《航海学》课程参考文献（地文资料）CHAPTER 36 TROPICAL CYCLONES

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 trop￾ics or subtropics. Although it generally resembles the extratropical cyclone of higher latitudes, there are impor￾tant 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 destruc￾tion 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 en￾countered 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 at￾mosphere. 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 appar￾ently organized convection, generally 100 to 300 miles in diameter, having a nonfrontal migratory character, and hav￾ing 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 iso￾bars 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 sus￾tained (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 circu￾lation, 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 ty￾phoon 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 cy￾clone 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 inte￾rior 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 fre￾quency 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 No￾vember; 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 re￾gions, 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 Amer￾ican 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 usu￾ally 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 di￾ameter 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 near￾AREA 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; how￾ever, winds of 130 knots have been encountered. They often develop as disturbances along the Intertropical Con￾vergence 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 cy￾clonic 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 dur￾ing October, the month of highest incidence during the tropical cyclone season. It is also during this period that tor￾rential rains from these storms, dumped over already rain￾soaked 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 De￾cember through March, although it is possible for a storm to form in any month. Tropical cyclones in this region usu￾ally 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 trop￾ical 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 is￾lands. 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 devel￾op 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 re￾gion 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 con￾densation 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 dol￾drums, 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 sys￾tem 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 low￾level air from the surrounding area begins to flow, acceler￾ating 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 hurri￾cane 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 twist￾ing 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 pres￾sure 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-al￾titude wind systems determines the intensity the hurricane will attain. If less air is pumped out than converges at low lev￾els, 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 devel￾opment, 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 al￾titudes 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 in￾flow 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 nec￾essarily a general one. The exact role of each contributor is not completely understood. The engine seems to be both in￾efficient 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 con￾vective 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 cy￾clone, 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-al￾titude 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, al￾though 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

510TROPICALCYCLONESHIGHALTITUDEWINDSSPIRALRAINBANDSFigure 3605.Cutaway view ofa hurricane greatly exaggerated in vertical dimension. Actual hurricanes are less than50,000feethighandmayhaveadiameterof several hundredmiles.miles per hour in the eye, at the opposite wall, winds in-high-pressure system aloft.Sea surface temperatures increaseagain,butcomefromtheoppositedirectionbecausethe vicinity are in the 28°-30°C range.Within48hourswindsincreaseto25knotsneartheofthecycloniccirculationofthestorm.Thissuddentrans-formationofstormintocomparativecalmandfromcalmcenter of definite circulation, and central pressure hasinto violencefrom anotherquarter is spectacular.The eye'sdroppedbelow1000millibars.Thedisturbanceisnowclas-abruptexistence in the midst ofopaquerain squalls and hur-sified as atropical depression.Soon the circulation extendsricane winds, the intermittent bursts of blue sky andout to100miles andupward to20.000feet.Windsnearthesunlight through light clouds in the core of the cyclone, andcenter increasetogaleforce,central