CHAPTER36TROPICALCYCLONESCAUSESANDDESCRIPTIONOFTROPICALCYCLONES3600.Introductionmilesfromthe center.Therapidity withwhichtheweathercandeteriorate with approachofthestorm,and theviolenceA tropical cyclone is a cyclone originating in the trop-of the fully developed tropical cyclone, are difficult toics or subtropics. Although it generally resembles theimagineiftheyhavenotbeenexperiencedextratropical cycloneof higher latitudes,there are impor-On his second voyage to the New World,Columbus en-tant differences, the principal one being the concentrationcountered a tropical storm.Although his vessels suffered noof a large amount of energy into a relatively small area.damage, this experience proved valuable during his fourthTropical cyclonesareinfrequentincomparisonwithmiddlevoyagewhen his ships werethreatenedbyafullydevelopedand highlatitude storms,but they have a record of destruc-hurricane.Columbus readthesigns ofanapproachingstormtion farexceeding that of anyothertype of storm.Becausefrom theappearanceof a southeasterly swell, thedirectionof theirfury,and becausetheyarepredominantly oceanicof the high cirrus clouds,and the hazy appearance of the at-theymeritspecialattentionbymarinersmosphere.Hedirected his vessels to shelter.TheA tropical stormhas adeceptively small size,andcommander of another group, who did not heed the signs,beautiful weather may be experienced only a few hundredlostmostofhisshipsandmorethan500menperishedFigure 3602.Areas in which tropical cyclones occur.The average number of tropical cyclones per5°square has beenanalyzedforthisfigure.Themainseasonfor intensetropical storm activityisalso shownforeachmajorbasin505
505 CHAPTER 36 TROPICAL CYCLONES CAUSES AND DESCRIPTION OF TROPICAL CYCLONES 3600. Introduction A tropical cyclone is a cyclone originating in the tropics or subtropics. Although it generally resembles the extratropical cyclone of higher latitudes, there are important differences, the principal one being the concentration of a large amount of energy into a relatively small area. Tropical cyclones are infrequent in comparison with middle and high latitude storms, but they have a record of destruction far exceeding that of any other type of storm. Because of their fury, and because they are predominantly oceanic, they merit special attention by mariners. A tropical storm has a deceptively small size, and beautiful weather may be experienced only a few hundred miles from the center. The rapidity with which the weather can deteriorate with approach of the storm, and the violence of the fully developed tropical cyclone, are difficult to imagine if they have not been experienced. On his second voyage to the New World, Columbus encountered a tropical storm. Although his vessels suffered no damage, this experience proved valuable during his fourth voyage when his ships were threatened by a fully developed hurricane. Columbus read the signs of an approaching storm from the appearance of a southeasterly swell, the direction of the high cirrus clouds, and the hazy appearance of the atmosphere. He directed his vessels to shelter. The commander of another group, who did not heed the signs, lost most of his ships and more than 500 men perished. Figure 3602. Areas in which tropical cyclones occur. The average number of tropical cyclones per 5° square has been analyzed for this figure. The main season for intense tropical storm activity is also shown for each major basin
506TROPICALCYCLONES3601.Definitions3602.AreasOfOccurrence"Tropical cyclone"is the term for cyclones originatingTropical cyclones occur almost entirely in six distinctareas,fourin theNorthern Hemisphere and two inthein the tropics or subtropics.These cyclones are classified bySouthern Hemisphere as shown inFigure 3602.The nameform and intensity as they increase in size.by which the tropical cyclone is commonlyknown variesAtropical disturbance is a discrete system of appar-somewhat with the locality.ently organized convection,generally 100 to 300 miles indiameter,havinga nonfrontal migratory character,and hav-1.NorthAtlantic.Atropical cyclonewith windsof64ing maintained its identity for 24 hours or more. It may orknots or greater is called a hurricane.maynotbeassociated withadetectabledisturbanceof the2.Eastern NorthPacific.The name hurricane is usedwind field.It has no strong winds and no closed isobars i.eas intheNorthAtlantic.isobarsthatcompletelyenclosethelow.3.WesternNorthPacific.Afullydeveloped