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fini"

obody who lived in southern Florida during the August 1992 passage

of Hurricane Andrew (Figure 12-l ) will ever forget the experience.

Andrew was fairly small but remarkably powerful, with wind gusts of

up to 280 km/hr (1 75 mph) that moved rapidly across the peninsula. ln one

regard Andrew was not as destructive as many other hurricanes; although it did

cause some local flooding, it dìd not yield extremely heavy rainfall. This con-

trasts strongly with Florida's experience with Hurricane lrene in October 1999.

lrene was typical of the strong tropical storms that form in the latter part of

hurricane season: large and slow moving. Although these storms lack the wind

speeds of storms such as Andrew, they can bring heavy rainfall for days and

cause extensive flooding. ln the case of lrene, much of southern Florida received

up to 27 cm (17 in.) of rainfall that produced widespread flooding. Nadia Gorriz of

Miami-Dade County was among the many victims. The floodwaters that created

a swamp around her house made it uninhabitable for humans but perfectly fine

for the snakes and fish that took up residence. She put the cleanup in perspective:

"We made it through Andrew. You just threw everything out and there wasn't

that much water. This is the worst cleanup. We've gone through two gallons of

Clorox ll. So far."

Hurricanes do not restrict their fury to coastal and inland regions; they

have been the nemesis of mariners for centuries. They have sunk an untold

number of ships and even played a role in World War ll when a single typhoon

(the equivalent of a hurricane over the western Pacifìc) sank or heavily damaged

several U.S. ships, destroyed hundreds of carrier-based aircraft, and killed

more than 800 sailors. The death toll exceeded that of most naval battles dur-

ing the war.

ln this chapter we first describe the setting for hurricanes and tropical

storms. We then describe their general characteristics, stages of development,

and typical patterns of movement, concluding with hurricane monitoring and

warning systems.

^ Modified satellite image showing the progression of Hurricane Andrew

as it crossed Florida and entered the Gulf of Mexico in August .1 992.

* h§ A$?T,ffi ffi #[F"]' 6._ F il.j E

372 CHAPTER 12 Tropical Storms and Hurricanes

Hurricanes Around the Globe:The Tropical SettingExtremely strong tropical storms go by a number ofdifferent names, depending on where they occur. Overthe Atlantic and the eastern Pacific they are known ashurricanes. Those over the extreme western Pacific arecalled typhoons; those over the Indian Ocean andAustralia, simply cyclones. In structure, the threekinds of storms are essentially the same, althoughtyphoons tend to be larger and stronger than the oth-ers. We will use the term hurricane for the general classof storm, regardless of location.

Most U.S. residents associate hurricanes with stormsthat form in the Atlantic Ocean or the Gulf of Mexico. Yetother parts of the world have a much greater incidenceof hurricanes (Table 72-l and Figure 12-2).The Atlantic

and Gulf of Mexico receive an average of 5.9 hurricane.each year, while the eastem North Pacific off the coast o:Mexico has an average of 9.0. Most tropical storms in theeast Pacific move westward, away from population cer.-ters, and so they receive little public attention. Some-times, however, they migrate to the northeast and brin:severe flooding and loss of life to western Mexico.

The region having the greatest number of theseevents-by far-is the western part of the North Pacificln a typical year, nearly 17 typhoons hit the region. Everduring the least active season between 1968 and 200ithere were 9 typhoons. At the other extreme, no hurri-canes form in the Southern Hemisphere Atlantic, everat tropical latitudes (except for a very unusual event ofthe coast of Brazil in 2004). As you will see late1, hurri-canes depend on a large pool of warm watet a condi-tion that does not arise in the relatively small Soutl^Atlantic basin.

Basin

Atlantic

NortheastPacific

NorthwestPacific

Northern lndianOcean

Southwestlndian Ocean

Least Average

4 10.6(1e83)

8 16.3(1e77)

17 26.7(1ee8)

2 5.4(1ee3)

7 13.3(1e83)

1

(1e88)

4(1ee1)

39(1e77)

19(1ee1)

68(1977)

10.6

58.7

29.0

87.7

Most'19

(1ees)

27(1ee2)

35(1e71)

11(1ee2)

18(1se7)

Tropical Storm (greater than17 mls sustained winds)

Hurricane/Typhoon/SevereTropical Cyclone (greater than33 m/s sustained winds)

Most Least Average

12 2 5.9(1e6e) (1e82)

16 4 (1969, 9.0(1990) 1e77)

24 I 16.9(1e71) (1ee8)

502.2(1998) (several)

11 2 6.7(1971 , (1974,1994, 1998)1997)

8 (1973,1 e85)

12(1ee8)

47(1e71)

22(1ee7)

64(1eeo)

0(several)

0(1e71)

24(1e77)

7(1e6e)

36(1e77)

33.7

14.5

48.3

lntense Hurricane (greater than50 m/s sustained winds)

Most Least Average

602.0(1996 and (several)2004)

10 o 4.1(1992) (several)

14 3 8.5(1e72) (1e74)

300.4(1999) (several)

602.7(1980, (several)1 994,2002)

5 (1e80,1981)

0(several)

0(several)

235(1ee2) (1e77)

1 1 (1992, O

1e94, (196e,2003) 1974)

347(1ee2) (1e77)

5.9

21.0

Australia/South- 13eastern lndian (1982)Ocean

Australia/South-westernPacific

NorthernHemisphere

SouthernHemisphere

Globally

18(1ee8)

76(1ee2)

38(1ee7)

106(1ee2)

b(2003)

15.1

( lt,.

\\ ,:,

90 120 150 180' 150>::--S<-__,,_.-

Hurricane Characteristics 373

AFR I CA

I

:t.l _-l_

AFRICA,

,0

Tropic of Cancer

PACIFIC

OCEAN

( 'r'r-I ! _/ /\ E:.i: \_

-.

, ' '-l'

January-March SOUTH

AMERICA

INDIAN

OCEAN

) AUSTRALIAiài'lili,lrti;i8i Tropic of Capricorn

... Typhoons,cyclones,and hurricanes

[-l Hurri""n".on"t,-' .t'0 1,500 3,000 Miles

_r--.-]--0 1,500 3,000 KilometeE

.'.;,,

ANTARCTICA

^ FIGUHE 12-z Hurricanes around the globe.

In Chapter 8 we saw that during much of the yearair spirals out of massive high-pressure cells thatoccupy large parts of the Atlantic and Pacific Oceans.

Middle- and upper-level air along the eastern side ofthese anticyclones sinks as it approaches the westcoasts of the adjacent continents. Because the air does

not descend all the way to the surface, a subsidenceinversion (see Chapter 6) forms above the surface. This

particular subsidence inversion is called the tradewind inversion. The air below the inversion, called themarine layer, is cool and relatively moist.

The thickness of the marine layer and the height ofthe inversion base vary across the tropical oceans. The

inversion is lowest along the eastern margins of the

oceans, where upwelling and cold ocean currents main-tain a relatively cool marine layer. Here the inversionmay be only a few hundred meters above the surface'

Farther to the west, the warmer surface waters heat the

marine layer and cause it to expand to a greater height.

Over the eastem part of the oceans, the low inversioninhibits vertical cloud growth, and low stratus clouds

often occupy the region. Farther to the west, the greaterheight of the inversion (or even its total disappearance)allows for more convection, and deep cumulus clouds

are more likely to form. For this reasory more hurricanesoccur along the western portion of the ocean basins'

H u rricane CharacteristicsHurricanes are the most powerful of all storms. The

energy urùeashed by just a single hurricane can exceed

the annual electrical consumption of the United States

and Canada. By definition, hurricanes have sustained

I- ATLANTIC

OCEAN

60

wind speeds of 120 km/hr (7a mph) or greater' Though

their wind speeds are less than those of tomadoes, hurri-canes are very much larger and have far longer lifespans.

Sea level pressure near the center of a §pical hurricane is

around 950 mb, but pressures as low as 870 mb have been

observed for extremely powerful hurricanes. The weak-est hurricanes have central Pressures of about 990 mb.

In contrast to tornadoes, whose diameters are typi-cally measured in tens of meters, hurricanes are

typically about 600 kilometers (350 mi) wide. Thus,

the typical hurricane has a diameter thousands oftimes greater than that of a tornado. Rememberingthat the area of a circle is proportional to the square ofits radius, and knowing that tornadoes and hurri-canes are roughly circular, we see that the area cov-ered by a hurricane is likely to be millions of timesgreater than the area covered by a tornado. Further-more, a tornado exists only for a couple of hours at

most, while a hurricane can have a lifetime of several

days or even a week or more.Though hurricanes are usually about one-third the

size of midlatitude cyclones, the pressure difference

across a hurricane is about twice as great' They therefore

have extreme horizontal pressure gradients that generate

powerful winds: Average hurricanes have peak windsof about 150 km/hr (90 mph), and the most intense hur-ricanes have winds up to 350 km/hr (210 mph). Inaddition to being smaller and more powerful than mid-latitude cyclones, hurricanes differ by not having the

fronts that characterize cyclones outside the tropics.Because hurricanes obtain most of their energy from

the latent heat released by condensation, they are most

common where a deep layer of warm water fuels them.

Given that tropical oceans have their highest surface

60 ;.§i!ì: i-*

June-October

374 CHAPTER 12 Tropical StormsandHurricanes

temperatures and evaporation rates in late summer andearly fall, it is not surprising thatAugust and Septemberare the prime hurricane months in the Northern Hemi-sphere, with January to March the main season in theSouthem Hemisphere.l

Hurricane Structure

";;;;"; ;; ;;;.;;;;;;;;

""', ;"; ;;;;; ;;";;:_tive cell. Instead they contain a large number of thun-derstorms arranged in a pinwheel formation, withbands of thick clouds and heavy thundershowers spi-raling counterclockwise (in the Northern Hemisphere)around the storm center (Figure 12-3). The bands ofheavy convection are separated by areas of weakeruplift and even descending air and less intense precip-itation. The wind speed and the intensity of precipita-tion both increase toward the center of the system (itseye), reaching a maximum 10 to 20 km (6 to 12 mi)away from the center, at what is called tlne eye wall (theeye and eye wall are described in the next section).

Figure 724 depicts generalized cross sectionsthrough a hurricane. The thickness of the cloud bands (a)corresponds well to the intens§ of rainfall @). On theother hand, the distributions of pressure (c) and windspeed (d) do not exhibit a similar banding. Both the pres-sure gradient and wind speed increase gradually towardthe center of the storm and then increase rapidly in thevicinity of the eye wall.

Hurricanes also differ from midlatitude cyclonesin that they are warm-core cyclones. As air flows

lThe United States National Hurricane Center defines the hurricaneseason as the period from June 1 to November 30. Tiopical storms inother months are rare events-from 7871, to 7996, only six stormsformed in December.

> Fi§URE 12-3 Across-sectionof a typical hurricane.

inward toward lower pressure, the warm ocean sur-face supplies large amounts of latent and sensibleheat to the overlying air. Because pressure within themoving air decreases as it flows toward the low, adia-batic expansion keeps the temperature from increas-ing dramatically, with the result that there is littletemperature difference across the base of the storm.Nevertheless, much thermal energy is added, result-ing in a "warm" central core. Aloft, after condensationand the release of latent heat, the warmth is reflectecìin temperature, so that temperatures near the centerare much higher than those of the surrounding air(Figure 12-5).

As a warm-core low, pressure within a hurricanedecreases relatively slowly with increasing altitude(see Chapter 10). Thus, the horizontal pressure gradi-ent within the storm gradually decreases with alti-tude. At about7.5 km (25,000 ft)-about the 400 mblevel-the air pressure is the same as that immedi-ately outside the storm. From this height to the lowerstratosphere, the hurricane has relatively high pres-sure. So unlike the lower part of the hurricane inwhich the air rotates cyclonically, the air in its upperportion spirals anticyclonically from its center (clock-wise in the Northern Hemisphere). Figure 12-6 shou-sa schematic of how surface winds typically florrinward, then rapidly rotate around the eye wall asthey ascend to higher levels, and then spiral outward.

In the upper reaches of the storm, the low tempera-tures cause water droplets to freeze into ice crystals. Asthe ice crystals spiral out of the storm center, they createa blanket of cirrostratus clouds that overlies andobscures the pinwheel-like structure of the storm. Thatexplains whyhurricanes on satellite images often appearto have a uniform thickness and intensity, when in factthey are strongly banded.

(

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rensthorthearrows

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Distance from center (km)

RAINFALL

Strongest a Strongest

Pressure-\ / -PressurePRESSUBE gradient \_/ gradìent

wlND SPEED .,'\ r Winds stronoer

f1 / \ \ on risht side lo;kinsin direction hurricane

is moving

Temoerature difference in oC

+10 , ir +11 !o +e

+5

+4

376 cFrAr:ttritr .iil Tropical Storms and Hurricanes

flcLil?H JÉ*6 Air trajectories initially rise graduallyas they approach a hurricane eye wall. Most rapid ascentoccurs in the eye wall until the air reaches the uppertroposphere and flows outward anticyclonically.

THE EYE AND THE EYE WALL

One of the rnost distinctive characteristics of a hurri-cane is its eye, a region of relatively clear skies, slowlydescending air, and light winds. Hurricane eyes averageabout 30 km (20 mi) in diameter, with most rangingfrom 20 to 50 km (15 to 30 mi). Eye diameters vary con-siderably among individual storms, with some as smallas 6 km (3.5 mi) and others almost as large as 100 km(60 mi). The change in the size of an eye through timegives some indication of whether the hurricane isintensifying or weakening. Generally, a shrinking eyeindicates an intensifying hurricane.