pressurefalls below990millibars,and toweringcumulonimbusclouds shieldathegalleriedwallsofcumulusandcumulonimbuscloudsare unforgettable.developing eye;atropical stormhasdeveloped.Every hurricane is individual, and the more or less or-Satellite photographs now reveal a tightly organizedderly circulation described here omits the extremetropicalcvclone.andreconnaissancereportsindicatemaxivariability and instabilitywithin the storm system.Pressuremumwindsof80knotsaroundacentralpressureof980andtemperaturegradientsfluctuatewildlyacrossthestormmillibars, a hurricane has developed. A ship to the rightasthehurricanemaintainsitserraticlife.IfitisanAugust(leftintheSouthernHemisphere)ofthehurricane'scenterstorm,itsaverage lifeexpectancyis12days,ifa JulyorNo-(lookingtowardthedirectionofstormmovement)reports30-foot seas.The hurricane is rapidly maturing as itcontin-vember storm, it lives an average of 8days.ues westward3606.LifeOfATropicalCycloneA few days later the hurricane reaches its peak.Thesatellite photographs show a textbook picture, as 120-knotReportsfromships in thevicinityofaneasterlywavewinds roar around a 940-millibar pressure center, hurri-cane-force winds extend 50 miles in all directions,and seas(awestward-movingtroughof lowpressureembedded inare reported up to 4o feet. There is no further deepeningdeepeasterlies)mayindicatethattheatmosphericpres-sureintheregionhasfallenmorethan5millibarsinthenow,but thehurricanebeginstoexpand.In2days,galespast24 hours.This is causefor alarm,because in the Trop-extendoutto200miles,andhurricanewindsoutto75ics pressure varies little, the normal diurnal pressuremiles.Thenthehurricaneslowsandbeginstorecurve:thisturning marks thebeginning of itsfinal phasechangeisonlyabout3millibars.Satellitepicturesmayin-dicate thickening middle and high clouds. Squalls areThehurricaneaccelerates,and,uponreachingtemper-reportedaheadoftheeasterlywave,andwindreportsin-ate latitudes, it begins to lose its tropical characteristics.dicate a cyclonic circulation is forming.The formerThe circulation continues to expand, but now cold air is in-easterly wave,now classified a tropical disturbance,istruding (cold air, cold water, dry air aloft, and land, aid inmoving westwardat10knotsunder thecanopyof a largethedecay of a tropical cyclone).The windsgradually abate

510 TROPICAL CYCLONES miles per hour in the eye; at the opposite wall, winds in￾crease again, but come from the opposite direction because of the cyclonic circulation of the storm. This sudden trans￾formation of storm into comparative calm, and from calm into violence from another quarter is spectacular. The eye’s abrupt existence in the midst of opaque rain squalls and hur￾ricane winds, the intermittent bursts of blue sky and sunlight through light clouds in the core of the cyclone, and the galleried walls of cumulus and cumulonimbus clouds are unforgettable. Every hurricane is individual, and the more or less or￾derly circulation described here omits the extreme variability and instability within the storm system. Pressure and temperature gradients fluctuate wildly across the storm as the hurricane maintains its erratic life. If it is an August storm, its average life expectancy is 12 days; if a July or No￾vember storm, it lives an average of 8 days. 3606. Life Of A Tropical Cyclone Reports from ships in the vicinity of an easterly wave (a westward-moving trough of low pressure embedded in deep easterlies) may indicate that the atmospheric pres￾sure in the region has fallen more than 5 millibars in the past 24 hours. This is cause for alarm, because in the Trop￾ics pressure varies little; the normal diurnal pressure change is only about 3 millibars. Satellite pictures may in￾dicate thickening middle and high clouds. Squalls are reported ahead of the easterly wave, and wind reports in￾dicate a cyclonic circulation is forming. The former easterly wave, now classified a tropical disturbance, is moving westward at 10 knots under the canopy of a large high-pressure system aloft. Sea surface temperatures in the vicinity are in the 28°-30°C range. Within 48 hours winds increase to 25 knots near the center of definite circulation, and central pressure has dropped below 1000 millibars. The disturbance is now clas￾sified as a tropical depression. Soon the circulation extends out to 100 miles and upward to 20,000 feet. Winds near the center increase to gale force, central pressure falls below 990 millibars, and towering cumulonimbus clouds shield a developing eye; a tropical storm has developed. Satellite photographs now reveal a tightly organized tropical cyclone, and reconnaissance reports indicate maxi￾mum winds of 80 knots around a central pressure of 980 millibars; a hurricane has developed. A ship to the right (left in