stormAt its next stageofdevelopmentitbecomes atropicalwith winds of 64knots or greater is called a ty-depression. A tropical depression has one or more closed iso-phoon or, locally in the Philippines, a baguio.bars and some rotary circulation at the surface.The highest4.North Indian Ocean.Atropical cyclone with windssustained(1-minutemean)surfacewind speed is33knotsof 34knots orgreater is called a cyclonicstorm.The next stage is tropical storm,Atropical storm hasSouth Indian Ocean. A tropical cyclone with winds5closed isobars and a distinct rotary circulation.The highest sus-of 34knots or greater is called a cyclone.tained (1-minute mean) surface wind speed is 34 to 63 knotsSouthwest Pacific and Australian Area.The name cy-6.When fully developed, a hurricane or typhoon hascloneisusedasintheSouthIndianOcean.Asevereclosed isobars,a strong and verypronounced rotary circu-tropical cyclone originating in the Timor Sea andlation,andasustained(l-minutemean)surfacewind speedmovingsouthwestandthensoutheastacrosstheinteof 64 knots or higher.rior of northwestern Australia is called a willy-willyFigure3603a. Storm tracks.The width of the arrow indicates the approximate frequency of storms; the wider the arrowthehigher thefrequency.Isolines on thebasemapshowtheresultantdirection toward which stormsmoved.Data for theentireyearhas been summarizedforthis figure
506 TROPICAL CYCLONES 3601. Definitions “Tropical cyclone” is the term for cyclones originating in the tropics or subtropics. These cyclones are classified by form and intensity as they increase in size. A tropical disturbance is a discrete system of apparently organized convection, generally 100 to 300 miles in diameter, having a nonfrontal migratory character, and having maintained its identity for 24 hours or more. It may or may not be associated with a detectable disturbance of the wind field. It has no strong winds and no closed isobars i.e., isobars that completely enclose the low. At its next stage of development it becomes a tropical depression. A tropical depression has one or more closed isobars and some rotary circulation at the surface. The highest sustained (1-minute mean) surface wind speed is 33 knots. The next stage is tropical storm. A tropical storm has closed isobars and a distinct rotary circulation. The highest sustained (1-minute mean) surface wind speed is 34 to 63 knots. When fully developed, a hurricane or typhoon has closed isobars, a strong and very pronounced rotary circulation, and a sustained (1-minute mean) surface wind speed of 64 knots or higher. 3602. Areas Of Occurrence Tropical cyclones occur almost entirely in six distinct areas, four in the Northern Hemisphere and two in the Southern Hemisphere as shown in Figure 3602. The name by which the tropical cyclone is commonly known varies somewhat with the locality. 1. North Atlantic. A tropical cyclone with winds of 64 knots or greater is called a hurricane. 2. Eastern North Pacific. The name hurricane is used as in the North Atlantic. 3. Western North Pacific. A fully developed storm with winds of 64 knots or greater is called a typhoon or, locally in the Philippines, a baguio. 4. North Indian Ocean. A tropical cyclone with winds of 34 knots or greater is called a cyclonic storm. 5. South Indian Ocean. A tropical cyclone with winds of 34 knots or greater is called a cyclone. 6. Southwest Pacific and Australian Area. The name cyclone is used as in the South Indian Ocean. A severe tropical cyclone originating in the Timor Sea and moving southwest and then southeast across the interior of northwestern Australia is called a willy-willy. Figure 3603a. Storm tracks.The width of the arrow indicates the approximate frequency of storms; the wider the arrow the higher the frequency. Isolines on the base map show the resultant direction toward which storms moved. Data for the entire year has been summarized for this figure