Along the margin of the eye lies the eye wall, thezone of most intense storm activity. The eye wall con-tains the strongest winds, thickest cloud cover, andmost intense precipitation of the entire hurricane.Directly beneath the eye wall, rainfall rates of 2500mm/day (100 inlday) are not uncommon. The abrupttransition from an eye wall to the eye causes a strongand rapid change in weather. Imagine a hurricane aboutto make a direct hit on a small island. As the hurricanecenter approaches the island, the intensity of the windand rain steadily increase, becoming most intense as theeye wall arrives. But when the eye reaches the island,the storm seems to suddenly dissipate, as blue skies andcalm conditions take hold. Of course, the storm has notdissipated at all. Rathel, there is just a brief lull until theopposite side of the eye wall covers the island andstorm conditions resume. Because the average hurri-cane eye is about 30 km (18 mi) in diameter and travelsat about 20 km/hr, (12 mph) the calm associated with

DID YCDU KNOW?Although hurricane wind speeds are, by definition,extremely fast, it still takes a parcel of air an averageof 8 days to flow into and out of a hurricane.

passage of the eye lasts about an hour or two. Clearly, iithe eye just grazes the island, the break in the storm willbe even shorter.

Some intense hurricanes develop double eye walls.as surrounding rain bands contract and intensify. As theinterior eye wall contracts, it can begin to dissipate andthe surrounding band can take its place. Figure 12-lshows this process occurring in Hurricane Emily in2005, with the outer band of rainfall (shown as greenarcs embedded with red areas of heavier activitr-rsurrounding the small eye wall in the center. The exis-tence of double eye walls often indicates a hurricane isachieving its maximum strength.

NASA scientists have recently uncovered the exis-tence of hot towers (Figure 12-8) embedded in someeye walls that last between 30 minutes and 2 hours.Hot towers are localized portions of eye walls that riseto greater heights (up to 72 km, or 7 mi) than the restof the eye wall. The researchers found that develop-ment of hot towers indicates a greater likelihood thatthe hurricane will intensify within the next 6 hours.

The air temperature at the storm's surface withinan eye is several degrees warmer than outside the evebecause compression of the sinking air causes it iowarm adiabatically. The air is also drier, becausewarming the unsaturated air lowers its relatir.ehumidity. Contrary to common belief, however, theair is not entirely cloud free within the eye; instead,fair-weather cumulus clouds are scattered through-out the otherwise blue sky.

Hurricane FormationHurricanes do not suddenly appear out of nowhere;they begin as much weaker systems that often migratelarge distances before turning into hurricanes. In thissection we examine hurricane development, with par-ticular emphasis on how it occurs in the Atlantic.

STEPS IN THE FORMATIONOF HURRICANES

Although most tropical storms attain hurricane statusin the western portions of the oceans, their earliest ori-gins often lie far to the east as small clusters of smallthunderstorms called tropical disturbances. Tiopicaldisturbances are disorganized groups of thunder-storms having weak pressure gradients and little or norotation.

j

HurricaneFormation 377

< F§*iJÈÌH 12-f Some hurricanesdevelop double eye walls, such as

Hurricane Emily in 2005. UsuallY

occurring as the hurricane achievesmaximum strength, both eYe wallscontract, with the innermost eye wall

eventually dissipating and giving wayto the outer eye wall.

< FISLJ[AH i**$ This hottower in Hurricane Rita in 2005was observed by NASAsTropical Rainfall MeasuringMission fl-RMM) sPacecraft.These localized areas of deePcloud cover and intenseprecipitation often Precedehurricane intensification.

j tà,

*' t"td-

Tropical disturbances can form in several differentenvironments. Some form when midlatitude troughsmigrate into the tropics; others develop as part of the

normal convection associated with the ITCZ. But most

tropical disturbances that enter the western Atlanticand become hurricanes originate in easterly waves,

large undulations or ripples in the normal trade windpattern. Figure 12-9 illustrates a typical easterly wave.

B"crrre pressure gradients in the tropics are normally

-t

tr

{iI

weaker than those of the extratropical regions, the east-

erly waves are better shown by plotting lines of winddirection (called streamlines) rather than isobars' The airin the wave initially flows westward, turns poleward,and then flows back toward the equator, u'ith the entire

wave pattern extending 2000 to 3000 km (1200 to 1800

mi) in length. On the upwind (eastern) side of the axis,

the streamlines become progressively closer together,

indicating that surface motions are convergent. With

378 CHAPTEH 12 Tropical Storms and Hurricanes

40'

NIllwdFEIIs

#\§$1"4 \b " -\'---'-r" < qr

ATLANTICOCEAN Tropical

disturbance

0 500 .1,000 Mites

500 1.000 KilomètéB

7?"-'-

A FIGURE 12-9 Easterly waves have surface convergence and cloud cover east of the axis and divergence to the west.

convergence there is rising motion (see Chapter 6); thusthe tropical disturbance is located upwind of the waveaxis (the dashed line) of the easterly wave. Surfacedivergence downwind of the wave axis leads to clearskies. (An explanation for why the streamlines are con-vergent and divergent is somewhat complicated; themain factor involves changes in relative vorticity thatoccur as the air moves poleward and equatorward.)

The tropical disturbances that affect the AtlanticOcearl Caribbean, and the Americas mostly form overwestem Africa, south of the Saharan desert. Being in thezone of the trade winds, these storms tend to migratewestward. When they reach the west coast of Africa,they weaken as they pass over the cold Canary currentover the eastem Atlantic. There the low water tempera-tures chill the air near the water surface and cause the airto become statically stable. If the disturbances migratebeyond the coastal zone of surface upwelling, howevel,warmer waters farther offshore raise the temperafureand humidity of the lower atmosphere and cause the airto become unstable. Then, as the storms continue west-ward, a small percentage develop into more intense andorganized thunderstorm systems. Easterly waves movewestward at about 15 to 35 km/hr (10 to 20 mph), and soit typically takes about a week to 10 days for an embed-ded tropical disturbance to migrate across the Atlantic.

The vast majority (probably more than 90 percent) oftropical disturbances die out without ever organizinginto more powerful systems. But some undergo a lower-ing of pressure and begin to rotate cyclonically. IrVhen atropical disturbance develops to the point where there is

at least one closed isobar on a weather map, the distur-bance is classified as a tropical depression. If the depres-sion intensifies further and maintains wind speed=above 60 krn/fu (37 mph), it becomes a tropical sto*r(At this point the system is named. See Box 12-1, Speci.;..Interest: Naming Hurricanes, for more information on thl.practice.) A further increase in sustained wind speeds tr120k;rn/fu (7amph) creates a true hurricane. lA/hile onÌ,.a small fraction of tropical disturbances ever becom.tropical depressions, a Larger proportion of depressior-.become tropical storms, and an even greater percentaqÉof tropical storms ultimately become hurricanes.

The location at which hurricanes are most likelr, tcform varies seasonally. Early in the Atlantic r"uro..weak fronts in the western ocean extend southn arcover warm tropical water. Wind shear across the fron'sprovides the circulation necessary for cyclone develop-ment. Later in the seasory fronts are confined to highe:latitudes and no longer play a role in cyclogenèsisInstead, warm waters are found progressively farther tithe east, so that disturbances leaving the African cont-nent can grow into full-scale cyclones. (Systems tha:become tropical storms in the tropical waters just o=westem Africa and become hurricanes before reachh:the Caribbean are often referred to as Cape Verde lttn.r:-canesl so named for the islands near which they orig-nate.) The net effect is that the birthplace of tropiia-cyclones moves from west to east across the tropica_ocean during the first half of the season. Lr the late fa[the breeding ground moves westward as frontal actir.ii.again emerges as a primary agent of cyclone genesis.

i.iro-

As with their Atlantic counterparts, Pacific hurri-canes move westward during their formative stages.

Many come near Hawaii, but most bypass the islandsor die out before reaching them. Unfortunately, this isnot always the case. In September 1992, HurricaneIniki battered the island of Kauai with wind gusts upto 258 km/hr (160 mph) and brought heavy floodingto the beach resort areas. The hurricane destroyed orseverely damaged half of the homes on the island anddevastated most of the tourist industry.

CONDITIONS NECESSARYFOR HURRICANE FORMATION

Aithough the dynamics of hurricanes are extremely com-

plex, meteorologists have long recognized the conditiorsthat favor their development. Great amounts of heat are

needed to fueI hurricanes, and the primary source of this

energy is the release of latent heat supplied by evaporation

from the ocean surface. Because high evaporation rates

depend on the presence of warm watel hurricanes formonly where the ocean has a deep surface layer (several tens

of meters in depth) with temperatures above 27"C (81'F).

The need for warm water precludes hurricane formationpoleward of about 20 degrees; sea surface temperatures are

usually too low there. Hurricanes develop most often in late

sununer and early fall, when tropical waters are warmest.Hurricane formation also depends on the Coriolis

force, which must be strong enough to prevent filling ofthe central low pressure. The absence of a Corioliseffect at the equator prohibits hurricane formationbetween 0o and 5" latitude. This factor and the need forhigh water temperatures explain the pattern shown inFigure 12-2, in which tropical storms attain hurricanestatus between the latitudes of 5o and 20o.

Stability is also important in hurricane development,with unstable conditions throughout the troposphere an

absolute necessity. Along the eastem margins of the

oceans, cold currents and upwelling cause the lower tro-posphere to be statically stable, inhibiting uplift. More-over, the trade wind inversion puts a caP on any mixingthat might otherwise occur. Moving westward, watertemperatures §pically increase and the trade wind inver-sion increases with height or disappears altogethel, andso hurricanes become more prevalent. Finally, hurricaneformation requires an absence of strong vertical windshear, which disrupts the vertical transport of latent heat.

Once formed, hurricanes are self-propagating (just as

severe storms outside the tropics are self-maintaining).That is, the release of latent heat within the cumulusclouds causes the air to warm and expand upward. The

expansion of the air supports upper-level divergence,

which draws air upward and promotes low pressure andconvergence at the surface. This leads to continued uplift,condensatiory and the release of latent heat.

So if hurricanes are self-propagating, can theyintensify indefinitely, until they attain supersonic

Hurricane Movement and Dissipation 379

speeds? No, because they are ultimately limited bythe supply of latent heat, which in turn is constrainedby the temperature of the ocean below and by theprocesses underlying evaporation and convection.The importance of ocean temperature suggests that ifthe oceans were to become warmer/ hurricanes wouldtheoretically become more intense. This topic has

received considerable attention lately because of thepossibility of climatic warming, which could be

accompanied by higher ocean temperatures.

Hurricane Movementand DissipationThe movement of tropical systems is related to the stage

in their development. Tropical disturbances and depres-

sions are guided mainly by the trade winds and, there-

fore, tend to migrate westward. The influence of the

trade winds often diminishes after the depressions inten-sify into tropical storms. Then the upperlevel windsand the spatial distribution of water temperature morestrongly determine their speed and direction (with thestorms tending to move toward warmer seas).

HURRICANE PATHS

Once fully developed, tropical storms become morelikely to move poleward, as shown in Figures 12-2 and12-10. Figure 12-11 shows the locations of all hurricanelandfalls from 1950 through 2007. (Box 12-2, Special

lnterest: 2004 and 2005: Two Historic Hurricane Seasons),

describes the paths and damage done by severalnotable hurricanes of these two remarkable hurricaneyears. Hurricanes and tropical storms often move inwildly erratic ways-for example, moving in a constantdirection for a time, then suddenly changing speed anddirection, and even backtracking along its previouspath. Figure 12-12plots some particularly erratic pathsalong the east coast of North America.