the Southern Hemisphere) of the hurricane’s center (looking toward the direction of storm movement) reports 30-foot seas. The hurricane is rapidly maturing as it contin￾ues westward. A few days later the hurricane reaches its peak. The satellite photographs show a textbook picture, as 120-knot winds roar around a 940-millibar pressure center; hurri￾cane-force winds extend 50 miles in all directions, and seas are reported up to 40 feet. There is no further deepening now, but the hurricane begins to expand. In 2 days, gales extend out to 200 miles, and hurricane winds out to 75 miles. Then the hurricane slows and begins to recurve; this turning marks the beginning of its final phase. The hurricane accelerates, and, upon reaching temper￾ate latitudes, it begins to lose its tropical characteristics. The circulation continues to expand, but now cold air is in￾truding (cold air, cold water, dry air aloft, and land, aid in the decay of a tropical cyclone). The winds gradually abate Figure 3605. Cutaway view of a hurricane greatly exaggerated in vertical dimension. Actual hurricanes are less than 50,000 feet high and may have a diameter of several hundred miles

511TROPICALCYCLONESas theconcentrated stormdisintegrates.Thewarm core sur-Notall tropical cyclones followthis averagepattern.vivesforafewmoredaysbeforethetransformationtoaMostfalter in the early stages, somedissipate overland, andlargeextratropical low-pressuresystemiscompleteothers remainpotentforseveral weeks.Figure3606.Satellite photographof a hurricaneFORECASTINGANDPREDICTINGTROPICALCYCLONES3607.WeatherBroadcastsAndRadiofacsimileTheUS.Navy,theNational Oceanic andAtmosphericAdministration,and theU.S.AirForcehavedeveloped ahighly effective surveillance system for the tropical cy-The marine weather broadcast and radiofacsimileclone-proneareasoftheworld.Routineandspecialweatherweathermaps arethemost importanttoolsforavoidingtrop-reports (fromland stations,ships at sea, aircrat, weatherical cyclones.Thesebroadcasts, covering all tropical areas,provide information about the tropical cyclone's locationsatellite imagery,radar reports from land stations; specialmaximumwindsand seas,andfutureconditions expected.reportsfrom shipsat sea,andthespeciallyinstrumented

TROPICAL CYCLONES 511 as the concentrated storm disintegrates. The warm core sur￾vives for a few more days before the transformation to a large extratropical low-pressure system is complete. Not all tropical cyclones follow this average pattern. Most falter in the early stages, some dissipate over land, and others remain potent for several weeks. FORECASTING AND PREDICTING TROPICAL CYCLONES 3607. Weather Broadcasts And Radiofacsimile The marine weather broadcast and radiofacsimile weather maps are the most important tools for avoiding trop￾ical cyclones. These broadcasts, covering all tropical areas, provide information about the tropical cyclone’s location, maximum winds and seas, and future conditions expected. The U S. Navy, the National Oceanic and Atmospheric Administration, and the U.S. Air Force have developed a highly effective surveillance system for the tropical cy￾clone-prone areas of the world. Routine and special weather reports (from land stations, ships at sea, aircraft; weather satellite imagery; radar reports from land stations; special reports from ships at sea; and the specially instrumented Figure 3606. Satellite photograph of a hurricane

512TROPICALCYCLONESweather reconnaissance aircraft of National Oceanic andthe Naval Meteorology and Oceanography Command andAtmosphericAdministrationandtheU.S.AirForce)enablethe National Weather Service.accurate detection, location, and tracking of tropical cy-Although the areas of forecasting responsibility areclones. International cooperation is effective.Data buoys,fairly well defined for theDepartmentofDefense,the inter-both moored and drifting,provide another source ofnational and domestic civilian system provides manyinformation,overlaps and is dependent upon qualitativefactors.For ex-The tropical warning services have three principalample, when a tropical storm or hurricane is travelingfunctions:westward and crosses 35°w longitude, the continued issu-ance of forecasts and warnings to the general public,1Thecollection and analysis of the necessary obser-shipping interests, etc., becomes the responsibility of thevational data.National Hurricane Center of theNational Weather Service2.The preparation of timely and accurate forecastsat Miami,Florida.When a tropical storm or hurricanecrosses 35oW longitudetravelingfromwesttoeast, theNa-and warnings.tional Hurricane Center ceases to issue formal public3.The rapid and efficient distribution of advisories,advisories,but will issuemarinebulletins onany dangerouswarnings, and all other