507TROPICALCYCLONESTropical cyclones havenotbeen observed inthe Southor recurve and accelerate in theNorth Atlantic.Some willAtlanticorintheSouthPacificeastof 140°Wrecurve afterreaching the Gulf of Mexico,while others willcontinue westwardto a landfall in Texas or Mexico.3603.Origin,SeasonAndFrequencyEastern North Pacific:The season is from Junethrough October,although a stormcanform in anymonthSeeFigures3603aand3603b.Origin,season,andfre-Anaverageof 15tropical cyclonesformeachyearwithquency of occurrence of the tropical cyclones in the sixabout 6 reaching hurricane strength. The most intenseareas areas follows:storms are often the early-and late-season ones, theseformNorthAtlantic:Tropical cyclones can affect theentirecloseto the coast and far south.Mid season storms formNorthAtlanticOceanin anymonth.However,theyareanywhere in a wide band from the Mexican-Central Amer-mostlya threat south of about 35°NfromJune throughNo-icancoasttotheHawaiianIslands.AugustandSeptembervember,August,September,andOctoberarethemonths ofarethemonthsofhighestincidence.Thesestormsdifferhighest incidence. See Figure 3603b.About 9 or 10 tropicalfrom their North Atlantic counterparts in that they are usu-cyclones(tropical stormsandhurricanes)formeachseason;ally smaller in size.However,they can be justas intense.5or6reachhurricane intensity(winds of 64knotsandWesternNorthPacifie:Moretropical cyclones formhigher)Afewhurricaneshavegeneratedwindsestimatedin thetropical western NorthPacificthananywhereelse inas high as 200knots.Early and late season storms usuallythe world.Morethan25tropical stormsdevelop eachyear,develop west of 50oW; during August and September, thisand about18becometyphoons.Thesetyphoons arethespawning ground extends to the Cape Verde Islands.Theselargestand mostintense tropical cyclones in the worldstormsusuallymovewestwardorwestnorthwestwardatspeeds of less than 15knots in the lower latitudes.AfterEachyear anaverageof fivegeneratemaximum windsovermovingintothenorthernCaribbeanorGreaterAntillesre-130knots;circulationscoveringmorethan600miles indi-gions, they usually either movetoward the Gulf of Mexicoameterarenotuncommon.MostofthesestormsformeastAREAAND STAGEJANFEBMARAPROCTNOVDECANNUALMAYJUNSEPNORTHATLANTIC4.2TROPICALSTORMS0.40.31.01.51.2/0.40.10.32.75.2HURRICANES0.41.51.30.3TROPICAL STORMS AND HURRICANES0.80.19.44.32.5JANFEBMARAPRMAYJUNJULL|AUGSEPOCTNOVDECANNUALEASTERN NORTH PACIFIC2.32.311.20.39.3TROPICALSTORMS1.52.80.35.8HURRICANES0.60.92.01.81.0TROPICAL STORMS AND HURRICANES15.2AT22JANFEBMARAPRMAYJUNJULAUGSEPOCTNOVDECANNUALWESTERN NORTHPACIFICTROPICALSTORMS020.67.5020.50.8TYPHOONS0.20.93.317.80.30.20.71.22.74.04.12.10.7TROPICAL STORMS AND TYPHOONS0.4050.995.613:1.325.3JANFEBMARAPRMAYAUGSEPOCT|NOVDECANNUALJUNSOUTHWEST PACIFIC AND AUSTRALIAN AREATROPICALSTORMS10.90.2HURRICANES0.30.10.10.730.53.8TROPICAL STORMS AND HURRICANES0.10.1JANFEBMARANNUALAPEMAYINJULAIGSOCNODECSOUTHWESTINDIANOCEANTROPICALSTORMS7.4204J.60.80.80.40.53.8HURRICANES1.311.2TROPICALSTORMSAND HURRICANES1.4JANFEBMARAPRMAYJUNAUGSEPOCTLNOVDECANNUALNORTHINDIANOCEANTROPICALSTORMS0.10.10.30.60.50.33.50.50.50.4140.10.40.22.30F0.20.10.10.6CYCLONES'0.50.10.10.30.60.51.05.7TROPICALSTORMSANDCYCLONESLess than.051Winds ≥ 48 Kts.Monthly values cannot be combined because single storms overlapping two months were counted once in each month and once in the annualFigure3603b.Monthlyandannualaveragenumberofstormsperyearforeachareaof thePhilippines,and move across thePacifictowardtheDecember.However,tropical cyclonesaremore commonPhilippines,Japan,and China,a few stormsform in theintheoff-seasonmonthsinthisareathananywhereelseSouthChina Sea.The season extends from April throughThepeak of the season is July through October, when near-
TROPICAL CYCLONES 507 Tropical cyclones have not been observed in the South Atlantic or in the South Pacific east of 140°W. 3603. Origin, Season And Frequency See Figures 3603a and 3603b. Origin, season, and frequency of occurrence of the tropical cyclones in the six areas are as follows: North Atlantic: Tropical cyclones can affect the entire North Atlantic Ocean in any month. However, they are mostly a threat south of about 35°N from June through November; August, September, and October are the months of highest incidence. See Figure 3603b. About 9 or 10 tropical cyclones (tropical storms and hurricanes) form each season; 5 or 6 reach hurricane intensity (winds of 64 knots and higher). A few hurricanes have generated winds estimated as high as 200 knots. Early and late season storms usually develop west of 50°W; during August and September, this spawning ground extends to the Cape Verde Islands. These storms usually move westward or west northwestward at speeds of less than 15 knots in the lower latitudes. After moving into the northern Caribbean or Greater Antilles regions, they usually either move toward the Gulf of Mexico or recurve and accelerate in the North Atlantic. Some