While hurricanes can hit any part of the Gulf ofMexico or Atlantic coasts at any time during hurricaneseason, they have a greater likelihood of taking particularpaths during different months. Figure 12-13 shows the

likelihood of hurricane passage for August (a), September(b), and October (c). In August, the most likely path ofhurricanes tracks over the West Indies. From there, hurri-canes are about equally likely to track toward the Texas

coast or along the Atlantic coast from Florida to NorthCarolina. Two prominent paths dominate in September'

One goes from between the Yucatan Peninsula and west-em Cuba, northward toward the cenkal Gulf of MexicocoasU the other moves northward from around Haiti, the

Dominican Republic, and Puerto Rico into the westernAtlantic. Hurricanes taking the more easterly track are

most likely to hit the middle Atlantic states if they makelandfall. October paths exhibit a greater tendency to track

380 CI.{APT§H 12 Tropical Storms and Hurricanes

: | ,.i9fl1 1---'..ltlrii.t:. I l

During hurricane season, several tropicalstorms or hurricanes can arise simulta-neously over various oceans. Meteorolo-gists identify these systems by assigningnames when they reach tropical stormstatus. The World Meleorological Orga-nizalion WMO) has created several listsof names for tropical storms over eachocean. The names on each list are

ordered alphabetically, starting with theletter A and continuing up to the letterW. When a depression attains tropicalstorm status, it is assigned the nextunused name on that year's list. At thebeginning of the following season,names are taken from the next list,regardless of how many names wereunused in the previous season, Six lists

have been compiled for the AtlanticOcean, and the names on each list areused again at the end of each 6-yearcycle. English, Spanish, and Frencl-names are used for Atlantic hurricanes. l:

all the available names on a season's listare used, subsequent storms will begiven the names of the letters of theGreek alphabet. Thus, the 22nd namec

Ana Alex Arlene Alberto

Bill Bonnie Bret Beryl

Claudette Colin Cindy Chris

Danny Danielle Don Debby

Erika Earl Emily Ernesto Erin

Fiona Franklin Florence Fernand

Gaston Gert Gordon Gabrielle

2009

Fred

Grace

Henri Hermine

lda lgor

Joaquin Julia

Kate Karl

Larry Lisa

Mindy Matthew

Nicholas Nicole

2010 2011 2012 2013

Andrea

Barry

Chantal

Dorian

2014"

Arthur

Bertha

Cristobal

Dolly

Edouard

Fay

Gustav

Hanna

lke

Josephine

Kyle

Laura

Marco

Nana

Omar

Paloma

Rene

Sally

Teddy

Vicky

Wilfred

Odette Otto Ophelia Oscar Olga

Katia Kirk Karen

Lee Leslie Lorenzo

Maria Michael Melissa

Nate Nadine Nestor

Paula Philippe Patty Pablo

Richard Rina Rafael Rebekah

Harvey Helene

lrene lsaac

Jose Joyce

Humberto

lngrid

Jerry

Sandy Sebastien

Tony Tanya

Valerie Van

William Wendy

Peter

Rose

Sam Shary Sean

Teresa Tomas Tammy

Victor Virginie Vince

Wanda Walter Whitney

List effective as of summer 2008. Subject to change if any names are retired following the 2OOB season.

storm for a season would be Alpha. This is

exactly what happened in October 2005

when Tropical Storm Alpha appeared in

the western Atlantic-the first time ever

that the list of names was unable to

accommodate all of the tropical storms in

a single season.Particularly notable hurricanes can

have their names "retired" by the WMO if

an affected nation requests the removal of

that name from the list. All replacements

are made with names of the same gen-

der and language. As of early 2OO8,70

names had been retired from the

Atlantic hurricane list. lf a hurricane with

a Greek letter merits special designation,it would do so with its year appendedto the letter. Thus if in 2013 Alpha is

very destructive, it can be noted as

Alpha 2013.The practice of naming hurricanes

appears to have begun during World

Hurricane Movement and Dissipation 381

War ll when meteorologists in the Pacific

assigned female names (possibly after

wives and girlfriends) to tropical storms

and typhoons. This practice was adopted

by the U.S. National Weather Service

(then called the Weather Bureau) in 1953

and maintained until 1979, when male

names were added to the lists' The

names for Atlantic and east Pacific hurri-

canes for the years 2OO9-14 are pre-

sented in Tables 1 and 2.

Enrique Estelle Eugene Emilia Erick

2009 2010

Andres Agatha

Blanca Blas

Carlos Celia

Dolores DarbY

Felicia

Guillermo

_Linda

Marty

Nora

Olaf Orlene

Patricia Paine

Rick BoslYn

Sandra SeYmour

Terry Tina

2012 2013

Aletta Alvin

Bud Barbara

Carlotta Cosme

Daniel Dalila

2011

Adrian

Beatriz

Calvin

Dora

2014*

Alma

Boris

Cristina

Douglas

Elida

Fausto

Genevieve

Hernan

lselle

Julio

Karina

Lowell

Marie

Norbert

Odile

Polo

Rachel

Simon

Trudy

Vance

Winnie

Xavier

Yolanda

Zeke

Hilda Howard Hilary Hector Henriette

lgnacio lsis lrwin lleana lvo

Jimena Javier Jova John Jullette

Kevin KaY Kenneth KristY Kiko

Frank Fernanda Fabio Flossie

Georgette Greg Gilma Gil

Lester Lidia Lane Lorena

Madeline Max Miriam Manuel

Newton Norma Norman Narda

OliviaOtis Octave

Pilar Paul Priscilla

Ramon Rosa RaYmond

Selma Sergio Sonia

Todd Tara Tico

Vivian Virgil Veronica Vicente Velma

Waldo Winifred WileY Willa Wallis

Xina Xavier Xina Xavier Xina

York Yolanda York Yolanda York

Zelda Zeke Zelda Zeke Zelda*

Effective as of summer 2008. Subject to change if any names are retired after the 2008 season.

A The tracks of all western Atlantic and eatern Pacific major hurricanes (Category 3 or higher), from 1g5l -2007for the Atlantic and 1949-2007 in the pacific.

EsmtftmcmtafiUmErfd

Hunmfleame Smnktss*flg5ts - 2tsW7

\eonn irou)\Bob it991 r

Sallir-Simpson HtrricaneIategories ..

Sustained

74-e5 Category 1

96-110 Category 2111-130 Category 3131-1ss lD Category 4>15s (], i_]r.r.r,i)

B.el (1999), Aikn (1980)

.t,r, \zoo2t

li ,.Hilda 1199)

oavLd 11979).Eio 11995)

xJù,na t2005)i King (1950)

È eo,., i,coo,

. O Flossy il956rDanry^i !9q/)I c,*i iz'cbir' '

Ftorenar trgBé)Earr r r95n)ser<v i tq6i, Gladyr t1968

" Bobll979)

Wima (2005ìGeoqes (1998i

A The locations of all hurricane landfalls over the continental United States from 1950 through 2007.Saffir-Simpson categories are those at the time of landfall.

crn.n r/on-r o

A FIGURE 12*12 Tropical storms and hurricanes have atendency to move north or northeast out of the tropics alongthe southeast coast of North America. Their paths are oftenerratic, as seen in these examples.

from eastem Mexico, northward to Florida and the rest ofthe southeastem United States.

Atlantic tropical storms and hurricanes can travelgreat distances along the North American east coast, butthey usually weaken considerably as they approach thenortheastem United States and the Maritime Provinces ofCanada. These storms usually lack the strong winds thatcharacterize hurricanes in the low latitudes but can stillbring intense rainfall and flooding. On rare occasions, the

storms can maintain their strong winds even as theymove considerable distances from the subtropics. Forexample, an intense September 1938 hurricane brought200 km/hr (120 mph) winds to Long Island, New York,as it moved toward New England. Its estimated 600 fatal-ities made it the fourth deadliest of all U.S. hurricanes.More recently, i. September 1985, Hurricane Gloriabrought considerable wind and flood damage to LongIsland and Connecticut.

Although hurricanes and tropical storms can moveinto the northeastern United States, along the West Coast

they do not migrate nearly as far north without weaken-ing to tropical depressions. The reason for this is the dif-ference in water temperatures along the two coasts. The

Pacific Coast is dominated by upwelling and the coldCalifomia current, while the warm Gulf Stream flowingalong the East Coast provides a greater supply of latentheat. Sometimes, storms off the coast of Mexico move to

the northeast across Baja California and into southernCalifomia. These storms lose their supply of latent heatand lose their intensity as they move irùand, but they can

Hurricane Destruction and Fatalities 383

DTD YOU I(NOW?A quick glimpse of Figure 12-2 reveals that hurricanesnever occur in the South Atlantic. Well, almost never.

On March 27,2004, for the first time in recordedhistory a hurricane hit the coast of Brazil. Tropicalcyclones have hit the Brazilian coast twice before, but"Catarina" (so named for the Brazilian state of SantaCatarina, which was hit hardest) packed 147 km/hour(90 mph) winds-making it a true hurricane. Thisstorm did not develop in exactly the same manner as

do most of those in the North Atlantic. lt appears tohave been a hybrid between a tropical system and amidlatitude cyclone, having initially formed along acold front in the South Atlantic. Considerable damageoccurred along the coastal region, but effectivewarnings to the public minimized the loss of life.

still bring heavy rains and flooding. In 1976 HurricaneKathleen caused massive flooding in the desert of south-em California that wiped out part of Lrterstate Highway 8.

EFFECT OF LANDFALL

After making landfall, a tropical storm may die outcompletely within a few days. Even as the storm weak-ens, though, it can still import huge amounts of watervapor and bring very heavy rainfall hundreds of kilo-meters inland. This is especially true when the remnantof a hurricane moving poleward joins with a midlati-tude cyclone moving eastward. Exactly this happened in1969 when one of North America's most notorious hurri-canes, Camille, moved northward from the coast ofMississippi (Figure 72-74). After its high winds andtidal flooding brought extreme damage to the GulfCoast, the storm moved northeastward toward the west-ern slopes of the Appalachians. There, orographic upliftcoupled with low pressure and high water vapor con-

tent of the remnant hurricane could easily have pro-duced serious flooding. But to make matters worse/ aneastward-moving cold front reached the mountains atthe same time as the former hurricane. The combinationof moist ai1, low pressure, an approaching front, and the

orographic effect set the stage for intense rains and flashflooding that killed more than 150 persons.

Hurricane Destructionand FatalitiesHurricanes can bring death and devastation in severalways, through any combination of strong winds, heavyrain, hurricane-spawned tornadoes, and the elevationof coastal waters combined with heavy surf.

384 CI.IAPTER 12 Tropical Storms and Hurricanes

, ,,-'i=i:.i-.....The period between 1995 and 2005 wasmarked by an unusually high number ofAtlantic tropical storms and hurricanes,with many making landfall. Of the ten sea-sons during this time span, all but twohad at least eight hurricanes-well abovethe annual average of 5.9. But 2OO4 and2005 were particularly noteworthy. Theyear 2004 was the costliest hurricaneseason to hit the United States up to thattime, bringing $42 billion in losses. Hurri-canes damaged one-fifth of all the homesin Florida and killed 1 17 people that year.lncredibly, that devastation was dwarfedby the hurricane season that hit the fol-lowing year. The year 2005 produced oneof the greatest natural disasters in U.S.history-Hurricane Katrina-and several

other major hurricanes that made landfall;in addition, it proved to be the most activetropical storm-hurricane season inAmerican history with 27 named storms(breaking a record set in 1933). Here wepresent a brief review of four hurricanesfrom 2004 and Hurricanes Rita andWilma in 2005 (Figures 1 and 2). Given itsenormous death toll and destruction, wediscuss Hurricane Katrina separately laterin this chapter.

Hurricane Charley,August 9-14,2OO4Hurricane Charley was the strongest hur-ricane to hit the United States since Hurri-cane Andrew 12 years earlier. At the timeit was also the second costliest hurricane

to strike the United States, bringing arestimated $14 billion in damages alongwith 10 fatalities that were direcfly attrib-utable to the storm.

Charley developed as a tropicadepression noftheast of Venezuela oiAugust 9. lt moved steadily to the west-northwest as it intensified, and b'.August 1 1 it achieved hurricane statuswith its eye a mere 65 km (40 mi) south-west of Jamaica. Charley's path begarto turn gradually toward the right, ancthe hurricane approached western Cuba24 hours later, with winds in excess c.190 km/hr (120 mph), making it a Cate-gory 3 hurricane. The storm weakenecas it passed over the island but still led tcfour deaths in Cuba and one in Jamaica

A FIGURE I The paths of four 2004 hurricanes.

After exiting northwest Cuba, thehurricane reintensified and continuedarcing to its right. lt made landfall on theGulf Coast of the Florida peninsula at

3:45 p.u. on August 13, just afterachieving wind speeds of 231 km/hr(1 43 mph)-a Category 4 hurricane.

The storm moved northeastward acrossthe state and across the eastern Florida

shoreline at about 1l:30 p.v. on August'14. lt made a second landfall over SouthCarolina and entered North Carolina asa weakenìng tropical storm. Charley

then became embedded within a pass-

ing midlatitude cyclone.Relatively little damage was done by

Charley's storm surge, which peaked at

about 1 .28 m (4.2 ft); most of the dam-age was incurred by strong winds and

the nine tornadoes the hurricane

spawned.

Hurricane Frances,August 2S-9eptember 8, 2004

Eleven days after the dissipation of Hur-

ricane Charley, a new system emerged

in the central North Atlantic Ocean atabout I2" N latitude. Tropical StormFrances moved west-northwestwardand intensified to a hurricane the next

day. The hurricane went through several

cycles of intensification and weakening.It achieved maximum wind speeds esti-mated at 231 km/hr (144 mph) onAugust 29, making it the second Cate-gory 4 hurricane that month. Frances hit

eastern Florida shortly after midnight onSeptember 5 as a Category 2 hurricane.Gradual weakening occurred as itpassed northwestward over Florida (hit-

ting a part of central Flodda that hadalready been severely damaged by Hur-

ricane Charley), and Frances entered theGulf of Mexico as a tropical storm. Afterturning northward, it made its secondlandfall in the Big Bend area of Floridaand continued northward. Even after los-

ing tropical storm status, Frances's rem-nants brought strong winds and rain tomuch of the northeastern United Statesand southeastern Canada. The esti-mated $9 billion in damages incurred by

Hurricane Destruction and Fatalities 385

Frances ranked it as the fourth costliesttropical cyclone to hit the UnitedStates-and the second major disasterin less than a month.