pertinent information.tropicalcycloneintheNorthAtlantic,ifitisofimportanceor constitutes a threat to shipping and other interests.TheseTo provide timely and accurate information andadvisories are included in National Weather Service Marinewarnings regarding tropical cyclones,the oceans haveBulletins broadcast to ships over radio station NAM Nor-been divided into overlapping geographical areas offolk,Virginia.Special advisoriesmay be issued atany time.responsibilityIntheAtlanticOcean,DepartmentofDefenseresponsibili-Fordetailed informationon theareasofresponsibilityofwith the Naval Atlantic Meteorologytyrestsandthe countries participating in the international forecastingOceanography Center in Norfolk, Virginiaand warning program, and radio aids, refer to SelectedWorldwideMarineWeatherBroadcasts,published jointlybyIntheeasternPacificeastoflongitude140°W,respon-NOAA/NATIONALHURRICANECENTER MARINEADVISORYNUM-BER13HURRICANE LADY 0400Z SEPTEMBER2119--HURRICANE WARNINGS ARE DISPLAYED FROM KEY LARGO TOCAPEKENNEDY.GALE WARNINGS AREDISPLAYEDFROM KEYWESTTOJACKSONVILLEANDFROMFLORIDABAYTOCEDARKEYHURRICANECENTERLOCATEDNEAR LATITUDE25.5NORTHLONGITUDE78.5WESTAT21/0400Z.POSITIONEXCELLENTAC-CURATEWITHIN1OMILESBASEDONAIRFORCERECONNAISSANCEANDSYNOPTICREPORTSPRESENTMOVEMENTTOWARDTHEWESTNORTHWESTOR285DEGREESAT10KT.MAXSUSTAINEDWINDSOF100KTNEARCENTERWITHGUSTS TO 160KTMAX WINDS OVER INLAND AREAS 35KTRADOF65KTWINDS90NE60SE80SW90NWQUADRADOF50KTWINDS120NE70SE90SW120NWQUADRADOF30KTWINDS210NE210SE210SW210NWQUADREPEAT CENTER LOCATED 25.5N 78.3W AT 21/0400Z.12HOURFORECAST VALID 21/1600Z LATITUDE 26.0N LONGI-TUDE80.5WMAXWINDSOF100KTNEARCENTERWITHGUSTSTO160KTMAX WINDS OVER INLANDAREAS 65KTRADIUSOF50KTWINDS120NE70SE90SW120NWQUAD24HOURFORECASTVALID22/0400ZLATITUDE26.0NLONGITUDE83.0WMAXWINDSOF75KTNEARCENTERWITHGUSTSTO120KT.MAXWINDS OVERINLANDAREAS 45KTRADIUSOF50KTWINDS120NE120SE120SW120NWQUAD.STORMTIDEOF9TO12FTSOUTHEASTFLACOASTGREATERMIAMI AREA TO THE PALM BEACHESNEXTADVISORYAT21/1000ZFigure3607.Exampleof marineadvisoryissuedbyNationalHurricaneCenter

512 TROPICAL CYCLONES weather reconnaissance aircraft of National Oceanic and Atmospheric Administration and the U.S. Air Force) enable accurate detection, location, and tracking of tropical cy￾clones. International cooperation is effective. Data buoys, both moored and drifting, provide another source of information. The tropical warning services have three principal functions: 1. The collection and analysis of the necessary obser￾vational data. 2. The preparation of timely and accurate forecasts and warnings. 3. The rapid and efficient distribution of advisories, warnings, and all other pertinent information. To provide timely and accurate information and warnings regarding tropical cyclones, the oceans have been divided into overlapping geographical areas of responsibility. For detailed information on the areas of responsibility of the countries participating in the international forecasting and warning program, and radio aids, refer to Selected Worldwide Marine Weather Broadcasts, published jointly by the Naval Meteorology and Oceanography Command and the National Weather Service. Although the areas of forecasting responsibility are fairly well defined for the Department of Defense, the inter￾national and domestic civilian system provides many overlaps and is dependent upon qualitative factors. For ex￾ample, when a tropical storm or hurricane is traveling westward and crosses 35°W longitude, the continued issu￾ance of forecasts and warnings to the general public, shipping interests, etc., becomes the responsibility of the National Hurricane Center of the National Weather Service at Miami, Florida. When a tropical storm or hurricane crosses 35°W longitude traveling from west to east, the Na￾tional Hurricane Center ceases to issue formal public advisories, but will issue marine bulletins on any dangerous tropical cyclone in the North Atlantic, if it is of importance or constitutes a threat to shipping and other interests. These advisories are included in National Weather Service Marine Bulletins broadcast to ships over radio station NAM Nor￾folk, Virginia. Special advisories may be issued at any time. In the Atlantic Ocean, Department of Defense responsibili￾ty rests with the Naval Atlantic Meteorology and Oceanography Center in Norfolk, Virginia. In the eastern Pacific east of longitude 140°W, respon￾NOAA/NATIONAL HURRICANE CENTER MARINE ADVISORY NUM￾BER 13 HURRICANE LADY 0400Z SEPTEMBER 21 19-. HURRICANE WARNINGS ARE DISPLAYED FROM KEY LARGO TO CAPE KENNEDY. GALE WARNINGS ARE DISPLAYED FROM KEY WEST TO JACKSONVILLE AND FROM FLORIDA BAY TO CEDAR KEY. HURRICANE CENTER LOCATED NEAR LATITUDE 25.5 NORTH LONGITUDE 78.5 WEST AT 21/0400Z. POSITION EXCELLENT AC￾CURATE WITHIN 10 MILES BASED ON