Weather Systems

IntroductionAir MassesFrontal SystemsMid-latitude CyclonesThunderstormsTornadoesHurricanesSummary

A great thunderstorm; an extensive flood; a desolating hurricane; a suddenand intense frost; an overwhelming snowstorm; a sultry day, - each ofthese different scenes exhibits singular beauties even in spite of thedamage they cause. Often whilst the heart laments the loss to the citizen,the enlightened mind, seeking for the natural causes, and astonished at theeffects, awakes itself to surprise and wonder.

St. John de Crévecoeur

2

Introduction• Weather is the state of the atmosphere at a given time and

place.• Extreme weather events threaten lives, disrupt

transportation systems, and cause destruction.• The National Weather Service makes over a million daily

weather observations that will be used in forecasts bymedia sources nationwide.

• Advanced technology such as weather satellites andDoppler radar allow the accurate prediction of dangerousweather systems like tornadoes and hurricanes.

No scientific phenomena concern us as much as the dailyevolution of weather systems. We live in a culture whereweather, the state of the atmosphere at a given time and place,helps us define regional cultural variations. States suchas California and Florida are defined in part by their warm,sunny weather. Seattle, Washington, is known for its rain,North Dakota and Minnesota for cold winter temperatures, andOklahoma for tornadoes. Superimposed on the regular rhythmsof the atmosphere are more extreme events that threaten lives,disrupt transportation systems, and cause destruction (Figs. 1,2). Approximately 90% of presidential disaster declarationsare weather related. In the dozen years between 1988-1999,there were 38 U.S. weather disasters that generated at least $1billion in damages. In 1998 alone there were seven billion-dollar weather disasters (Fig. 2).

Figure 1.Extreme weatherevents in thelower 48 states.Note hurricanesare found alongAtlantic and Gulfcoasts, andtornadoes aremost commonover the GreatPlains and upperMidwest.

3

The first half of the chapter is divided into three sections thatdescribe how common weather systems develop across muchof the nation. Weather in any region is influenced by theatmospheric changes that occur when masses of air withcontrasting properties interact. The characteristics of airmasses vary with location ranging from dry and cold to warmand humid. The daily clash of air masses over North Americagenerates our common weather patterns characterized by high-and low-pressure systems bounded by warm and cold fronts.These frontal systems are relatively narrow, curvilinear zonesthat mark a transition from one air mass to another. Weatherexperienced over much of the central and eastern U.S. is theresult of the west-to-east migration of regional-scale low-pressure systems, termed mid-latitude cyclones, and theirassociated warm and cold fronts. Mid-latitude cyclones affectmuch of the continental landmass for up to a week at a time.Meteorologists attempt to predict the path of these mid-latitudecyclones and their frontal systems by monitoring theirassociated atmospheric conditions such as moisture,temperature, pressure, and wind direction. Using thesecharacteristics they can predict the potential weather for two tofive days in the future. However, these dynamic systems aresubject to change, and the short-term, relatively accurateforecast becomes a long-term calculated guess as the forecastextends beyond two or three days.

Figure 2. Top:Cost of damagesassociated withweather events,1998. Total costwas over $16billion. Bottom:Proportion offatalitiesassociated withspecific weather-relatedphenomena.

4

Such is our devotion to understanding how future weatherpatterns will affect us that millions daily tune in to the cableweather news station, the Weather Channel. Regardless ofwhere we get our information on the weather forecast, almostall of it comes from the same place, the National WeatherService (NWS). The NWS processes over one million weatherobservations per day. These basic observations may bereprocessed by commercial weather companies (e.g.,Accuweather) to generate maps and graphics for publicdistribution to a variety of media sources.

The NWS began life on February 9, 1870, as part of the SignalService Corps in the Department of War. It initially had theunwieldy title of The Division of Telegrams and Reports forthe Benefits of Commerce and was given the charge “toprovide for taking meteorological observations at the militarystations in the interior of the continent and at other points inthe States and Territories . . . and for giving notice . . . of theapproach and force of storms.” The fledgling service made itsfirst simultaneous observations at just 24 sites on November 1,1870. Within two years it was creating national weather maps(Fig. 3), and by 1878 daily observations were being collected at284 sites and relayed cross-country by telegraph.