Hurricane Ivan,September Y24,2OO4While Hurricane Frances was about2 days away from landfall over easternFlorida, Tropical Storm lvan formed in

the east-central North Atlantic at about10' N. Over the next 3 weeks, lvan

became a truly remarkable system forboth its intensity-achieving Category 5status on three separate occasions-aswell as for its highly unusual path. lvanwas the southernmost hurricane onrecord in the Atlantic. The island ofGrenada was the first land area to beseverely hit by lvan. Category 3 winds hitthe southern portion of the island onSeptember 7, destroying 14,000 homesand killing 39 people. On September 10,

lvan passed south of Jamaica as a Cate-gory 4 hurricane. Though its strongestwinds occurred to the south of the

,r" go' ";

/

\_. *rr{.'.},t -Yr

*i.-J.-

ì -t; ,-z- f, q----r

,l',1

)

60 55' 35 30 25 .o 15

tji

+(r', !1I;1

't-\\." N

L,,\, _+,

i(

Oci. 25 i

Sept.WILMA

24

Sept. 2225

Sept. 2420. Ja23

18

NAra\ §===,b': '=!{r-i. ,-TA

6

)*--t. t\ - "\\,,.-'rl"\i

r-i'.{ìt) -a.--,F*]

J-

/.-.0 500 '1,000 Mìles

ffi0 500 1,000 Kilometers

oa

Tropical storm stage

Hurricane stage

Tropical Depression

Position at 4:00 PM. E.S.T.

Position at 4:00 A.N4. E.S.T.

FTGURE 2 The paths of Hurricanes Katrina, Rita, and Wilma in 2005.

386 CHAPTER 12 Tropical StormsandHurricanes

island, the hurricane still destroyed b600homes and damaged 47,OOO others andkilled 17 people. Grand Cayman lslandwas next, with near-Category 5 windsdamaging or destroying 95 percent ofthe island's homes.

lvan then headed for the UnitedStates and made landfall on September16 near Gulf Shores, Alabama, as a Cat-egory 3 hurricane. The combination of a3 to 5 m (10 to 15 ft) storm surge, heavyrain, strong winds, and numerous torna-does hit the Alabama-Florida coastlineespecially hard. The system weakenedas it moved northward and northeast-ward, but it was still bringing heavy rain-falls as far north as New Hampshire2 days later.

Most storms would have done alltheir damage by this point, but this wasno ordinary storm. Remnants of the for-mer hurricane began to move southwardalong the Atlantic coast, reaching theFlorida shoreline on the morning ofSeptember 21 . Thundershowers occurredover southern Florida as the systemmoved westward across the peninsula.Then after returning to the Gulf of Mex-ico, the system redeveloped into a tropi-cal storm on September 22, lvan thenmoved to the northwest and made land-fall once again as a tropical depressionover soulheastern Louisiana, where itrapidly died out.

lvan was responsible for 92 deaths-25 of them in the United States. U.S.damages were estimated at well over$14 billion-about the same as Hurri-cane Charley had brought a month anda half earlier.

Hurricane Jeanne,September 1+28,2OO4By far the deadliest of the 2004 Atlantichurricanes was Jeanne, which killedmore than 3000 people. Most of its fatal-ities were associated with flooding in theCaribbean, especially in Haiti, as highrainfall rates coupled with slow stormmovement led to very large precipitationtotals. Although Puerlo Rico had fewerfatalities than did Haiti, it suffered record-breaking floods.

Jeanne moved northward after it hitHispaniola, undertook a clockwise loop inthe Atlantic, and headed almost directlytoward Florida. Jeanne had Category 3wind speeds just before landfall, with amaximum gust of 206 whr (128 mph)observed at Fort Pierce lnlet. Hurricane-force winds extended well inland, includ-ing a part of central Florida that hadalready taken direct hits from HurricanesFrances and lvan. North of Tampa,Jeanne weakened to a tropical storm andcontinued northward, eventually bringingheavy rains to Georgia, the Carolinas, andthe Delmarva Peninsula as a tropicaldepression. Estimated damages in theUnited States are believed to haveapproached $7 billlon.

Hurricane Rita,September 1*25,2OO5Following the devastation of HurricaneKatrina (see Box 7 2-3, Special lnterest:Hurricane Katrina) only a few weeks ear-lier, Hurricane Rita became the secondCategory 5 hurricane to develop in theGulf of Mexico that year-the first time in

recorded history that two such powerfulstorms had ever occurred in the Gulf inthe same season (Hurricane Wilma alsoachieved Category 5 status in theCaribbean Sea that year, but it lost Cate-gory 5 intensity before movlng into theGulf). At its worst, Rita was a monsterstorm. lts maximum wind speeds toppecout at 280 km/hr (175 mph), hurricane-force winds extended 1 10 km (70 mi) awayfrom the center, and tropical storm-forcewinds reached out to 295 km (185 mi).Its minimum sea level air pressure oi897 mb was the third lowest everobserved in the Atlantic Ocean.

Hurricane Rita's first brush with lanooccurred as it passed south of theFlorida Keys on September 20 as a Cat-egory 2 hurricane. Though Rita's pathtook it too far to the south to deliver itsworst punch, it still did considerabledamage to the Keys, downing trees anccreating a 1.5 m (5 ft) storm surge thattopped U.S. Highway I and causecsome flooding to business and homes.As it moved westward, the hurricanerapidly intensified and reached Category5 status on the afternoon of September21. With the public very much aware ofthe devastation wrought by HurricaneKatrina, citizens along much of the Gui.Coast took the call for evacuation ver-,seriously, causing massive traffic prob-lems in east Texas as an estimatec3 million people headed inland.

.See this book's CD for movies showing theprogression of Hurricanes Katrina, Rita, ancWilma.

WINDBy definition, hurricane winds exceed 120 km/hr (75 mph),and many are much faster. It is not surprising, then,that hurricane-force winds can cause extensive damageand even destroy well-built homes. An example of thishappened on August 24, 1992, when HurricaneAndrew devastated much of southern Florida. Itapproached southern Florida almost directly from theeast and cut westward across the state, with its eyepassing about 40 km (25 mi) south of Miami Beach.There was relatively little damage due to coastal flood-ing or heavy rainfall, but winds completely leveled thetown of Homestead, killed 24 people, and left 180,000homeless in the state before crossing into the Gulf ofMexico.

HEAVY RAIN

Hurricanes produce staggeringly intense rainfall, withrates on the order of meters per day found beneath theeye wall. The rate for a location that remains stationarr-beneath a passing storm is smaller, but still huge-onthe order of 25 crrt/day (10 in./day).

Heavy rain made Hurricane Mitch the deadliesthurricane to hit the Westem Hemisphere in the last 200years, killing thousands of people in Central America inOctober 1998 (some effects of Mitch were described atthe beginning of Chapter 3). Mitch hit Central Americafrom the east. Although Mitch's winds weakened sub-stantially, the remnant system brought heavy rainfallacross the region as it tracked northward toward theGulJ of Mexico. Intense rains lasted for several days, and

Rita struck the Texas coast just west:f Louisiana as a Category 3 storm on

September 23. lt delivered hurricane-'crce winds to areas as far inland as 24O

<m (1 50 mì) and tropical storm-force.vinds as far north as southern Arkansas.

Storm surges as high as 4.6 m (15 ft),:rough smaller than originally feared,

:aused very serious flooding and the:otal destruction or major damage ofseveral communities. ln addition, a2.4 m3 ft) storm surge in New Orleans'eopened several breaches in the levees

:rat had been temporarily repaired fol-owing Hurricane Katrina. Overall dam-age from Hurricane Rita was very large

cut far short of that brought about by-'lurricane Katrina. lnitial reports indi-

:ated a total loss of life of about 1 'l 9

ceople. Only 6 of the deaths appear torave resulted directly from the hurricane;

:ne rest were indirectly related, such as

:nose caused by a fire that engulfed a

cus full of elderly patients evacuating-louston.

Hurricane Wilma,October 17-25,2OOsr just about any other year, Hurricane,Vilma would have been the most'emarkable of hurricanes. Tropical

)epressìon 24 intensified to Tropical

Storm Wilma in the early morning of'./onday, October 17, about 280 km

I75 mi) southeast of Grand Caymansland in the Caribbean Sea. Thoughsustaìned wind speeds were a mere55 km/hr (40 mph), the National Hurricane

lenter had already predicted that the

siorm would become a major hurricane

sometime within the next 5 days. Thisforecast understated the magnitudeand rapidity of what was to haPPen.

Wilma remained a tropical storm until

the next morning. At 11 n.u. EDT Tues-

day, October 18, the troPical stormbecame Hurricane Wilma-the 12th ofthe season-as a Category t hurricanewith sustained wìnds of 12O km/hr(75 mph). By 5 n.u. EDT WednesdaY,

the hurricane packed sustained windsof 280 km/hr (175 mph)-comPletingan explosive transition from tropicalstorm to Category 5 hurricane ìn lessIhan 24 hours! Hurricane reconnais-sance aircraft observed a sea level airpressure of 884 mb-the lowest everobserved in the Atlantic.

Wilma then headed for the Yucatan

Peninsula. lt made a direct hit on

Cozumel lsland, southeast of the main-

land, on Friday morning, October 21,

and continued northwestward towardthe Cancun area. The storm remained

over the area through much of Friday

and Saturday before turning into the Gulf

of Mexico and heading toward theFlorida coast. An estimated 22,000tourists and many more residents of theresort area had to find shelter from the200 km/hr (125 mph) winds and heavY

downpours. Meanwhile, 700,000 peoplehad been evacuated from the west sideof Cuba in anticipation of the heavy

flooding that hit that portion of the island.

At least 19 people were killed ìn theCaribbean area by the hurricane.

Wilma moved rapidly across the Gulf

of Mexico as it headed toward thesouthwestern coast of Florida. lt hit the

Hurricane Destruction and Fatalìties 387

western Gulf Coast on Monday morning,October 24,2005, as a Category 3 hurri-

cane. Storm surges up to 2.75 m (9 ft)caused extensive flooding, especially in

Key West. Widespread wind damagealso occurred across much of the GulfCoast. The hurrìcane moved very rapidlyinland toward the Northeast. Away fromthe Gulf Coast, high winds brought themost severe damage, especially on theright hand side of the hurricane. ln fact,

the rapid movement of the storm con-tributed to the surprisingly high windspeeds encountered over much of south-ern Florida. The Miami and Ft. Lauderdale

areas experienced considerable damage,including the shattering of windows in

large buildings and the downing of large

trees. Roads were ìmpassible soon afterthe storm, millions of people lost electricpower for extended time periods, andresidents were advised to boil their tapwater before drinking it. Earliest reportsidentified ten fatalitìes.

Hurricane Wilma then proceedednorthward in the western Atlantic.Though its strong winds did not hit

coastal areas, it fed additional moistureinto a nor'easter, hitting Canada and thenortheastern United States. Thisincreased the amount of snow and rain-fall for an area that had already experi-enced extreme precipitation amounts forthe month. Across the United States,total insured damages caused by Wilmawere estimated to be between $6 billìon

and S10 billion. On Tuesday, October 26,

the National Hurricane Center quit track-ing the storm as it moved safely awayfrom North America.

parts of Honduras and Nicaragua received estimatedprecipitation totals of 85 cm (35 in.), causing extensivetlooding and mudslides in this mountainous region.

Irt2002 Tiopical Storm Allison demonstrated that alropical storm need not attain hurricane status tohecome a major disaster. Allison hit the south Tèxas

coast on ]une 5 and hovered over the area for nearly a

rreek. Its heavy rainfall (as much as 96 cm, or 40 in.)caused major flooding across Texas and Louisiana,killing 24 people and flooding more than 46,000 homesand businesses.

TORNADOES

\lany hurricanes also contain clusters of tomadoes, mostoften in the right-forward quadrant (Figure 12-15). Theyr,rsually occur far enough away from the center that they

are surrounded by relatively weak winds. It appears thatslowing of the wind by friction at landfall contributes totomado formation. Flurricane-spawned tomadoes tendto have shorter lifespans than tomadoes in the centralUnited States.

STORM SURGES

In addition to the threat of heavy rain, strong winds, andtomadoes, coastal regions are vulnerable to a special

problem called the storm surge, a rise in water levelinduced by the hurricane. Two processes create a stormsurge, the major one being the piling up of water as

heavy winds drag surface waters forward. Strong windsblowing toward a coast force surface waters landwardand thereby elevate sea level, while also bringing hear,y

surf. The low atmospheric pressure in a hurricane also

388 CHAPTER 12 Tropical Storms and Hurricanes

A FIGURE 12-,I3and October (c).

10

August

f -J urery

| | More lit<ety

! Most tiretV

+ Prevailing tracks

^0(a)

September

fl t-itetyl=l More likel

! lrost tiketv

-> Prevailing tracks

/' soulH,t'

^}lEBlCA

0 500 1000 Miles

ffi0 500 1000 Kilometere

Eoùator

i--- 40 30o roo

(b)

10

tIT

October

LikelyMore likely

Most likely

Prevailing tracks

..".-r'

-Lii"ì} -,il ;'.> -

t" ^Fntcai

1

o'o

0 500 1000 Mil6s

0 500 1000 KilomeleB

Equato,10

(c)

The predominant paths of Atlantic hurricanes in August (a), September (b),

A FIGUBE 12-14 The movement of Hurricane Camille.

contributes to the storm surge, in the same way that the

height of a column of mercury in a barometer respondsto variations in atmospheric pressure. For every millibarthe pressure decreases, the water level rises 1 cm (0.4 in.)'For most hurricanes along a coastal zone, the stormsurge elevates the water level only a meter or two. But inextreme circumstances, storm surges can increase the

water level by as much as7 m (23 ft), as was the case forHurricane Camille along the coast of Mississippi in 1969.