AIR FORCE RECONNAISSANCE AND SYNOPTIC REPORTS. PRESENT MOVEMENT TOWARD THE WEST NORTHWEST OR 285 DEGREES AT 10 KT. MAX SUSTAINED WINDS OF 100 KT NEAR CENTER WITH GUSTS TO 160 KT. MAX WINDS OVER INLAND AREAS 35 KT. RAD OF 65 KT WINDS 90 NE 60 SE 80 SW 90 NW QUAD. RAD OF 50 KT WINDS 120 NE 70 SE 90 SW 120 NW QUAD. RAD OF 30 KT WINDS 210 NE 210 SE 210 SW 210 NW QUAD. REPEAT CENTER LOCATED 25.5N 78.3W AT 21/0400Z. 12 HOUR FORECAST VALID 21/1600Z LATITUDE 26.0N LONGI￾TUDE 80.5W. MAX WINDS OF 100 KT NEAR CENTER WITH GUSTS TO 160 KT. MAX WINDS OVER INLAND AREAS 65 KT. RADIUS OF 50 KT WINDS 120 NE 70 SE 90 SW 120 NW QUAD. 24 HOUR FORECAST VALID 22/0400Z LATITUDE 26.0N LONGITUDE 83.0W. MAX WINDS OF 75 KT NEAR CENTER WITH GUSTS TO 120 KT. MAX WINDS OVER INLAND AREAS 45 KT. RADIUS OF 50 KT WINDS 120 NE 120 SE 120 SW 120 NW QUAD. STORM TIDE OF 9 TO 12 FT SOUTHEAST FLA COAST GREATER MIAMI AREA TO THE PALM BEACHES. NEXT ADVISORY AT 21/1000Z. Figure 3607. Example of marine advisory issued by National Hurricane Center

513TROPICALCYCLONESsibility for the issuance of tropical storm and hurricaneThe Naval Pacific Meteorology and Oceanographyadvisories andwarnings for thegeneral public,merchantCenterCenter-West/Joint Typhoon WarningCenter(NPMOC-W/JTWC) in Guam is responsible for all U.S.shipping,andother interests rests with the National Weath-erServiceEasternPacificHurricaneCenter,SanFrancisco,tropical storm andtyphoon advisories and warnings fromCalifornia.TheDepartment of Defenseresponsibilityreststhe180thmeridianwestward tothemainland ofAsia.Awith the Naval Pacific Meteorology and Oceanographysecondary area ofresponsibility extends westward tolongi-Center,Pearl Harbor,Hawai.Formal advisories and warn-tude 90°E.Whenever a tropical cyclone is observed in theings are issued daily and are included in the marinewestern North Pacific area,serially numbered warnings,bulletinsbroadcastbyradiostationsKFS,NMC,andNMQbearing an“immediate"precedenceare broadcastfromtheNPMOC-W/JTWCat0000,0600,1200,and1800GMT.In the central Pacific(between themeridian and longi-Theresponsibilityforissuinggaleand storm warningstude 140°W),the civilian responsibility rests with thefortheIndianOcean,ArabianSea,BayofBengal,WesternNational Weather Service Central Pacific Hurricane Cen-Pacific,andSouthPacificrestswithmanycountries.Inter.HonoluluHawaii.DepartmentofDefensegeneral,warningsof approachingtropical cyclones whichresponsibilityrests withtheNaval Pacific Meteorology andmaybehazardous will includethefollowing informationOceanography Center in Pearl Harbor.Formal tropicalstormtype,central pressuregiven in millibars,wind speedstormand hurricane advisories and warnings areissued dai-observed withinthe storm,stormlocation,speedand direc-ly and areincluded in the marine bulletins broadcast bytion ofmovement, the extentofthe affected area, visibilityradiostationNMOandNRV.and the state of the sea, as well as any other pertinent infor-Tropical cyclone informationmessagesgenerally con-mationreceived.All stormwarningmessagescommencetain position ofthestorm,intensity,direction and speedofwith the international call sign TTTmovement, and a description of the area of strongwinds.Thesewarnings arebroadcastonspecifiedradiofrequencyAlso included is aforecastoffuturemovement and intensi-bands immediatelyupon receiptofthe information and at spety.When the storm is likely to affect any land area, detailscific intervalsthereafter,Generally,thebroadcast interval isonwhenand whereitwillbefeltand dataontides,rain.every6to8hours,dependingupon receiptofnew information.floods,and maximum winds are also included.Figure3607Bulletins and forecasts are excellent guides to thepresent and future behavior of the tropical cyclone, and aprovides anexampleofamarineadvisoryissued bytheNa-tional Hurricane Center.plotshouldbekeptofall positions.AVOIDINGTROPICALCYCLONES3608.ApproachAndPassageOfATropicalCyclonethe barometer starts a long,slowfall.Atfirst thefall is sogradual that it only appears toalter somewhat the normalAn early indication of theapproachof a tropical cy-dailycycle (twomaxima andtwominima intheTropics)Astherateoffall increases,thedailypattern is completelyclone isthepresenceof a longswell.Intheabsenceof alost in the more or less steady fall.tropical cyclone,thecrestsofswell inthedeepwatersoftheThe cirrus becomes more confused and tangled,andAtlantic pass at the rate ofperhaps eight per minute.Swellgenerated by a hurricane is about twice as long,the creststhengraduallygives waytoa continuous veil of cirrostratus.passingattherateofperhapsfourperminute.SwellmaybeBelowthis veil,altostratusforms,andthen stratocumulus.observed several days before arrival ofthe stormThese clouds graduallybecome more dense,and as they doWhen thestorm center is 500 