will recurve after reaching the Gulf of Mexico, while others will continue westward to a landfall in Texas or Mexico. Eastern North Pacific: The season is from June through October, although a storm can form in any month. An average of 15 tropical cyclones form each year with about 6 reaching hurricane strength. The most intense storms are often the early- and late-season ones; these form close to the coast and far south. Mid season storms form anywhere in a wide band from the Mexican-Central American coast to the Hawaiian Islands. August and September are the months of highest incidence. These storms differ from their North Atlantic counterparts in that they are usually smaller in size. However, they can be just as intense. Western North Pacific: More tropical cyclones form in the tropical western North Pacific than anywhere else in the world. More than 25 tropical storms develop each year, and about 18 become typhoons. These typhoons are the largest and most intense tropical cyclones in the world. Each year an average of five generate maximum winds over 130 knots; circulations covering more than 600 miles in diameter are not uncommon. Most of these storms form east of the Philippines, and move across the Pacific toward the Philippines, Japan, and China; a few storms form in the South China Sea. The season extends from April through December. However, tropical cyclones are more common in the off-season months in this area than anywhere else. The peak of the season is July through October, when nearAREA AND STAGE JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC ANNUAL NORTH ATLANTIC TROPICAL STORMS * * * * 0.1 0.4 0.3 1.0 1.5 1.2 0.4 * 4.2 HURRICANES * * * * * 0.3 0.4 1.5 2.7 1.3 0.3 * 5.2 TROPICAL STORMS AND HURRICANES **** 0.2 0.7 0.8 2.5 4.3 2.5 0.7 0.1 9.4 JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC ANNUAL EASTERN NORTH PACIFIC TROPICAL STORMS * * * * * 1.5 2.8 2.3 2.3 1.2 0.3 * 9.3 HURRICANES * * * * 0.3 0.6 0.9 2.0 1.8 1.0 * * 5.8 TROPICAL STORMS AND HURRICANES **** 0.3 2.0 3.6 4.5 4.1 2.2 0.3 * 15.2 JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC ANNUAL WESTERN NORTH PACIFIC TROPICAL STORMS 0.2 0.3 0.3 0.2 0.4 0.5 1.2 1.8 1.5 1.0 0.8 0.6 7.5 TYPHOONS 0.3 0.2 0.2 0.7 0.9 1.2 2.7 4.0 4.1 3.3 2.1 0.7 17.8 TROPICAL STORMS AND TYPHOONS 0.4 0.4 0.5 0.9 1.3 1.8 3.9 5.8 5.6 4.3 2.9 1.3 25.3 JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC ANNUAL SOUTHWEST PACIFIC AND AUSTRALIAN AREA TROPICAL STORMS 2.7 2.8 2.4 1.3 0.3 0.2 * * * 0.1 0.4 1.5 10.9 HURRICANES 0.7 1.1 1.3 0.3 * * 0.1 0.1 * * 0.3 0.5 3.8 TROPICAL STORMS AND HURRICANES 3.4 4.1 3.7 1.7 0.3 0.2 0.1 0.1 * 0.1 0.7 2.0 14.8 JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC ANNUAL SOUTHWEST INDIAN OCEAN TROPICAL STORMS 2.0 2.2 1.7 0.6 0.2 * * * * 0.3 0.3 0.8 7.4 HURRICANES 1.3 1.1 0.8 0.4 * * * * * * * 0.5 3.8 TROPICAL STORMS AND HURRICANES 3.2 3.3 2.5 1.1 0.2 **** 0.3 0.4 1.4 11.2 JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC ANNUAL NORTH INDIAN OCEAN TROPICAL STORMS 0.1 * * 0.1 0.3 0.5 0.5 0.4 0.4 0.6 0.5 0.3 3.5 CYCLONES1 * * * 0.1 0.5 0.2 0.1 * 0.1 0.4 0.6 0.2 2.2 TROPICAL STORMS AND CYCLONES1 0.1 * 0.1 0.3 0.7 0.7 0.6 0.4 0.5 1.0 1.1 0.5 5.7 * Less than .05 1Winds ≥ 48 Kts. Monthly values cannot be combined because single storms overlapping two months were counted once in each month and once in the annual. Figure 3603b. Monthly and annual average number of storms per year for each area
508TROPICALCYCLONESly70 percent ofall typhoons develop.There is a noticeablesoaked areas, cause disastrous floods.seasonal shift in storm tracks in this region. From JulySouthIndianOcean:Overthewaters west of 100°Ethrough September, storms move north of the Philippinesto the east African coast, an averageof 1l tropical cyclonesandrecurve,whileearly-andlate-seasontyphoonsmoveon(tropicalstorms andhurricanes)form each season,anda more westerly track through the Philippines beforeabout 4 reach hurricane intensity.The season is from De-recurving.cember through March, although it is possiblefor a stormNorth Indian Ocean: Tropical cyclones develop into form in any month.Tropical cyclones in this region usuthe Bay of Bengal and Arabian Sea during the spring andallyform south of 1oos.The latitude of recurvature usuallyfall.Tropical cyclones in this area form between latitudesmigrates from about 20°s in January to around 15°s in8Nand15°N,exceptfrom