Hurricane Destruction and Fatalities 389

DID YOU KNOW?Hurricane damage is not all water and wind. For

example, in Navarre Beach, Florida, Hurricane Katrinaleft behind meters of sand along a 2O-mile (12 km)

stretch of road (Figure 12-16). Months of work wereneeded to clear these storm-produced drifts.

SURFACE WEATHEB MAPAT 7:00 A.M., E.S.T.

AUG.17,1969

(b)

SURFACE WEATHEBAI7:00 A.M.. E.S.T

4UG.19,1969

Al7i00 A.M., E.S.T

AUG. 20, '1969

, \\ lI

SI]RFACE WEATHER MAPAT 7:oO A.M., E.S.L

ì"+.. 0- 11-.--gE.* -

, i, t--G--?**-,*

AUG. 21, 1969

(e)

390 CHAPTER 12 Tropical StormsandHurricanes

> FIGURE 12-15 Tornadoes most often form inthe right-forward quadrant of hurricanes (based onthe direction in which the storm is moving). Thefigure is based on data from 373 hurricane-embedded tornadoes between 1948 and 1972 in theNorthern Hemisphere. Each dot represents atornado. The circled X indicates the mean position oftornadoes relative to the storm center.

Storm surges along low-lying coastal plains can beextremely devastating where the rise in sea level bringswaters far inland. Furthermore, the heavy waves gen-erated by the strong winds pound away at structures,with debds carried by the waves adding to the prob-lem. In the case of Hurricane Camille, tÈe storm com-pletely destroyed the Richelieu Apartments along theMississippi coast (Figure 12-17). Despite previous

> FIGURE '12-16 A stretch ofroad at Navarre Beach, Florida,cut off by blowing sand fromHurricane Katrina.

warnings to evacuate, about a dozen residents decidedto have a "hurricane patty," figuring that if the stormsurge got too high they would just move up to the thirdfloor of the building. Unfortunately for them, the stormsurge undermined the apartment foundation, andmore than two dozen died when the building com-pletely collapsed. Storm surges are most destructivewhen they coincide with high tides, especially over

Direction ofstorm movemenl

0

aa

atatat.

Hurricane Forecasts and Advisories 391

(b)

The Richelieu Apartments before (a) and after (b) the passage of Hurricane Camille in 1969.

(a)

A',

bays and inlets that have an extreme range of heightbetween high and low tide.

Hurricane winds and storm surges are most intense

on the right hand side of the storm relative to the direc-

tion it is moving. To understand why, imagine that a

hvpothetical hurricane in the Gulf of Mexico is movingnorthward at 50 km/hr, with a uniform eye wall having100 km/hr-120 mph-winds (Figure 12-18). If this

hurricane makes landfall along the coast of Mississippiarrd Alabama, the winds to the east of the eye move at

150 km/hr (the sum of wind speed and storm speed).

\long the western part of the eye wall, the winds are

.rnly 150 km/hr (200 km/hr minus 50 km/hr). The righthand side of the storm will also have a greater storm

surge because the winds push the water onshore rather:han offshore.

Though storm surges Present the greatest poten-:ial for catastrophic coastal destruction and have

;laimed thousands of American lives over the last few:enturies, they do not account for the majority of.\merican hurricane fatalities. A study published by a:esearcher at the National Hurricane Center revealed

:lrat between7970 and2002, more than half the fatali-:ies from tropical storms and hurricanes in North.\merica resulted not from storm surges, but rather:rom freshwater flooding from heavy rain. Only about.rne-quarter of the fatalities associated with tropicalstorms and hurricanes (or their remnants) occurred in.rrastal counties. For several decades prior to 2005 and

ilurricane Katrina, there had been a decrease in the

-i-rcidence of storm surge fatalities. (See Box 12-3, Spe-

-:.rl Interest: HtLrricttne Kstrina, for more information on

:l-ris historic event.) The reduction in deaths associated'.rith storm surges is partially the result of an episodicjecrease in the number of strong hurricanes hitting:rrprLrlated coastal regions during the 3O-year period,,:.lt-rng with a better ability to predict the movement of:.r-rrricanes and improved evacuation procedures. (Wejiscr-rss trends and cycles in hurricar-re activity later in::ris chapter.)

Hurricane Forecastsand AdvisoriesResponsibility for tracking and predicting Atlantic and

east Pacific hurricanes lies with the National HurricaneCenter (NHC) in Miami, Florida. During hurricane sea-

son, this office of the National Weather Service obtains

constantly updated surface reports and satellite data to

determine current storm conditions. Sophisticatednumerical models on a suPercomputer predict the for-mation, growth, and movement of tropical storms and

hurricanes. When active hurricanes approach land,specially equipped aircraft fly into the storms and

piovide reconnaissance data from airborne radar and

dropsondes, packages containing temperature/ pressure/

and moisture sensors and transmitters released fromthe plane into the storm.

The NHC uses the standard computer models

for conventional weather forecasting (discussed inChapter 13), as well as others developed specifically forhurricanes. The latter fall into three categories:

stntistical, dynanùcal, and lrybrid. Statistical models

apply information on past hurricane tracks and use

DID YOU ](NOW?Meteorologists once believed that seeding cloudswith silver iodide (see Chapter 7) could be an

effective way to reduce hurricane strength' The

idea was to seed parts of the cloud outside the eye

wall. lf the seeding successfully led to enhancedconvection outside the eye wall, the new zone ofheavy activity could compete with and therebyweaken the strength of the eye wall. Seeding wasundertaken sporadically between 1962 and 1983

on 8 days in four different hurricanes. Although theresults once appeared promlsing, the methodultimately proved unviable.

392 CI.IAPTER 12 Tropical Storms and Hurricanes

. 1; P.:.:r.' :=11-r'' ,ixjillr5;=-::-. -. .

At the end of August 2005 we witnessedone of the major natural catastrophes inAmerican history: Hurricane Katrina.Katrina was the first of three Category 5hurricanes to form in the Gulf of Mexicoor the Caribbean in 2005, bringing with itsubstantial flooding in southern Florida,the inundation of New Orleans, and adevastating storm surge in coastalLouisiana, Mississippi, and Alabama.Two months later, the storm's death tollwas complete: More than 1300 peoplehad perished in the United States. Dam-age estimates exceed $100 billion. ln

this section we present a chronology ofwhat happened meteorologically. ln abook such as this, we cannot even beginto analyze comprehensively the humanimpact of the disaster.

Wednesday, Au g ust 24, 2OO5:TropicalStorm Eastof South FloridaAt 11 nv. EDìl Tropical Storm Katrina waslocaled to the east of south Florida (Figure

1 a shows the location of the storm, as well

as the official NHC predictions for futuremovements and status). Hurricane warn-ings had recently been issued for thesoutheast Florida coastline, with antici-pated landfall near Miami. The storm wasforecast to be a Category t hurricane by 8p.ur, the following day very near the shore-line. Though the cone depicted a fairly

extensive range of possible positions, theforecast models the center used were in

close agreement, and the storm movedmuch as predicted over the short term.

Thu rsday, August 25, 2OO5:Hurricane Katrina LandfallAt 5 p.u. EDT on August 25 (Figure 1b),

Katrina had developed into a hurricaneand was near landfall at the positionforecast the night before (though it hadmoved somewhat faster than expected).Landfall occurred at about 6:30 cw. nearthe Broward/Miami-Dade County stateline (Figure 2). At this point, forecastersofficially called for the cyclone to movedirectly westward and, upon entering the

Gulf Coast the following afternoon, toturn toward the nofthwest and follow atrack somewhere in the eastern Gulfalong the west coast of Florida.

What is not revealed by Figure 1b isthat the forecast models showed highlydiffering forecasts. Several suggestedthe path similar to the official NHC fore-cast, but three others indicated a south-westward movement across southernFlorida. The storm did indeed move tothe southwest, reaching the westernFlorida coast seven hours after initiallandfall. The hurricane moved rapidlyacross the state, which reduced theamount of weakening normally under-taken as a hurricane passes over land.So even though Katrina traveled acrosssouthern Florida mainly as a tropicalstorm, it was able to reintensify into ahurricane when it reached the Gulf.

Katrina did considerable damage toFlorida directly and indirectly due to heavyrain that exceeded 26 cm (16 in.) in places.The heavy rainfall led to major flooding andtrees toppled by the combination of satu-rated soils and strong winds. Six peopledied in Florida from Katrina, which alsocaused S100 million in damages and $423million in agricultural losses. But there wasmuch, much more to follow

Friday, August 26,2OO5= !n theEastern Gulf of MexicoShortly after midnight, Katrina enteredthe Gulf of Mexico. By dawn it hadbecome clear to forecasters that the hur-ricane had the potential to become ahuge danger to parts of the Gulf Coast.Early morning forecasts called for thestorm to initially move to the west andthen begin to arc northwestward towardthe coast anywhere from the Florida pan-handle to extreme eastern Louisiana(Figure 1c). But by late evening, the sys-tem had moved farther to the southwestthan anticipated, and by 11 e.v. a verydifferent track was predicted-one thatwould put Katrina on a collision coursewith New Orleans and the Mississippicoast (Figure 1d). The hurricane was nowset to pass over a region of very warmGulf waters, which the NHC described as

". . . like adding high octane fuel to thefire." All the forecast models predictecfurther intensification of Katrina, one c'them calculating that wind speeds woulctop out at more lhan 243 km/hr (1 5'mph)-a strong Category 4. Also note-worthy was the fact that all the compute'models were giving the same guidance ir

terms of the storm's path, creating littledoubt of what was to come.

Saturday, August 27, 2OO5=

On GourseBy Saturday night (Figure 1e)the prev-ous day's forecasts had proven ver'.accurate, and there was little change ir-

the expected path of the storm. TheNHC issued a hurricane warning tha:included some of the following text:

. POTU\]TIALLY CATASTROPHIC

HURRICANE KATRINA MENACING THE

NORTHERN GULF COAST

A HURRICANE WARNING IS IN EFFECT

FOR THE NORTTI CEVIR.AL GULF COAST

FROM MORGAN CITY LOUISIAXTA EASTWAF:

TO THE ALABAMA/FLORfDA BOFDER

INCLUDING T'HE CITY OF NE{^T ORLEANS

AND T,AKE PONTCTARTR.LIN.

MAXIMUM SUSTAINED I{INDS ARE NEAR

175 MPH WITH HIGHER GUSTS.KATRINA IS A POTM{T]ALLY CATA_

STROPHIC CATEGORY FIVE HURRICANE C].

THE SAFFTR_SIMPSON SCALE. SOME

FLUCTUATIONS IN STRM.TGTH ARE LIKE::DURING THE NEXT 24 HOIJRS.

HI.'RRICANE FORCE W]NDS EXTEND OL]II_

WARD UP TO 105 MILES FROM THE CEN_

TB. AND TROPICAL STORM FORC:

WINDS EXTM{D OI]IIWARD UP TO 205MILES.

COASTAT- STORM SURGE FLOODING OF 1:TO 22 FEET ABOVE NORMAL TIDE LEVE-.'. LOCAILY AS HIGH AS 28 FEET

ALONG WITH I,ARGE AND DANGEROUS BAI_TERING WAIBS CAN BE EXPECTE:

NEAR AND TO THE EAST OF hJHERE THE

CENTER MAKES LANDFALL. SIGNIFICAN:STORM SURGE FLOODING WILL OCCUR

ELSEI^]HERE AIONG THE CEMIR.AL AND

NORTHEASTERN GULF OF MEXICO COAST.

The most vulnerable city in the UnitecStates, New Orleans, Louisiana, was

L

Becent (and Future?) Trends in Hurricane Actrvlty 393

= (r)

, The position and forecasted movement of Hurricane Katrina at various times. The area in white shows the

..-ge of possible locations for the hurricane center al the forecast times shown.