to1.000milesaway,theso,theweatherbecomesunsettled.Afine,mist-likerainbebarometer usually rises a little, and the skies are relativelygins tofall,interrupted from time to timeby rain showers.clear.CumuluscloudsifpresentatallarefewinnumberThebarometerhasfallenperhapsatenthofaninchand their vertical development appears suppressed.The ba-Asthefall becomesmorerapid,thewind increases inrometer usually appears restless, pumping up and down agustiness,and its speed becomes greater, reaching perhapsfew hundredths of an inch.22 to 40 knots (Beaufort 6-8).On the horizon appears a darkAs thetropical cyclone comes nearer,acloud sequencewall of heavy cumulonimbus, called thebar ofthe stormbegins whichresemblesthatassociatedwiththeapproachThis is the heavy bank ofclouds comprising the main massofawarm frontinmiddlelatitudes.Snow-white,fibrousof thecvclone.Portionsof thisheavycloud becomede-“mare's tails"(cirrus)appear when the storm is about 300tached from time to time, and drift across the skyto600miles away.Usuallythese seemto converge,moreaccompanied by rain squalls and wind of increasing speedor less, in thedirection from which the storm is approachBetween squalls, the cirrostratus can be seen throughing.This convergence is particularlyapparent at about thebreaks in the stratocumulus.time of sunrise and sunset.As the bar approaches, thebarometerfalls more rapidlyShortlyafterthecirrus appears, but sometimesbefore,and wind speed increases.The seas, which have been gradu-

TROPICAL CYCLONES 513 sibility for the issuance of tropical storm and hurricane advisories and warnings for the general public, merchant shipping, and other interests rests with the National Weath￾er Service Eastern Pacific Hurricane Center, San Francisco, California. The Department of Defense responsibility rests with the Naval Pacific Meteorology and Oceanography Center, Pearl Harbor, Hawaii. Formal advisories and warn￾ings are issued daily and are included in the marine bulletins broadcast by radio stations KFS, NMC, and NMQ. In the central Pacific (between the meridian and longi￾tude 140°W), the civilian responsibility rests with the National Weather Service Central Pacific Hurricane Cen￾ter, Honolulu, Hawaii. Department of Defense responsibility rests with the Naval Pacific Meteorology and Oceanography Center in Pearl Harbor. Formal tropical storm and hurricane advisories and warnings are issued dai￾ly and are included in the marine bulletins broadcast by radio station NMO and NRV. Tropical cyclone information messages generally con￾tain position of the storm, intensity, direction and speed of movement, and a description of the area of strong winds. Also included is a forecast of future movement and intensi￾ty. When the storm is likely to affect any land area, details on when and where it will be felt, and data on tides, rain, floods, and maximum winds are also included. Figure 3607 provides an example of a marine advisory issued by the Na￾tional Hurricane Center. The Naval Pacific Meteorology and Oceanography Center Center-West/Joint Typhoon Warning Center (NP￾MOC-W/JTWC) in Guam is responsible for all U.S. tropical storm and typhoon advisories and warnings from the 180th meridian westward to the mainland of Asia. A secondary area of responsibility extends westward to longi￾tude 90°E. Whenever a tropical cyclone is observed in the western North Pacific area, serially numbered warnings, bearing an “immediate” precedence are broadcast from the NPMOC-W/JTWC at 0000, 0600, 1200, and 1800 GMT. The responsibility for issuing gale and storm warnings for the Indian Ocean, Arabian Sea, Bay of Bengal, Western Pacific, and South Pacific rests with many countries. In general, warnings of approaching tropical cyclones which may be hazardous will include the following information: storm type, central pressure given in millibars, wind speed observed within the storm, storm location, speed and direc￾tion of movement, the extent of the affected area, visibility, and the state of the sea, as well as any other pertinent infor￾mation received. All storm warning messages commence with the international call sign “TTT.” These warnings are broadcast on specified radio frequency bands immediately upon receipt of the information and at spe￾cific intervals thereafter. Generally, the broadcast interval is every 6 to 8 hours, depending upon receipt of new information. Bulletins and forecasts are excellent guides to the present and future behavior of the tropical cyclone, and a plot should be kept of all positions. AVOIDING TROPICAL CYCLONES 3608. Approach And Passage Of A Tropical Cyclone An early indication of the approach of a tropical cy￾clone is the presence of a long swell. In the absence of a tropical cyclone, the crests of swell in the deep waters of the Atlantic pass at the