Service Offices keep the samekind of watch for the same destructive storms,and warning is given to ships and coastlinesin their path. But. because the Pacifictradewinds carry most storms away from land,this warning work is perhaps best known toaviators and manners

38

Babe, Clara... that changes eachyear, and is different for the NorthAtlantic and the Eastern NorthPacific. Typhoons also receive namesin alphabetical order. but beca,Isethey occur tnroughout the year,these storms are named cor.,utive-ly without regard to the ,ear `that is,a calendar year of typhoon activitycould begin with typhoon Wanda.

The first hurricane warning inthe United States was flashed in1873, when the Signal Cr )s warnedagainst a storm approach' g thecoast between Cape May, NewJersey, and New London, Connectcut. Today, naming a story is asignal which brings a considerablymore elaborate warning system toreadiness. Long distance communica-tions lines and preparedness plansare flexed.

As an Atlantic hurricane driftscloser to land, it comes undersurveillance by weather reconnais-sance aircraft of the U.S. Air Force,the famous "Hurricane Hunters," whobump through the turbulent interiorsof the storms to obtain precise fixeson the position of the eye, andmeasure winds and pressure fields.Despite the advent of satellites, theaircraft probes are the most detailedinformation hurricane forecastersreceive. The hurricanes are alsoprobed by "flying laboratories" fromNOAA's Research Facilities Center inMiami. Finally, the approachingstorm comes within range of a radarnetwork stretching from Browns-ville to Boston, and from Miami tothe Lesser Antilles.

Through the lifetime of thehurricane, advisories from theNational Hurricane Center and otherwaming offices, give the storm's'position and what the foretaste-, inMiami expect the storm to do. Asthe hurricane drifts to within a day

. ._

or two of its predicted landfall, theseadvisories begin to carry hurricanewatch and warning messages, tellingpeople when and where the ci

hurricane is expected to stfike, andwhat its effects are likely to be. Notuntil the storm has decayed 'Dyerland and its cloudy dements andgreat cargo of moisture haveblended with other brands ofweather, does the hurricane

.emergency end.This system works Well. The

death toll in the United States fromhurricanes has dropped steadily asNOAA's hurricane tracking andwarning apparatus has matured. Al-though the accuracy of hurricaneforecasts has improved over theyears any significant improvementsmust come from quantum jumps inscientific understanding.

The forecasters also know thatscience will never provide a full solu-tion to the problems of hurricanesafety. The rapid development ofAmerica's coastal areas has placedmillions of people with little or nohurricane experience in the path ofthese lethal storms. For thisvulnerable coastal population, theanswer must h.- communitypreparedness and public education inthe hope that education andplanning before the fact will savelives and lessen the impact of thehurricane.

i..

'"?

C

aosart4ikinfeMiri". .madam*

NOAA's VVP3D Orion flies a hug ,anemission dunng Project Stormfury, aneffort to determine whether man canmodify humcanes through cloud seedingwith beneficial results

40

No human effort to change theatmosphere has quite the dimensionsof Project Stormfury, NOAA's ex-perimental attempt to mitigate thewinds of hurricanes. As a scientificachievement, learning to modify hur-ricanes would rate very high. As acomplex, carefully executed fieldexperiment, in the atmosphere, it hasfew equals. '

Stormfury is an effort to learnenough abnut hurricanes to be ableto say whether they c'an be alteredthrough cloud seeding, with benefi-cial results. The experimentdeveloped from studies performedsince 1956 by what is now NOAA'sNational Hurricane and ExperimentalMeteorology Laboratory, in Miami.Basically a kind of atmospheric judo,it pits scientists and aircraft andcloud-seeding techniques against thegiant forces of a hurricane. Theobject is to use the giant's ownenergy against it, setting in motionrearrions within the storm that finallyred.. 2 its intensity,

The rationale for Stormfury isthat by reducing hurncane windsand, so, greatly reducing the force ofthe winds on structuresdamage inareas struck by the big storms couldbe significantly diminished. Also, because waves and storm surge areclosely coupled to wind fields in ahurricane, reduction of windspresumably would also moderatethese more destructive elements Inthose terms. Stormfury is anexperimental investment which scientists expect to pay br itself manytimes over, if what is called theStormfury hypothesis is correct.