Curenl Center Location26o ll 73.0wSustanìed wlnd 50 mph

C!trent MovementWat 3 inPh

O C!rent Cenrer Locatlona Forecast center Posillons

H Suslalnedehrd,T3mphS Suslained vlnd 39 73 mDh

f--\ Pordiliat Day r 3 lracI arer'Hurricane Waffrilì9

r Tropical Stoiln warningTroDlcal Slonn Watch

n lurricane Katrina\uoust 25. 2005, Ptr, EDT Thursday{WS TPClllational Hurlcane center

lurrenl Center Localion 26.1 N /9.9 WJil sustained wind 75 mph

:urcnt Movemef tvJ at 6 rnph

O cutrcrt cenrer Local ona Forecast Centtr Posilioils

H Suslahed v/ind , 73 inPh

S Sustailred !/ind 3s.7J rn!hN Porenrlal Ddy r 3 lra(hArec

Huricane Warnlng

- lropical Slonn lvarnirqrrÒDlcal Sloiln Walch

SC

GAr1

t'^__re.a

^{.'l§/

\'

\,, ,*)

(S)

5 Thu ,, r1 ;'"'"'

rruo dt Jo oolr @ '

- ***- \atr ii5 250 r'5 5oo \ \A,Drvr Drsrin € scdt6 ( s,ntùLo illlos ) \{'*r" /u os 8oh _ ,)

{#',\7,.1

(b)

Auoust 25. 20055 AL, EDT FridayrJV/S IPC/National l!urricane Center

Cuiient Center Localion 25.3 l{ 3l.s WMu Sustahed wind 75 inphCur.enl Movem.nlw at 5 mpl

O curent Cenler LocationO Forecasr Cenler Positions

^ H Susrùilìed qird ' /! rnph

L \ Porerriùl Day r I lrach arÈd

r T.opl.al Sioiin WanritgIropi.al slonn watch

",:-4*:k.,. \lr

11 PM EDT Fflday

Cutreni C€nter t ocatloil 24.6 ll 83.6 WSustai.ed Wind 105 inph

rent t/ovcm.nrWSVr' al 3 rnDh

6) cutrenr cenrer Locariona Forecasl cÈnler Pos lions

^ H sus!. red viil,l 73 rnPlrf _\

Porenriol Ddy r I Trò(k ^rca

- lropical Stonn

.\lFfM Mon

:ij1-;J'-<

Auousi 27. 2005to ÉM cDT saturdayNWS TPC/l{ational Hrrlcaile Cetrter

cnrent Center I ocalion 25 o N 86.2 WSuslailìÈd Vr'hd I l5 mDh

Movement Vr'NW at 7 inph

O currenr Cenler LocationO rorecasl Csnter Positiotrs

H Sustained v/hd > /3 mphs srsrained \vlnd 39 73 hùh

N Potential Day 1 3 rrack areaHurlcane Vr'anrhgHunlcaile Walch

r lropical Ston. Wanrinq

t 28,2005CDT Sundaya1!arioilal llnrricaf E Cùntei

Cenler LocallÒn 27.6 fl 6s.4 wSùsratrrcn Wnd 160 rnpl

[lovenrenÌ llflw al ]o inph

é) curenr cenEr Locat ona Forecast CeDter Posillons

H Sùslained \{hd ' 73 mDh

S Sùslanredeiild3s 73 mphlJ sùsratrred vird . Jg rtruh

N Poknrial Dùy r r lccl arr;Hurricane wanlDg!uril.anp Watch

394 C*{AF}T'EFì-i È Tropical Storms and Hurricanes

> F,Gt RE 2 Radar image of Hurricane Katrina as itapproached Miami. This is shown as a movie on thisbook's CD.

> Ff,S{JRE S Radar image of Hurricane Katrinaas it approached the Louisiana coast. This isshown as a movie on this book's CD.

BAtr REFLECTIVIIYKLIX. NEW ORLEAN' LA08/291206 09127:08 GMILAT:30/20/13NLoN: 891a9/30 wELEV: I38.OFTrcDEivcPrA/ l2l

EtflANGLE|0.s0cux:5t d&RAKE2{ANM

about to be hit by a catastrophic storm.Bounded by Lake Pontchartrain to thenorth and surrounded by the winding Mis-sissippi River, the city-much of which is

below sea level-had long been known to

be extremely vulnerable. The levees pro-tecting much of the city were believed tobe able to withstand a direct hit from aCategory 3 hurricane, but they had neverbeen seriously tested, as they would be

soon. Meanwhile, coastal Mississippi ar_Alabama were lined up on the right han_side of an enormously powerful hurricar=track.

those tracks as predictors for current storms. Dynami-cal models take information on current atmosphericand sea surface conditions and apply the governinglaws of physics to current data. Hybrid models com-bine elements of statistical and dynamical models. Themodels repeatedly forecast the movement and internalchanges of hurricanes for short time increments andthen print information on projected storm positions, air

pressure, and wind at 6-hour intervals. Not sur-prisingly, model forecasts become less accurate as learitime increases and are unreliable for more than abor,r:72 hours.

Hurricane forecasting requires a tremendousamount of data and places a great demand on compute:hardware. For both reasons, the NHC has recently under-gone substantial modernization. The National Oceanic an,-

Sunday, August 28: LandfalllmminentBy late Sunday night the hurricane wasjust offshore (Figure 1f). Those who had

set out to evacuate were long gone.

Those unable to leave were directed toshelters or hunkered down to take theirchances at home. That morning, windshad easily exceeded the threshold for aCategory 5 hurricane, with sustaìnedwinds of 282 km/hr (175 mph). The hur-ricane had become as intense as Hurri-

cane Camille, which devastated coastalMississippi in 1969, and was even largerrFigure 3). Katrina's minimum air pres-

sure of 902 mb was the fourth lowestever observed for an Atlantic storm.Television news networks that had beenfollowing the storm nonstop relayed

warnings from the NHC: There wasabsolutely no doubt that bad things'ryere in store for the Gulf Coast.

Monday, August 29: Landfall

lurricane Katrina made landfall earlyMonday morning. Coastal Louisiana wasoattered first as a huge storm surge over-

look the area. Though the storm had

,veakened enough to be a strong Cate-

Eory 4, everybody knew that a historic

Jisaster was occurring. The eye tracked

.r:st to the northeast of New Orleans, put-

- ng the cily on the less-threatening "left

side" of the storm. This initially gave the'alse ìmpression that the city had nar-'cwly escaped a disaster. This, of course,

oroved entirely incorrect. As New Orleans,,,as subiected to wind speeds on the:rder of 160 km/hr (100 mph), Lake)ontchartrain waters rose along the lev-

:es, which were unable to hold themcack. Eighty percent of the city came

-nderwater (Figure 4).

For several weeks after the disaster,

.ne U.S. Army Corps of Engineers argued

that the levees were overtopped in places

and that turbulent floodwaters under-mined the base of the concrete walls

atop the levees. Later assessments (still

subject to further analysis at the time this

book was going to press) suggested that

this may have occurred in some loca-

tions, but in other places the walls gave

way without having been overtopped by

the storm surge.ln some areas of coastal Mississippi,

the flood surge wreaked total devastation.

Hurricane Forecasts and Advisories 395

Farther inland, wind destroyed all of ormost of many communities. Figure 2 in

Box 12-2 plots the entire track of Hurri-

cane Katrina, and this book§ CD includesa movie compiled from satellite imagesthat shows the entire movement of Kat-rina from the time it approached Florida tolandfall in Louìsiana and Mississippi. An

enormous amount of information is alsoavailable on the Web. Check out the Web

sites at the back of this chapter for links tovaluable resources.

.ltnrospheric Administration (NOAA) has deployed newgeostationary satellites (meaning they remain over fixedIrrcations on Earth), GOES 10 and 72, to provideimproved data acquisition. NOAA has also purchased a

new jet airplane that can fly higher into a hurricane thanits older propeller airplanes. Furthermore, an improvedslrpercomputer can run increasingly complex models-'rith increased accuracy.

The improvement in hurricane forecasting inrecent years has been substantial. In 1965 the average

error in the 24-hour forecast position of an Atlantic hur-ricane was about 250 km (155 mi). By the end of thetwentieth century, that figure had been cut to about160 km (100 mi). As you would expect, there is less

accuracy for the 48-hour forecasts, with an average

error of 260 km (160 mi). Still, the movement of hurri-

A iijtÉ{;,,i}-l* ,i Downtown New Orleans under water.

396 CHAPTER 12 Tropical Storms and Hurricanes

Louisiana

Wind onleft handside of Hurricanehurricane movement

A FIGURE 12-18 The varying intensities of the wind on the left and right sides of hurricanes. Thedirection and speed of the wind components are signified by the direction and length of the arrows,respectively. A hypothetical hurricane moves northward at 50 km/hr (30 mph). Along the right handside, the 200 km/hr (120 mph) winds are in the same direction as the movement of the storm, so thereis a net wind speed of 250 km/hr (1 55 mph). On the left side, the internal winds blow opposite thedirection of storm movement, so the net winds are southward at 150 km/hr (90 mph). Because theirwinds spiral toward the eye in a clockwise direction, Southern Hemisphere hurricanes have strongestnet winds on the left side.

ISpeed anddirection ofhurricanemovement

INetwind

HurricanemovementtI

Wind onright handside ofhurricane

Netwind

I

canes is particularly difficult to predict. (Longer-rangeforecasts are discusse d rn Box124, F orecasting: Seasonaland Long-Term Hurricane F orecasts.)

HURRICANE WATCHESAND WARNINGS

When forecasters at the NHC predict that an approach-ing hurricane will reach land in more than 24 hours, theyissue a hurricane watch. If it is expected to make landfallwithin 24hours, they issue a hurricane waming. Hurri-cane wamings do not guarantee that a particular stormwill hit any particular locale. Rather, they are issued in

terms of probabilities that a hurricane will make landfa-_at given points. For example, a hurricane waming ma..cover a coastline 500 km (300 mi) long, with a 25 percer:probability for a"hit" near the center of the waming are:and a 5 percent chance along the margins. Overall, tl-.."hit rate" for hurricane warnings is about 20 percent. I:other words, there is a one in five chance of the hurricar..making landfall somewhere in the waming area.

The erratic nature of hurricanes makes ther:-notoriously difficult to predict. When predicting hu:-ricane movements, forecasters must weigh the effecr.of issuing watches or warnings for hurricanes tha:never make landfall versus the consequences of fai--ing to issue a watch or warning for a storm that ult--

DID YOU KNOW?Between 1992 and 2001 the average forecast errorfor the 24 hr hurricane landfall position was 149 km(92 mi). lf you live near a threatened coastal zone,you must keep in mind that the endangered areaextends far beyond the point of landfall. Hurricane-force winds can occur for distances well exceeding160 km (100 mi) from the eye in any direction. Soeven if the point of expected landfall is far away or ifthe eye of the storm is still offshore, you may besubject to extremely hazardous hurricane conditions.

mately does hit. Obviously, the failure to warn peopleto evacuate may lead to unnecessary loss of life andproperty. On the other hand, false warnings haveserious ramifications, especially if they occur repeat-edly. Repeated false warnings can make the public socomplacent that people will eventually disregardn,arnings that prove accurate.

Evacuations based on these advisories haveimmense economic costs for the general public, gov-ernment agencies, and industry. Local residents andsmall businesses have their lives thoroughly disruptedas they board up windows and prepare to evacuate ortake sheltet while large industries (such as petroleummining and processing) incur costs measured in tens ofmillions of dollars from having to shut down andreopen their plants.

HURRICANE INTENSITY SCALE

In addition to alerting the public to the location and pro-jected movement of hurricanes, meteorologists use a

simple scale to categorize their intensity. The Saffir-Simpson scale (Table 12-2) classifies hurricanes into fivecategories based on the highest current 1-minute aver-age winds in the hurricane. Generally, higher-categoryhurricanes have lower central Pressures and largerstorm surges. Though storm surges are usually the moredestructive element of hurricanes upon landfall, thescale is based on wind speeds because storm surges are

affected by nonmeteorological factors such as coastalconfiguration and the steepness of the offshore continen-tal shelf. Extremely violent hurricanes are rare, with only3 Category 5 and 16 Category 4 hurricanes having hit themairùand United States between 1900 and 2005.

Of course, Category 4 and 5 hurricanes are far moredeadty and devastating than lower-category hurricanes.The effects of Hurricane Camille (Category 5) in 1969 havealready been described. Since theru 16 Category 5 hurri-canes have occurred in the GulJ of Mexicq Caribbean Sea,

or westem Atlantic, and orùy three of those struck land atfulICategory 5 intensity: Andrew lri.1992, and Dean andFelix, both n 2007. But even though only a handful ofCategory 5 hurricanes made landfall at that maximum

Recent (and Future?) Trends in Hurricane Activity 397

level, the majority of these extreme hurricanes havemade landfall at some point in their lifetimes, often withcatestrophic consequences. The great Galveston hurri-cane of 1900 that killed 6000 persons (see Box 12-5, Special

lnterest: The Galaeston Hurricane of 1.900) is believed tohave been a Category 4 hurricane. Table 12-3 lists all theCategory 4 and 5 Atlantic-Gulf of Mexico{aribbeanhurricanes between 1900 and 2007, andBox12-6 Special

lnterest: Recent Deadly Cyclones describes some notewor-t§ cyclones elsewhere.

Recent (and Future?) Trendsin Hurricane ActivityThe exkemely destructive seasons of 2004 and 2005

greatly heightened the public's awareness of the dangerhurricanes pose. Florida made repeated headlines as fourhurricanes hit it in 2004, and the Gulf Coast witnesseddestruction of historic proportions in 2005, complimentsof Hurricanes Katrina and Rita. These events stimulatedpublic and media interest about whether the recentupsurge in Atlantic hurricane activity was related to nat-ural periodic cycles, global warming, or both.

One thing we know for certain is that the AtlanticMultidecadal Oscillation (AMO), a25- to O-year oscilla-tion in water temperatures (Figure 12-79), has been a

major factor in the increase in Ailantic hurricane activity-and especially in strong hurricanes. Figure 12-20 plots theannual number of Atlantic-named systems (tropicalstorms and hurricanes), hurricanes, and Categories 3

through 5 hurricanes from 1851 through 2007. Theperiod between the early 1970s and mid-1990s wasone of relatively low Atlantic sea surface temperaturesand hurricane activity. In fact, the years 1991 through1994 had less Atlantic hurricane activity than anyother 4-year period on record (despite the Category 5

Hurricane Andrew in 1992). Then came an abrupttransition to a very active period beginning in 1995 thatcoincided with a shift in the AMO, and the 7995-99

period proved to be the most active S-year period onrecord (41 hurricanes)-at least until that record wassurpassed in 2001-05 (44 hurricanes).