JunethroughSeptember,whenApril.Aftercrossing3oos,these storms sometimes becomethelittleactivitythatdoesoccurisconfinednorthofaboutintenseextratropical lows.15°N.These storms are usually short-lived and weak,how-Southwest Pacific and Australian Area:These trop-ever,winds of130knotshavebeen encountered.Theyical waters spawn an annual average of15 tropical cyclonesoften develop as disturbances along theIntertropical Con-4 of which reachhurricane intensity.The season extendsvergenceZone(ITCZ)thisinhibitssummertimefromaboutDecemberthroughAprilalthoughstormscardevelopment, since theITCZ is usually over land duringform in anymonth.Activityis widespread in January andthismonsoon season.However, it is sometimes displacedFebruaryanditisinthesemonthsthattropicalcvclonesaresouthward, and when this occurs, storms will form over themost likely to affect Fiji, Samoa, and the other eastern is-monsoon-flooded plains of Bengal.On the average, six cy-lands.Tropical cyclones usuallyform in the waters fromclonicstormsformeachvear.Theseincludetwostormsthat105°Eto160°W,between5°and20°s.Storms affectinggenerate winds of 48knots orgreater.Another 10 tropicalnorthern and western Australia often develop inthe Timorcyclones neverdevelopbeyondtropical depressions.TheorArafuraSea.whilethosethataffecttheeastcoastforminBay of Bengal is the area of highest incidence.However,itthe Coral Sea.These storms are often small, but can devel-is not unusualfora stormtomove across southern India andopwinds inexcessof130knots.NewZealandissometimesreintensify in the Arabian Sea. This is particularly true dur-ing October, the month of highest incidence during thereachedbydecayingCoral Sea storms,andoccasionallybytropical cyclone season.It is alsoduringthis period thattor-anintensehurricane.Ingeneral.tropicalcvclonesinthisre-rential rainsfrom thesestorms,dumped overalreadyrain-gionmovesouthwestward andthenrecurvesoutheastward.ANATOMYOFTROPICALCYCLONES3604.Formationtem as an atmospheric heat engine,present a morecomprehensiveand convincingview.Hurricaneformationwas oncebelieved toresultfromTheybegin with a starter mechanism in which eitheran intensification of convective forces which producetheinternal orexternal forces intensify the initial disturbance.The initial disturbance becomes a region into which low-toweringcumulonimbuscloudsofthedoldrums.Thisviewof hurricane generation held that surface heating causedlevel airfromthe surroundingareabeginstoflowacceler-warm moist air toascendconvectivelytolevels where con-atingtheconvectionalreadyoccurringinsidethedensationproducedcumulonimbusclouds,which,afterandisturbance.The vertical circulation becomes increasinglyinexplicabledropin atmosphericpressure,coalesced andwell organized as watervapor in the ascending moist layerwere spun into acyclonicmotionby Coriolis force.is condensed (releasing large amounts of heat energy toThis hypothesis left much unexplained.Although somedrive the wind system), and as the system is swept into ahurricanes developfrom disturbances beginning in the dol-counterclockwise cyclonic spiral.But this incipient hurri-canewould soon fill upbecauseof inflowat lower levels,drums, very few reach maturity in that region. Also, the highunless the chimney in which converging air surges upwardincidenceofseeminglyidealconvectivesituationsdoesnotmatch the low incidence of Atlantic hurricanes.Finally,theis provided the exhaust mechanism of high-altitude windshypothesisdid notexplain thedrop in atmospheric pressureThesehigh-altitude winds pumpascending air out ofsoessentialtodevelopmentof hurricane-forcewinds.the cyclonic system, into ahigh-altitude anticyclone,whichThere is still no exact understanding of the triggeringtransports the air well awayfromthedisturbance, beforemechanism involved inhurricanegeneration,the balanceofsinking occurs.Thus,a large scale vertical circulation is setconditions neededto generate hurricane circulation,and theup, in which low-level air is spiraled up the cyclonic twist-relationshipsbetweenlarge-and small-scale atmosphericing of the disturbance, and, after a trajectory over the sea,processes.But scientists today,treating the hurricane sys-returnedto loweraltitudes somedistancefromthestorm