This hypothesis has evolved asscl%ntists have obtained a betterunderstanding of the storms. Initially,the object was to seed clouds justoutside the ring o: maximum windsin the outer portions of the eyewall

41

and adjacent spiral bang., causingsupercooled waterwater cooledbelow freezing but still in liquidformto freeze, releasing largequantities of heat enemy into theclouds near the eye. inis largerelease of latent heat of fusion out-side the ring of maximum winds, thehypothesis went, would flatten thepres-iire gradient, causing thehurricane to spread into a stormsystem of lesser intensity.

Today, the Stormfury hypothesishas been refined by measurementsbrought back by new airbornesensors, and analysis in the Miamilaboratory. The present seeding stra-tegy is to seed clouds outside theeyewall, in an effort to redistributethe mass flow of the storm. Here,seeding still converts supercooledwater to ice, triggering the release ofenormous quantities of energy intothe storm outside the eyewall. Butthe effect is to cause immatureclouds there to develop, condensingmore water vapor at greaterdistances from the eye, the heat ofcondensation released by this effectis also ,...actable to stimulate cloudgrowth Thus, these outer cloudsgrow at the expense of the eyewall,the storm's chimney system. Ineffect, Stormfury would create asecondary e,,,e wall which wouldreplace the initial one, effectivelyspreading the storm center. Thiswould flatten the pressure gradientacross the eye, and tum a severehurcane into a moderate one. Theeffects of seeding, Stormfuryresearchers believe, would persistfrom six to twelve hours, after whichthe hurricaneother thingsremaining equalwould regain itsonginal intensity.

Stormfury has been chronicallyshort on experimental opportunities,however. Public concern that seedingmight cause a storm to change itstrack, or increase intensity afterseeding, or reduce seasonal rainfallfrom hurricanes, or simply "goagainst nature," has partly shapedthe experiment; but recent researchand past observations indicate suchfears are groundless. Scientificconcern that the hurricane behandled objectivelythat is,seeded and monitored over theneutral environment of the oceanhas also been a powerful shapingfornc. These factors led to stringentrui2s which prohibit seeding whenthe storm has more than a tenpercent chance of making landfallwithin 24 hoursconstraints whichhave put many storms out ofreach.

From 1961 through 1971,Stormfury, then a joint Departmentof Commerce-Department of Defenseexperiment, was able to "treat" onlyfour storms. Esther in 1961, Beulahin 1963, Debbie in 1969, and Gingerin 1971.

Although this sample is quitesmall, seeding appears to haveproduced some interesting effect..The seeded,portion of Esther'seyewall faded from the radarscope(which "sees" water droplets), in-dicating either a change of liquidwater to ice crystals or thereplacement of large droplets bymuch smaller ones. Beulah alsoappeared to respond to seeding. thecentral pressure of the eye rose soonafter injection of silver iodide parti-cles and the area of maximum windsmoved away from the storm center,however, natural oscillations withinthe hurricane could have producedeffects of this magnitude.

Hurricane Debbie was the firststorm subjected to multiple seedings(August 18 and 20, 1969), receivingmassive charges of silver iodide attwo-hour intervals, five times eachday. Appreciable changes inmaximum wind speeda 30-percentreduction on the 18th, 15 percent onthe 20thand other parametersrelated to the structure of Debbieoccurred after seeding. Analyses ofpast storms indicated the rate ofdecrease in wind speed observed onAugust 18 would be very rare in anunseeded hurricane; and windspeeddecreases on August 20 could beexpected to occur naturally in few-er than or e tenth of a sample ofunmodifiei storms. The fact that thestorm's wind diminished on bothseeding days suggests themodification experiments wereeffective.

Results obtained from hu-ricaneGinger, seeded on Septembe, 25and 28, 1971, were ambiguous Thesilver iodide seeding agent wasdispersed into the rainsector bandsnear the eyewall, rather than justoutside the eyewall, and some cloudchanges appeared to be related tothe seeding. But Ginger was toopoorly organized for scientists to separate seeding effects from thestorm's natural noise level.

Late in 1972, the decision wasmade to concer :ate on hurncaneresearch, temporarily suspending theseeding operations in Stormfury. Thisintenm penod would also be used toreplace the aging piston powered aircraft used to carry NOAA's scientistsand instnimentation into ourricaneswith new, turbine-powered onesequipped with the best instrurnentation available through present-daytechnology.