rate of perhaps eight per minute. Swell generated by a hurricane is about twice as long, the crests passing at the rate of perhaps four per minute. Swell may be observed several days before arrival of the storm. When the storm center is 500 to 1,000 miles away, the barometer usually rises a little, and the skies are relatively clear. Cumulus clouds, if present at all, are few in number and their vertical development appears suppressed. The ba￾rometer usually appears restless, pumping up and down a few hundredths of an inch. As the tropical cyclone comes nearer, a cloud sequence begins which resembles that associated with the approach of a warm front in middle latitudes. Snow-white, fibrous “mare’s tails” (cirrus) appear when the storm is about 300 to 600 miles away. Usually these seem to converge, more or less, in the direction from which the storm is approach￾ing. This convergence is particularly apparent at about the time of sunrise and sunset. Shortly after the cirrus appears, but sometimes before, the barometer starts a long, slow fall. At first the fall is so gradual that it only appears to alter somewhat the normal daily cycle (two maxima and two minima in the Tropics). As the rate of fall increases, the daily pattern is completely lost in the more or less steady fall. The cirrus becomes more confused and tangled, and then gradually gives way to a continuous veil of cirrostratus. Below this veil, altostratus forms, and then stratocumulus. These clouds gradually become more dense, and as they do so, the weather becomes unsettled. A fine, mist-like rain be￾gins to fall, interrupted from time to time by rain showers. The barometer has fallen perhaps a tenth of an inch. As the fall becomes more rapid, the wind increases in gustiness, and its speed becomes greater, reaching perhaps 22 to 40 knots (Beaufort 6-8). On the horizon appears a dark wall of heavy cumulonimbus, called the bar of the storm. This is the heavy bank of clouds comprising the main mass of the cyclone. Portions of this heavy cloud become de￾tached from time to time, and drift across the sky, accompanied by rain squalls and wind of increasing speed. Between squalls, the cirrostratus can be seen through breaks in the stratocumulus. As the bar approaches, the barometer falls more rapidly and wind speed increases. The seas, which have been gradu-

514TROPICALCYCLONESATOPQEHGHESTCLOUDTOWEALTOSTRATUSFOOWnFCUNMATOECLOUDTOTAAMFofFigure3608.Typical hurricanecloud formationsTypical cloud formations associated with a hurricaneally mounting, become tempestuous.Squall lines, one afterthe other,sweeppast in ever increasing number and intensity.areshowninFigure3608With the arrival of the bar,the day becomes very dark,3609.Locating The Center OfATropical Cyclonesqualls become virtually continuous,and the barometerfallsprecipitously,witharapidincreaseinwindspeed.Thecentermaystillbe100to200milesawayinafullydevel-If intelligent action is to be taken to avoid thefull furyoped tropical cyclone.Asthe center of the storm comesofatropical cyclone,earlydeterminationofitslocation andcloser, the ever-stronger wind shrieks through the rigging.directionoftravelrelativetothevesselisessential.Thebul-andaboutthesuperstructureofthevessel.Asthecenterap-letins and forecasts are an excellentgeneral guide, but theyproaches, rain falls in torrents. The wind fury increases. Theare not infallible,and maybe sufficiently in error to induceseas become mountainous. The tops of huge waves area mariner ina critical positionto alter course so as to unwit-blownofftominglewiththerainandfilltheairwithwatertinglyincreasethedangertohisvessel.OftenitispossibleVisibility is virtuallyzero in blinding rain and spray.Evenusing onlythoseobservationsmadeaboard ship,toobtainathelargest and most seaworthyvesselsbecome virtually un-sufficientlycloseapproximationtoenablethevessel toma-manageable,and may sustain heavy damage.Less sturdyneuver to the best advantagevessels may not survive.Navigation virtually stops as safe-Thepresence of an exceptionallylong swell is usuallyty of the vessel becomes the only consideration.Thethe firstvisible indication of theexistence ofatropical cy-awesomefury of this condition can only beexperiencedclone.In deep water it approachesfrom the generalWords are inadequateto describe it.direction of origin (theposition of the storm center whenIfthe eyeofthe storm passes over thevessel, the windsthe swell was generated).However, in shoaling water thisis a less reliable indication because thedirection is changedsuddenly drop to a breeze as the wall of the eye passes. Thebyrefraction,thecrestsbeing morenearlyparallel totherain stops,and the skies clear sufficientlyto permit the sunor stars to shinethrough holes in the comparatively thinbottom contours.cloud