508 TROPICAL CYCLONES ly 70 percent of all typhoons develop. There is a noticeable seasonal shift in storm tracks in this region. From July through September, storms move north of the Philippines and recurve, while early- and late-season typhoons move on a more westerly track through the Philippines before recurving. North Indian Ocean: Tropical cyclones develop in the Bay of Bengal and Arabian Sea during the spring and fall. Tropical cyclones in this area form between latitudes 8°N and 15°N, except from June through September, when the little activity that does occur is confined north of about 15°N. These storms are usually short-lived and weak; however, winds of 130 knots have been encountered. They often develop as disturbances along the Intertropical Convergence Zone (ITCZ); this inhibits summertime development, since the ITCZ is usually over land during this monsoon season. However, it is sometimes displaced southward, and when this occurs, storms will form over the monsoon-flooded plains of Bengal. On the average, six cyclonic storms form each year. These include two storms that generate winds of 48 knots or greater. Another 10 tropical cyclones never develop beyond tropical depressions. The Bay of Bengal is the area of highest incidence. However, it is not unusual for a storm to move across southern India and reintensify in the Arabian Sea. This is particularly true during October, the month of highest incidence during the tropical cyclone season. It is also during this period that torrential rains from these storms, dumped over already rainsoaked areas, cause disastrous floods. South Indian Ocean: Over the waters west of 100°E, to the east African coast, an average of 11 tropical cyclones (tropical storms and hurricanes) form each season, and about 4 reach hurricane intensity. The season is from December through March, although it is possible for a storm to form in any month. Tropical cyclones in this region usually form south of 10°S. The latitude of recurvature usually migrates from about 20°S in January to around 15°S in April. After crossing 30°S, these storms sometimes become intense extratropical lows. Southwest Pacific and Australian Area: These tropical waters spawn an annual average of 15 tropical cyclones 4 of which reach hurricane intensity. The season extends from about December through April, although storms can form in any month. Activity is widespread in January and February, and it is in these months that tropical cyclones are most likely to affect Fiji, Samoa, and the other eastern islands. Tropical cyclones usually form in the waters from 105°E to 160°W, between 5° and 20°S. Storms affecting northern and western Australia often develop in the Timor or Arafura Sea, while those that affect the east coast form in the Coral Sea. These storms are often small, but can develop winds in excess of 130 knots. New Zealand is sometimes reached by decaying Coral Sea storms, and occasionally by an intense hurricane. In general, tropical cyclones in this region move southwestward and then recurve southeastward. ANATOMY OF TROPICAL CYCLONES 3604. Formation Hurricane formation was once believed to result from an intensification of convective forces which produce the towering cumulonimbus clouds of the doldrums. This view of hurricane generation held that surface heating caused warm moist air to ascend convectively to levels where condensation produced cumulonimbus clouds, which, after an inexplicable drop in atmospheric pressure, coalesced and were spun into a cyclonic motion by Coriolis force. This hypothesis left much unexplained. Although some hurricanes develop from disturbances beginning in the doldrums, very few reach maturity in that region. Also, the high incidence of seemingly ideal convective situations does not match the low incidence of Atlantic hurricanes. Finally, the hypothesis did not explain the drop in atmospheric pressure, so essential to development of hurricane-force winds. There is still no exact understanding of the triggering mechanism involved in hurricane generation, the balance of conditions needed to generate hurricane circulation, and the relationships between large- and small-scale atmospheric processes. But scientists today, treating the hurricane system as an atmospheric heat engine, present a more comprehensive and convincing view. They begin with a starter mechanism in which either internal or external forces intensify the initial disturbance. The initial disturbance becomes a region into which lowlevel air from the surrounding area begins to flow, accelerating the convection already occurring inside the disturbance. The vertical circulation becomes