Has global warming also influenced the increase

in hurricane activity? Currently there is little, if any,

evidence to indicate that the increase in Atlantic tropi-cal storms and hurricanes is due to a longer-term trendin sea surface temperatures. In fact, the increase inAtlantic tropical storm and hurricane activity has notbeen observed in the other ocean basins around theglobe-despite the fact that worldwide tropical oceantemperatures have increased by about 0.5"C (1'F)between 7970 and2004.

The situation might be different with regard to theeffect on intense hurricanes of long-term sea surfacewarming. Research published in 2005 showed a near

398 CHAPTER 12 Tropical Storms and Hurricanes

Scientists are curently developing tech-niques for predicting the number andintensi§ of hurricanes and tropical stormson a seasonal basis. The best-knownforecasts are those put out by theresearch group at Colorado State Univer-sity, headed by Professor William Gray(now semiretired). Previously issued inNovember, June, and August (andsince 2003, updated in September andOctober), the forecasts predict the num-ber of tropical storms, hurricanes, intensehurricanes, and the number of days witheach, that will occur each season in thewestern Atlantic, Caribbean Sea, andGulf of Mexico. The forecasts do notpredict characteristics of individualstorms, such as date or landfall.

The seasonal forecasts are based on anumber of meteorological and oceano-graphic conditions around the globe,including the presence or absence of El

Nifro conditions, seasonal rainfall in west-ern Africa, temperature, stratospheric andupper-tropospheric wind directions, andair pressure over the Caribbean Sea. Thetheoretical underpinnings that make theseclimatological conditions good hurricanepredictors are supported by empiricalobservations. For example, a recent study

showed that during El Niflo years, an aver-age of 1.04 hurricanes hit the U.S. main-land, in contrast lo lhe 2.23 that hit duringLa Nifra years and the 1 .61 that hit in yearsthat are neither El Nifro nor La Nifla years.

As shown in Table 1, the group hashad mixed success in recent years. Theirpredictions for 2003 were reasonablygood, except for the underestimation ofthe number of intense hurricane days.They also successfully predicted theextremely active 2005 season, especiallywith regard to the number of intensehurricanes and hurrican days. The 2004and 2006 forecasts could not be consid-ered successful, however. ln 2004 theforecast team called for an above-averagehurricane year and was largely on tar-get with regard to the number of tropi-cal storms and hurricanes. But theybadly underestimated the number oftropical storm days, hurricane days,intense hurricanes and intense hurri-cane days. ln 2006 and 2007 theyerred on the side of overprediction. ln2006 they predicted 35 hurricane days;Only 20 occurred. ln 2OO7 they againforecasted 35 hurricane days and only11 actually occurred-the lowest num-ber since 2002.

Even beyond the accuracy issues, long-range huricane forecasting has someinherent problems. One of these involvesthe issues of where and when. For exam-ple, say an intense hurricane year is cor-rectly forecast, as in 2005. There wasnothing in that seasonal forecast to indr-

cate one of those hurricanes would neari,,

decimate a mqor city, New Orleans.

Louisiana. There is also a possibility that aforecast of a relatively inactive seasorcould cause coastal residents to be under-prepared. So even if only a single hurricanewere forecast and that forecast turned ou:to be accurate, it would be useless at bes:and possibly counterproductive if in tha:year the lone hurricane made a direct hion a major city. Morevoer, incorrect sea-sonal forecasts could diminsh the public s

confldence in all forecasts, including short-term ones that have a much greate-degree of precision and accuracy.

The NOAA Climate Prediction Cente.(CPC) and the National Hurricane Cente.likewise jointly issue their own seasonaforecasts each spring. These forecasts areavailable online (see the Web link at theend of the chapter) but come with specifrcstatements emphasizing the uncertaintiesand shortcomings of such predictions.

> FIGURE 12-19 A timeseries of an index representingthe Atlantic MultidecadalOscillation from 1856 throughthe spring of 2008.

0.6

o.4

E

E 0.2(d

I o.oo

9 -0.2

Monthly values for the AMO index, 1856-2008

1860 1880 1 900 1920

Year

1 940 1960 1980

Recent (and Future?) Trends in Hurricane Activity 399

< FIGURE12.20The number ofAtlantic tropicalstorms (purple),Category 1-2hurricanes (orange),

and Category 3-5hurricanes (green)

by year, 1851-2004.otr6(.)aaEq)

€oC-Jz

b o ro o lr) o ro o ro o tr) o to o tl? I to o to o r-) a lQ I n o to o to o Lo

H H H X ai É EÉ E Ed H Ei ; ; § s H E É a E E E E E § E 3 E E E Er;;r-c\lN

2oo3 2004 2oo5 2006 2007

predicted Observed predicted Observed Predicted Observed Predicted Observed Predicted Observed

26159 15

75

8

35

4

10

14

34

6

11

2

6

Numberof 14 14TropicalStorms

Numberof 60 71

TropicalStorm Days

Number of 8 7

Hurricanes

Numberof 25 32HurricaneDays

Number of 3 3lntenseHurricanes

Number of 5 17

lntense Hur-ricane Days

NetTropical 120% 173%CyclongActivity

90 95 116 75 50

source: Data from http://hurricane.atmos.colostate.edu/Forecasts/2008/june2008/jun2008.pdf.

*An overall indicator of tropical storm and hurricane activity. lt is defined as the average seasonal percentage of the total number of tropical storms, tropical

storm days, hurricanes, hurricane days, intense hurricanes, and intense hurricane days in the year, relative to their long-term averages'

13

55

7

30

3

6

125%

2035

14

48

10

5546

14 20

229o/o 295% 263% 14Oo/o 85% 160% 94%

171822

I

I

400 CHAPTER 12 Tropical Storms and Hurricanes

Pressure Wind Speed Storm Surge

Category mb km/hr mph m ft Damage

1 >980 119-154 74-95 1-2 4-S Minimal. No major damage to mostbuilding structures.

2 965-979 155-178 96-1 10 2-3 6-8 Moderate. Some roof, door, andwindow damage. Some trees blowndown. Considerable damage tomobile homes.

3 945-964 179-210 11 1-130 3-4 9-12 Extensive. Some structural damageto small residences. Some large treesblown down. Some mobile homesdestroyed.

4 g2}-g44 211-250 131-155 4-6 13-18 Extreme.Some complete roofstructure failures on smallresidences. Many shrubs, trees, andall signs are blown down. Completedestruction of mobile homes.

5 <920 >250 >155 >6 >18 Catastrophic.Some completebuilding failures. All shrubs, trees,and signs blown down.

doubling in the number of Category 4 and 5 hurri-canes in the western North Pacific, western SouthPacific, Indian, and Atlantic oceans since 1970 thatcoincides with increasing water temperatures. Inter-estingly, the North Atlantic experienced the smallestincrease in major hurricanes among those basins.Another highly cited 2005 study pointed to a substan-tial increase in the overall energy released by Atlantichurricanes in recent decades, reflected in both hurri-cane intensity and duration. That article noted, how-ever, that only part of the increase in Atlantichurricane activity could be ascribed to increasing sea

surface temperatures (an assertion that was not widelyreported by the popular press). And yet another 2005article used the results of computer simulations toobtain the same conclusions based on empirical obser-vations. Thus, there is better reason to suspect thatglobal warming might influence the intensity of hurri-canes than the number of hurricanes.

Based on the above considerations, it appearsthat an increase in hurricanes and intense hurricanesmay be a fact of life for residents along the Gulf ofMexico and the Atlantic coast in the years to come-

at least for a few decades. Whether we are witnessinga longer term trend is currently unknown. Factorsbeyond sea surface temperatures could affect theintensity of hurricanes in a world with higher air ancìsea surface temperatures, and some of these factorscould make it harder for hurricanes to develop in a

warmer world.We are more vulnerable than ever to the destruc-

tive potential of hurricanes for the simple reason tha:there has been enormous population growth along theAtlantic and Gulf coasts in recent decades. This rr-il-continue to make evacuations of large populationsmore difficult in response to approaching hurricane=(as witnessed during the evacuation of southeast Texa:as Hurricane Rita approached in 2005).

Without question, hurricanes are among the mos:exciting of natural phenomena, a fitting subject for cor.-cluding this section on weather disturbances. Theremaining sections of this book will examine huma:activities and meteorology, and climate and climat.change. In our next chapter we turn our attention t;weather forecasting.

Recenl (and Future?) Trends in Hurricane Activity 401

Category 4 Hurricanes

Name

Claudette

Felix

Luis

Opal

Edouard

Hortense

Georges

Bret

Cindy

Floyd

Gert

Lenny

lsaac

Keith

lris

Michelle

Liti

Fabian

Charley

Frances

Karl

Dennis

Year

1 991

1995

1995

1995

1996

1996

'1998

1 999

1 999

1999

1 999

1 999

2000

2000

2001

2001

2002

2003

2004

2004

2004

2005

Month

September

August

August

September, October

August, September

September

September

August

August

September

September

November

September, October

September, October

October

November

October

September

August

August

September

July

August

October-November

September

September

July

August

September

October

August

September

MaximumSustained

Winds (km/hr)

215

220

220

240

230

220

250

230

220

250

240

250

220

220

230

220

230

230

230

230

230

240

MaximumSustained

Winds (mi/hr)

130

't40

140

150

145

140

155

145

140

'155

150

155

144

140

145

140

145

145

145

145

145

150

MinimumPressure (mb)

946

929

935

919

933

935

937

944

942

921

930

933

943

941

938

934

940

939

947

937

938

930

Category 5 Hurricanes

Andrew

Mitch

lsabel

lvan

Emily

Katrina

Rita

Wilma

Dean

Felix

1 992

1 998

2003

2004

2005

2005

2005

2005

2007

2007

280

285

270

270

260

280

285

295

280

280

175

180

165

165

160

175

'180

185

175

175

922

905

915

910

929

902

895

902

905

929

402 CHAPTER 12 Tropical Storms and Hurricanes

;'i.l ;";q,.,,1ii;fu i]::.. 11 1:,

Some natural disasters are so embeddedin our folklore that virtually everybodyknows about them. We have all heardabout the San Francisco eadhquake of1906 and the Great Chicago Fire of1871 . Yet the single deadliest natural dis-aster in U.S. history the Galveston

fexas) hurricane of 1900, seems to havebeen lost from the national memory. lnjust a few hours, rising sea waters andheavy surf drowned 6000 persons onGalveston lsland-a narrow strip of landthat peaks at less than 3 m (9 ft) abovesea level (Figure 1).

The loss of life resulted not from lackof warning but rather from a failure totake the lhreat serjously. Two days ear-lier, a strong storm was reported movingwestward into the Gulf of Mexico offCuba, and ships returning from the Gulfof Mexico reported encountering thestorm offshore the day before it madelandfall. Furthermore, the local weatherforecaster, lsaac Cline, observed thecombination of winds and heavy surfalong the local beach and deduced thatthe storm would move onshore. But evi-dence of the impending landfall seemsto have been largely unheeded, in partbecause some meleorologists erro-neously believed it was virtually impossi-ble for a storm in the Caribbean to trackacross the Gulf. Scientists (includingCline) were also erroneously convincedthat the gently sloping seafloor offshorewould protect Galveston from majorflooding in the event of a hurricane.

There is some uncertainty as to whenpeople started to take the hurricane seri-ously. According to Cline's account ofthe disaster, he rode lhrough Galvestonlsland urging residents to evacuate, butrecent research casts doubt about thedegree to which he actually warned thepopulace. Regardless of how urgent

Clines warnings were, however, fewpeople evacuated, and some residentseven rode to the beach to watch theheavy waves crash against the shore.

When the hurricane arrived, the peo-ple of Galveston had no way to escape.Within hours the rising seas completelycovered the island so that the only poten-tial sheller was in taller, well-built struc-tures. Even these failed to withstand thepounding of waves and debris. Cline latergave the following account of his ordeal:

By 8 clr. a number of houses haddrifted up and lodged to the east andsoutheast of my residence, and thesewith the force of the waves acted as abattering ram against which it wasimpossible for any building to stand forany length of time, and at 8:30 P.M., myresidence went down with about fiftypersons who had sought it for safely,and all but eighteen were hurled lnto

eternity. Among those lost was mywife, who never rose above the waterafter the wreck of the building.

Cline and his brother were luckier ancgrabbed onto floating debris that helpecthem stay afloat. After 3 hours, thefloodwaters subsided and the Clineswere on solid land, among the survivors

The horror did not end with the pas-sage of the hurricane, There were st6000 bodies to deal with. Some wer:taken out to sea on barges, but man-washed back to shore. Ultimately, mcs.of the bodies were cremaled where the.were found.

With our current ability to lrack an:forecast the movement of approachin:hurricanes, there is no reason for :repeat of this type of loss of life in Nor:-America. But hurricanes will alwai.present a threat to Gulf and Atlani :coasts that must be respected.

^ FIGURE 1 The Galveston hurricane of 1900 was the deadliest natural

disaster in U.S. history.

