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 increase again, but come from the opposite direction because of the cyclonic circulation of the storm. This sudden transformation 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 hurricane 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 orderly 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 November 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 pressure in the region has fallen more than 5 millibars in the past 24 hours. This is cause for alarm, because in the Tropics pressure varies little; the normal diurnal pressure change is only about 3 millibars. Satellite pictures may indicate thickening middle and high clouds. Squalls are reported ahead of the easterly wave, and wind reports indicate 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 classified 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 maximum 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 continues 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; hurricane-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 temperate latitudes, it begins to lose its tropical characteristics. The circulation continues to expand, but now cold air is intruding (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 survives 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 tropical 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 cyclone-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