increasingly well organized as water vapor in the ascending moist layer is condensed (releasing large amounts of heat energy to drive the wind system), and as the system is swept into a counterclockwise cyclonic spiral. But this incipient hurricane would soon fill up because of inflow at lower levels, unless the chimney in which converging air surges upward is provided the exhaust mechanism of high-altitude winds. These high-altitude winds pump ascending air out of the cyclonic system, into a high-altitude anticyclone, which transports the air well away from the disturbance, before sinking occurs. Thus, a large scale vertical circulation is set up, in which low-level air is spiraled up the cyclonic twisting of the disturbance, and, after a trajectory over the sea, returned to lower altitudes some distance from the storm
509TROPICALCYCLONESEASTERLYTRADEWINDSHIGHALTITUDEWINDSFigure 3604. Pumping action of high-altitude winds.This pumping action-and the heat released by the ascendingin decks of cumulus and cumulonimbus clouds to the con-airmayaccountforthesuddendropofatmosphericpres-vectivelimitofcloudformation,wherecondensingwatersure at the surface,which produces the steep pressurevapor is swept off as ice-crystal wisps of cirrus clouds.gradient along which winds reach hurricane proportions.Thunderstorm electrical activity is observed in these bands,It is believed that the interactionoflow-level and high-al-both as lightning and as tiny electrostatic dischargestitude wind systems determines the intensity the hurricaneIn the lower fewthousand feet, air flows in through the cy-will attain.Ifless air is pumped out thanconverges at lowlev-clone, and is drawn upward through ascending columns of airels, thesystemwillfill and die out.Ifmoreispumped outthannear the center.The size and intensity decrease with altitude,flows in, the circulation will be sustained and will intensifythecycloniccirculationbeinggraduallyreplacedabove40,000Scientists have found that any process which increasesfeetby ananticycloniccirculationcenteredhundreds ofmilesaway,which is the exhaust system ofthe hurricaneheat enginethe rate oflow-level inflow isfavorableforhurricanedevel-opment,providedtheinflowingaircarriessufficientheatandAt lower levels, where the hurricane is more intense,moisture to fuel the hurricane's power system. It has alsowinds on therim ofthe storm followa wide pattern, like thebeen shown that air above the developing disturbance,at al-slower currents around the edge of a whirlpool; and, liketitudesbetween20,000and40,000feet,increases1°to3°inthose currents, these winds accelerate as they approach thetemperatureabout24hours before the disturbance developscenter of the vortex.The outer band has light winds at theinto a hurricane.But it is not known whether low-level in-rim ofthestorm,perhaps nomore than25knots; within30flow and high-level warming cause hurricanes. They couldmiles of the center, winds may have velocities exceedingvery well bemeasurable symptoms ofanothereffect which130knots.The inner band is theregion ofmaximum windactually triggers the storm's increaseto hurricane intensity.velocity, where the storm's worst winds are felt, and whereTheview of hurricanes as atmospheric engines is necascendingairischimneyedupward,releasingheattodrivethe storm. In most hurricanes, these winds reach 85knots,essarily ageneral one.The exactroleof eachcontributor isandmorethan170knots inseverestorms.not completely understood.The engine seems to be both in-efficient and unreliable, a myriad of delicate conditionsInthehurricane,windsflowtowardthe lowpressure inmust besatisfiedfor the atmosphereto produce a hurricanethewarm,comparatively calm core.There,converging airTheir relative infrequency indicates that many potentialis whirled upward by convection, the mechanical thrustinghurricanesdissipatebeforedeveloping into stormsofother converging air,and thepumping action ofhigh-al-titude circulations. This spiral is marked by the thick cloudwalls curling inward toward the storm center,releasing3605.PortraitOfAHurricaneheavyprecipitationandenormousquantities ofheatenergyAt the center, surrounded by a band in which this strongIn the early life of the hurricane, the spiral covers anvertical circulation is greatest, is the eye ofthe hurricane.area averaging 100 miles in diameter with winds of 64knots andgreater,and spreadsgale-force winds overa 400-On the average, eye diameter is about 14 miles, al-thoughdiametersof 25miles arenotunusual.From themilediameter:The cyclonicspiralismarkedbyheavycloud bands from which torrential rainsfall,separated byheatedtowerofmaximumwindsand cumulonimbusareasoflightrainor norainat all.Thesespiral bands ascendclouds,winds diminish rapidlyto something less than 15