4C19

4

Ars

NOAA's C.130, still a key participant in hurricane research and Project Stormfury, isone of two such aircraft carrying an Air Forcedeveloped Airborne WeatherReconnaissance System (AWRS), a computerized set of instruments and dataanalyzers controlled from the panel shown at lower left. At lower right, a scientistlaunches a bathythermograph, to probe the cold wake some hurncanes leave behind

20 them in the sea.

43

Thus, the hurricane seasonsfrom 1973 through 1976 sawStormfury scientists fly purely re-search missions into westernhemisphere hurricanes. These long,bumpy missions, and newgenerations oc airborne instruments,brought back data whichresearchers have turned into asignificantly better understand-ing of hurricanes and theirsusceptibility to seeding.

Throughout, the emphasis hasbeen on leaming the dynamiccontext of the entire hurricanesystem, from.the microphysicalprocesses that control the giantstorm, to the immense circulationsand interactions of the hurricaneitself. These probes confirmed thathurricanes have the rightcombination of liquid water anddeveloping (but still immature) cloudtowers to make seeding feasible, andthat hurricane winds from thesurface to altitude"were aboi!,t. whatscientists had believed. They alsoconfirmed that revised flight pattemsthrough the hurricane, redesigned tokeep the aircraft in rainbands justoutside tb.2 region of greatest windsfor extended periods of time, couldbe flown safely. Parallel research ledto development of significantly improved mathematical models of thebig storms.

Meanwhile, NOAA's Miami-basedResearch Facilities Center has beenrevitalized with the addition of twonew Lockheed WP3D Orion aircraft,built to NOAA's special requirements.The Orions and theirinstrumentation systems are the mostadvanced airborne platforms for en-vironmental research in the world,and represent a major step in man'sability to look at the internal dynam-ics of hurricanes. NOAA's C130also carries an advanced airborneweather instrumentation sy;tern.

Stormfury-Americas. The weather-modification side of Stormfury hasbeen revived in StormfuryA--.ericas,which schedules seeding of selectedwestern hemisphere hurricanes latein the decade of the 1970's.

The principal operational difference between StormfuryAmericasand its predecessor missions is thatit is in many ways a simplerexperiment. The missions are flownwith similarly stringent condrtionsthat is, storms with more than a ten-percent chance of coming within 50miles (80 kilometers) of landfall within24 hours of seeding are not eligiblefor seeding. It employs thethree NOM aircraft, an additionalWC-130 on loan to NOM from theAir Force, and the NationalAeronautics and SpaceAdministration's Galileo tl, a high-fly-ing, heavily instrumented Convair990 In the storm, advancedinstrumentation systems aboard theseaircraft permit scientists to seedwhere it will have the greatest effect.and then monitor that area to detectseedingconnected changes.

Stormfury-Americas is a newStormfury, guided by deeperknowledge of the hurricane, andbacked by greater sophistication ofcomputer models instruments,aircraft, and other equipment. It willbe through such projects thathumanity discovers whether scienceand technology can be used to tamehurricanes, the greatest storms onearth.

44

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Holliday, Charles R., "Operations ofthe Joint Typhoon Waming Center,"Mariners Weather Log. 16(6):350-355, November 1972, Washington,D.C.Howard, R.A., et al., "Decision to SeedHurricanes," Science. 176(4040):1191.1205, June 16, 1972, Wash-ington, D.C.

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NOAA, Space: EnvironmentalVantage Point, U.S. GovernmentPrinting Office. Thirty-six pages,1971, Washington, D.C.

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nadoes," Conference on SevereLocal Storms, 8th, October 15-17,1973, Denver. Reprints. AmericanMeteorological Society. p. 108-114,1973, Boston

Sadler, James C., et al., Tropical Cy-clones of the North IndianOcean," Environmental Prediction' Re-search Facility, ENVPREDRSCHFACTechnical Paper No. 2.73. Fifty-eight pages, March 1973.

Simpson, R.H. "The Complex Killer,"Oceanus. Vol. 17, pg. 22.27,Spring 1974, Woods hole Massa-chusetts.- "Hurricane Prediction Skill:Progress and Prospects," Interna-tional Tropical Meteorology MeetingJanuary 31-February 7, 1974, Nai-robi, Kenya. Preprints, Pt. 1, Amen-can Meteorological Society. pp. 145-150, 1974, Boston.

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Technical Conference, "TyphoonModification: Proceedings of theTechnical Conference." World Mete-orological Organization. WMONo.408, 143 p., 1975, Geneva.

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