cover. Visibility improves.Mountainous seas ap-When the cirrus clouds appear,their point of conver-proachfromall sides incompleteconfusion.Thebarometergenceprovides an indication ofthedirection of thestormreaches its lowestpoint,whichmaybe1/2or 2 inches be-center.If the storm is topass well toone sideoftheobserv-low normal in fullydevelopedtropical cyclones.As theer,thepointofconvergence shiftsslowlyinthedirectionofwall on the opposite sideof the eye arrives,thefull fury ofstormmovement.If the storm center will pass neartheob-the wind strikes as suddenly as it ceased, but from the op-server,this point remains steady.When the bar becomesposite direction.The sequence of conditions that occurredvisible,itappearstorest upon thehorizonfor several hoursduringapproachofthe stormis reversed, andpassesmoreThe darkest part of this cloud is in the direction ofthe stormquickly,asthevariousparts ofthestormarenotas wide incenter.Ifthestorm istopassto oneside,thebar appears totherearofastormasonitsforwardsidedrift slowly along the horizon.If the storm is heading di-

514 TROPICAL CYCLONES ally mounting, become tempestuous. Squall lines, one after the other, sweep past in ever increasing number and intensity. With the arrival of the bar, the day becomes very dark, squalls become virtually continuous, and the barometer falls precipitously, with a rapid increase in wind speed. The center may still be 100 to 200 miles away in a fully devel￾oped tropical cyclone. As the center of the storm comes closer, the ever-stronger wind shrieks through the rigging, and about the superstructure of the vessel. As the center ap￾proaches, rain falls in torrents. The wind fury increases. The seas become mountainous. The tops of huge waves are blown off to mingle with the rain and fill the air with water. Visibility is virtually zero in blinding rain and spray. Even the largest and most seaworthy vessels become virtually un￾manageable, and may sustain heavy damage. Less sturdy vessels may not survive. Navigation virtually stops as safe￾ty of the vessel becomes the only consideration. The awesome fury of this condition can only be experienced. Words are inadequate to describe it. If the eye of the storm passes over the vessel, the winds suddenly drop to a breeze as the wall of the eye passes. The rain stops, and the skies clear sufficiently to permit the sun or stars to shine through holes in the comparatively thin cloud cover. Visibility improves. Mountainous seas ap￾proach from all sides in complete confusion. The barometer reaches its lowest point, which may be 1 1/2 or 2 inches be￾low normal in fully developed tropical cyclones. As the wall on the opposite side of the eye arrives, the full fury of the wind strikes as suddenly as it ceased, but from the op￾posite direction. The sequence of conditions that occurred during approach of the storm is reversed, and passes more quickly, as the various parts of the storm are not as wide in the rear of a storm as on its forward side. Typical cloud formations associated with a hurricane are shown in Figure 3608. 3609. Locating The Center Of A Tropical Cyclone If intelligent action is to be taken to avoid the full fury of a tropical cyclone, early determination of its location and direction of travel relative to the vessel is essential. The bul￾letins and forecasts are an excellent general guide, but they are not infallible, and may be sufficiently in error to induce a mariner in a critical position to alter course so as to unwit￾tingly increase the danger to his vessel. Often it is possible, using only those observations made aboard ship, to obtain a sufficiently close approximation to enable the vessel to ma￾neuver to the best advantage. The presence of an exceptionally long swell is usually the first visible indication of the existence of a tropical cy￾clone. In deep water it approaches from the general direction of origin (the position of the storm center when the swell was generated). However, in shoaling water this is a less reliable indication because the direction is changed by refraction, the crests being more nearly parallel to the bottom contours. When the cirrus clouds appear, their point of conver￾gence provides an indication of the direction of the storm center. If the storm is to pass well to one side of the observ￾er, the point of convergence shifts slowly in the direction of storm movement. If the storm center will pass near the ob￾server, this point remains steady. When the bar becomes visible, it appears to rest upon the horizon for several hours. The darkest part of this cloud is in the direction of the storm center. If the storm is to pass to one side, the bar appears to drift slowly along the horizon. If the storm is heading di￾Figure 3608. Typical hurricane cloud formations