TROPICAL CYCLONES 509 This pumping action-and the heat released by the ascending air may account for the sudden drop of atmospheric pressure at the surface, which produces the steep pressure gradient along which winds reach hurricane proportions. It is believed that the interaction of low-level and high-altitude wind systems determines the intensity the hurricane will attain. If less air is pumped out than converges at low levels, the system will fill and die out. If more is pumped out than flows in, the circulation will be sustained and will intensify. Scientists have found that any process which increases the rate of low-level inflow is favorable for hurricane development, provided the inflowing air carries sufficient heat and moisture to fuel the hurricane’s power system. It has also been shown that air above the developing disturbance, at altitudes between 20,000 and 40,000 feet, increases 1° to 3° in temperature about 24 hours before the disturbance develops into a hurricane. But it is not known whether low-level inflow and high-level warming cause hurricanes. They could very well be measurable symptoms of another effect which actually triggers the storm’s increase to hurricane intensity. The view of hurricanes as atmospheric engines is necessarily a general one. The exact role of each contributor is not completely understood. The engine seems to be both inefficient and unreliable; a myriad of delicate conditions must be satisfied for the atmosphere to produce a hurricane. Their relative infrequency indicates that many potential hurricanes dissipate before developing into storms. 3605. Portrait Of A Hurricane In the early life of the hurricane, the spiral covers an area averaging 100 miles in diameter with winds of 64 knots and greater, and spreads gale-force winds over a 400- mile diameter. The cyclonic spiral is marked by heavy cloud bands from which torrential rains fall, separated by areas of light rain or no rain at all. These spiral bands ascend in decks of cumulus and cumulonimbus clouds to the convective limit of cloud formation, where condensing water vapor is swept off as ice-crystal wisps of cirrus clouds. Thunderstorm electrical activity is observed in these bands, both as lightning and as tiny electrostatic discharges. In the lower few thousand feet, air flows in through the cyclone, and is drawn upward through ascending columns of air near the center. The size and intensity decrease with altitude, the cyclonic circulation being gradually replaced above 40,000 feet by an anticyclonic circulation centered hundreds of miles away, which is the exhaust system of the hurricane heat engine. At lower levels, where the hurricane is more intense, winds on the rim of the storm follow a wide pattern, like the slower currents around the edge of a whirlpool; and, like those currents, these winds accelerate as they approach the center of the vortex. The outer band has light winds at the rim of the storm, perhaps no more than 25 knots; within 30 miles of the center, winds may have velocities exceeding 130 knots. The inner band is the region of maximum wind velocity, where the storm’s worst winds are felt, and where ascending air is chimneyed upward, releasing heat to drive the storm. In most hurricanes, these winds reach 85 knots, and more than 170 knots in severe storms. In the hurricane, winds flow toward the low pressure in the warm, comparatively calm core. There, converging air is whirled upward by convection, the mechanical thrusting of other converging air, and the pumping action of high-altitude circulations. This spiral is marked by the thick cloud walls curling inward toward the storm center, releasing heavy precipitation and enormous quantities of heat energy. At the center, surrounded by a band in which this strong vertical circulation is greatest, is the eye of the hurricane. On the average, eye diameter is about 14 miles, although diameters of 25 miles are not unusual. From the heated tower of maximum winds and cumulonimbus clouds, winds diminish rapidly to something less than 15 Figure 3604. Pumping action of high-altitude winds