]ì''i:*;;,,r.1. .,

-lurricanes that make landfall over theJnited States can be deadly, but nothing

rn the scale of what has been witnessed

r other parts of the world due to cyclones

and §phoons. ln 1970 Bangladesh (then

cart of Pakistan) was hit by a tropical

:yclone that killed between 300,000 and

500,000 people. This dìsaster led to the

:onstruction of more than 2500 concrete

shelters on pillars (Figure 1)to protect res-

Cents against future cyclone hits. These

shelters have undoubtedly saved hun-

Jreds of thousands of lives, first in 1991

.vhen a cyclone hit the country with heavy

'ain, Category 5 winds, and a 9 m (30 ft)

storm surge. Despite the fact that this was:re strongest cyclone to hit the country inrrore than a century the death toll was

about 70,000-a horrific number but far

smaller than that of the 1970 cyclone. The

shelters once again saved tens of thou-sands of lives in November 2007 when

another Category 5 cyclone, Sidr, hit thelountry Estimates of the number of fatali-

. es have varied between 3000 and- 0.000-another terrrible number but far

ess than that which would have occurred

.'rithout the shelters.ln May 2008 Tropical Cyclone Nargis

=igure 2) made ìnternational news when: hit Myanmar (formerly called Burma) at

.s peak strength, packing peak winds:stimated at 213 km/hr (132 mph) and

croducing heavy rains and a 3.7 m (1 2 ft)

siorm surge. The disaster was intensi-

'ed by the ruling military dictatorship that:enìed entry to international relief workers:rying to bring food and medical supplies

:c the country. At least 77,OOO PeoPle:ied from Nargis-perhaps as many as- C0,000-which would make it the dead-est cyclone to hit Asia since the 1991

siorm. Two to three million people were

:ft homeless.Much of the coastline of South Asia is

cerfectly situated for disasters such as

:nese. Low-lying coastal areas, often^ear rivers that can ovedlow their banks,

and a poorly developed infrastructure forshelter and evacuation put the lives of-lillìons of people in great danger. lt is a

:''agic fact that events such as the onestescribed here not unlikely to be re-

ceated at great human cost.

Becent (and Future?) Trends in Hurricane Activity 403

^ FIGURE 't One of more than 2500 cyclone shelters set up in

Bangladesh after the catestrophic cyclone of '1970.

^ FlGLrRH 2 Satellite image of Tropical Cyclone Nargis

approaching Myanmar.

4O4 CHAPTEH 12

SUMMARY

Tropical Storms and Hurricanes

Hurricanes (and their counterparts such as typhoonsand tropical cyclones) are extremely powerful stormsthat originate in tropical regions and migrate into themiddle latitudes. They bring enormous destruction andloss of life to many coastal regions of the world. Thehurricane that hit Galveston Island, Texas, in 1900 wasthe greatest single natural disaster to hit North America,with a death toll of 6000. This figure pales in compari-son to the hundreds of thousands of fatalities associatedwith individual tropical cyclones in southern Asia.

Most hurricanes begin their life cycles as uneventfultropical disfurbances, small clusters of thunderstorms.When they intensify and organize into a rotating bandof cloud cover and thunderstorm activity, they arecalled tropical depressions. Further intensification resultsin their being classified as tropical storms, or hurri-canes if their sustained wind speeds exceed 120 km/hr.Because strong tropical storms can form only overoceans having high surface temperatures, tropicaldepressions most often become tropical storms andeventually hurricanes over the western portions ofthe ocean basins.

Hurricanes are smaller than midlatitude cyclonesbut much larger than tornadoes. They can last for aweek or more and travel thousands of kilometersbefore dissipating. The heaviest thunderstorm activityoccurs within bands of thick cloud cover that spiraltoward the center of the system in a pinwheel pattern.The intensity of the storm increases toward its centeruntil reaching the eye wall, the concentric zone of max-imum activity that surrounds the eye. The eye of a hur-ricane is strikingly different from the rest of thehurricane because it is marked by generally clear skies,

light winds, and higher air temperatures. Often it ishard to discern the true structure of a hurricane fromabove, because the anticyclonically rotating outflow ir.the upper troposphere creates a blanket of cirrostratu:clouds overlying the thicker cumulus.

Hurricanes can produce damage in several wavsCopious amounts of rain can bring intense floods, ancstrong winds can bring down structures. The most serj-ous threat posed by a hurricane is the storm surge, theelevated rise in sea level due to low atmospheric pre=-sure and the piling up of water by strong winds. Whe:-the storm surge coincides with a high tide, the flood-waters (coupled with heavy surf) can penetrate consid-erable distances inland.

The National Hurricane Center of the Nationa-Weather Service uses a sophisticated network of satel-lites, research aircraft, and computer hardware anùsoftware to issue advisories on the likelihood of hurrl-cane landfall. The erratic nature of hurricanes make.predicting them particularly difficult, but recent moi-ernization at the National Weather Service has substar.-tially increased forecast accuracy.

Hurricanes have become more frequent in theAtlantic Ocean since the mid-1990s, and several partic-ularly devastating hurricanes left their mark on Unite:States coastal areas in 2004 and 2005. This increase:activity is partly due to a shift in the Atlantic Mult-decadal Oscillation, a25- to }-year cycle on ocean ten--peratures, that occurred around 1995. While th.potential impact of global warming on hurricanes i.not entirely understood, it is likely that global warmir:will lead to more intense hurricanes rather than mo:.frequent hurricanes.

Review Questions 405

WEATHER IN MOTIONThe 2005 Hurricane SeasonSatellite movie showing all tropical storms and hurri-canes in the Atlantic from |une 1 to November 1, 2005.

Hurricane Katrina IA satellite movie showing Hurricane Katrina from thetime it approaches Florida to its second landfall inLouisiana and Mississippi.

Hurricane Katrina 2Radar loop of Hurricane Katrina as it hits Florida.

Hurricane Katrina 3Radar loop of Hurricane Katrina prior to hitting theGulf coast.

Hurricane RitaView the movement of Category 5 Hurricane Rita as itmoves over the Gulf of Mexico.

Hurricane WilmaHurricane Wilma as it moves toward Florida.

A Fly-Through of Hurricane MitchThis movie opens with a view of this devastating hur-ricane obtained by satellite sensors. As the moviezooms in on the hurricane, the overlying cirrus cloudsare stripped away so that the interior structure of thestorm is evident. This movie was compiled using

microwave, visible, and infrared imagery obtained bythe Tropical Rainfall Measuring Mission (TRMM).

Hurricane SliceThis brief movie shows the pinwheel structure of a hur-ricane after clearing away the overlying cirrus.

Hurricane Eye WallAview of a hurricane eye wall as seen from a hurricanechase plane. Note the abrupt transition from clear airwithin the hurricane eye and the massive wall of cloudencircling the eye.

Hurricane DennisA computer-enhanced movie showing the movementof Hurricane Dennis along the Atlantic Coast.

Storm SurgeThe combination of low pressure and strong onshorewinds can bring elevated sea levels and strong wavesashore. This scene shows what a moderate storm surgecan look like.

Hurricane DamageAview of severe inland damage in the wake of a majorhurricane, as seen from a helicopter.

lnterview with Chase Plane PilotHave you ever wondered what it must be like to flyinto a hurricane? Take a look at this film clip and hearfirsthand from an expert.

KEY TERMShurricane page372

typhoon page372

ryclone page372

trade windinversion page373

marine layer page373eye page376

eye wall page376

double eye wallspage376

hot towers page376

tropicaldisturbance page376

easterly waves page 377

tropical depressionpage378

tropical storm page378

storm surge page387

hurricane watchpage396

hurricane warningpage396

Saffir-Simpsonscale page397

Atlantic MultidecadalOscillation page397

406 CHAPTER"l 2 Tropical Storms and Hurricanes

REVIEW QUESTIONS1. Describe the geographic distribution of hurricanes,

typhoons, and cyclones. \vVhat environmental con-ditions at these locations favor the development ofsuch storms?

2. \Arhich region has the greatest incidence of majortropical storms?

5.

6.

What is the trade wind inversion, and what impactdoes it have on the formation of hurricanes?

Describe the size, sea level air pressure, and windspeed of a typical hurricane.

When are hurricanes most likely to form?

Describe the cloud and precipitation patterns asso-

ciated with hurricanes, including those associated

with the eye and eye wall.

Describe the various ways in which hurricanes dif-fer from midlatitude cyclones.

What are tropical disturbances, and how do east-erly waves influence them?

Describe the characteristics that distinguish tropi-cal disturbances, tropical depressions, tropicalstorms, and hurricanes from each other.

CRITICAL THINKING

L0. What ocean surface characteristics are required forthe intensification of storms into hurricanes and

the maintenance of hurricanes?

11. Is there a "typical" path that hurricanes take afterforming? Explain.

VVhat feature associated with hurricanes causes the

greatest destruction to coastal regions? Is this also

true of inland regions?

Why is the right hand side of a hurricane (relativeto its direction of movement) the most dangerous?

Where are tornadoes most likely to be embeddedin a hurricane?

What are hurricane watches and warnings? Arethey exact corollaries to tornado watches andwarnings?

Why are forecasters concerned with issuing hurri-cane advisories for areas that do not eventualh'get hit?

What is the highest hurricane category on the Saffir-Simpson scale? How frequently do hurricanes ofthat magnitude occur?

12.

1.3.

't4.

15.

4.

1.6.

17.

7.

8.

9.

1.

2.

3.

4.

Why don't hurricanes cross the equator?

If two hurricanes pass just to the west of Cuba over a

2-week period, what reasons might one have forexpecting the second one to be weaker than the first?

How might previous drought conditions affect theintensity of a former hurricane as it passes over thesouthern United States?

El Nifro conditions are believed to suppress hurri-cane development in the Atlantic. How might thephenomenon affect hurricane formation and move-ment in the Pacific?

It has been postulated that an increase in globaltemperatures could lead to an increase in the num-ber and intensity of tropical storms and hurricanes.Global temperatures were particularly high duringthe 1990s and early 2000s, and there has been an

increase in Atlantic hurricane activity since 1995.

Does this association prove the connection betweer,temperature and hurricane activity? Explain r,r'hr

or why not.

If global warming continues, thermal expansion oithe oceans and the melting of glaciers will lead to a

higher sea level. How would this affect ihe threat o:storm surges relative to wind damage and floodingl

Experts believe that New York City is the third mos:dangerous city in the United States with regard tt'hurricanes, despite the fact that there has been nt'major hurricane-inflicted damage on the area (othe:than some wind damage and coastal erosion fron'.

Hurricane Gloria in 1985). What factors could b.responsible for this vulnerability? After answerin!this question, refer to http://www2.sunysuffolk.edu'mandias/38hurricane/ for an informative discussioi-of this issue.

7-

5.

UsefulWeb Sites 4O7

PROBLEMS AND EXERCISESCompare the area of a hurricane that measures600 km in diameter to a midlatitude cyclone hav-ing a diameter of three times greater (1800 km).

During the tropical storm season, use the Web sites

described below to note the positions of currentsystems and the probabilities of landfall at variouscoastal locations. Describe how successful the pre-dictions proved to be.

Refer to the forecast for the upcoming tropical stormseason at http://tropical.atmos.colostate.edu/. \zVhat

existing conditions have led the forecast team tomake its prediction? Also, use this Web site to deter-mine how successful last year's forecast was.

4. Refer to http://www.ncdc.noaa.gov/oa/climate/severeweatherÀurricanes.html and read the SpecialReports on the past year's hurricane activity. Were

any tropical storms particularly noteworthy?

QUANTITATIVE PROBLEMSYou can gain a deeper understanding of hurricanes byrvorking out some numerical problems. These are avail-able from this book's Web site, ww'w.mygeoscienceplace

USEFUL WEB SITES

.com. Log on to the site and go to the Chapter 12 section

for some thought-provoking problems.

http ://www.nhc.noaa. gov/Official Web page for theTropical Prediction Center.

http ://www.wunderground.com/tropical/Site opens with a map showing current tropical activityand provides numerous links for satellite images, advi-sories, outlooks, and discussions.

http //www.nrlmry.navy.mil/tc-p ages/tc-home.htmlComprehensive information provided by the Monterey\aval Research Laboratory.

http://cimss.ssec.wisc.edrl/tropic/tropic.html-\rchived and real-time imagery and a large amount oftext in-formation from the University of Wisconsin-\ladison Tiopical Cyclone Research Team.

http//www.solar.if a.hawaii.edu/IropicaUtropical.htmlCurrent hurricane information and several data archives.

http ://www.usatoday.com/weather/hurricane/hurricane-resources.html>ome basic information from US A Today.

http ://www.ncdc.noaa.gov/oa/climate/severeweather/hurricanes.htmlClimatological information and data on past hurri-canes from the National Climate Data Center.

http://tropical.atmos.colostate.edu/Official Web site of the Tropical Meteorology Project at

Colorado State University. Includes much information,such as the seasonal tropical storm forecast and a descrip-tion of factors used to predict the upcoming season.

http ://www2.sunysuf f olk.edu/mandias/38hurricane/A comprehensive and interesting review of the hurri-cane of 1938 and what it tells us about the vulnerabilityof the New York area to future events.

http://www.ncdc.noaa. gov/oa/repoÉs/tech-report-2005012.pdfAn excellent analysis of Hurricane Katrina.

http://www.cpc.noaa.gov/products/outlooks/trurri-cane.shtmlThe latest seasonal hurricane forecast issued each

spring by the NOAA Climate Prediction Center.

Hurricane Center