The current NWS mission is to provide “weather, hydrologic,and climate forecasts and warnings for the United States, itsterritories, adjacent waters and ocean areas, for the protectionof life and property and the enhancement of the national

Figure 3. Earlynational weathermap, createdSeptember 1,1872, shows ahigh-pressuresystem over theNortheast. Imagecourtesy of NOAAphotolibrary.

5

economy.” Today the NWS uses sophisticated satellitetechnology to keep tabs on developing weather systemsworldwide. The Geostationary Operational EnvironmentalSatellite (GOES) Program began in 1968 and today has twosatellites in synchronous orbit above Earth that provide weathercoverage for 60% of the planet's surface (Fig. 4). The NWShas over one hundred Doppler radar sites nationwide that areused to track rapid changes in regional storms. The nationwideexpansion of Doppler radar installations resulted in an increasein the warning times given for sudden weather phenomenasuch as tornadoes and flash floods that claim hundreds of livesannually.

The latter half of the chapter is divided into three sections thatreview extreme weather events in the U.S., thunderstorms,tornadoes, and hurricanes. Thunderstorms form as warm,humid air is forced aloft, either in advance of cold fronts thatare migrating toward the east or as a result of differentialwarming of air near Earth's surface. The high winds, hail,heavy rains, and lightning associated with these storms claimapproximately a hundred lives a year in the U.S. Furthermore,over the much of central and eastern U.S., thunderstormsproduce even more violent tornadoes (Fig. 5), the highestvelocity winds on Earth. The use of new technology, such asDoppler radar, has increased the average lead time for tornadowarnings in the U.S. from 5 minutes (1986) to 12 minutes in1998.

Figure 4.Geostationarysatellitesgeneratethousands ofimages perday. Imagecourtesy of NOAAphotolibrary.

Figure 5. Atornado nearDimmit, westTexas, 1995.Image courtesyof NSSL's photoalbum.

6

Dangerous weather phenomena such as tornadoes andhurricanes cannot be stopped but with detailed observationsmeteorologists can provide timely warnings to protect peoplefrom the onslaught of these hazardous winds. The sheer sizesof hurricanes, hundreds of kilometers across and bigger thanmost states, mean that they will have significant impact onpeople and property when they come on shore. The mostexpensive natural disaster in U.S. history occurred in 1992when Hurricane Andrew wrecked havoc across southernFlorida, causing $30 billion in damages (Fig. 6). Damagescould easily have been doubled if the storm had made landfallin the highly developed areas further north. Continued coastaldevelopment makes a future $50 to $100 billion disasterinevitable.

A•

•

•

Figure 6.Property damagein southernFlorida resultingfrom HurricaneAndrew, 1992.Image courtesy ofNOAA.

Think about it . . .Examine the map at the end of the chapter that illustratesthe distribution of extreme weather events for theconterminous U.S. during 2000. What patterns can youidentify in the weather characteristics displayed on themap?

ir Masses Air masses are large regions of the lower atmosphere with

uniform characteristics that are originally defined by asource area.

Air masses are identified by temperature (polar vs. tropical)and the nature of the source area (continental vs. maritime)

North American weather patterns are dominated bycontinental polar and maritime tropical air masses.

7

• Air masses are modified as they move over areas withdifferent temperatures or topography than the source area.

Air masses represent large regions (1,000s km2) of the lowertroposphere with relatively uniform properties (temperature,moisture content). The characteristics of individual air massesare dependent upon the attributes of a source area and themodification of the air mass that occurs as a result ofmovement from the source region. Weather in any region isinfluenced by the changes that occur in the air mass over timeand the interactions that occur at fronts, the boundariesbetween contrasting air masses.

Source AreasAn air mass develops when the atmosphere is located above arelatively uniform land or water surface for several days. Thelower atmosphere assimilates some of the properties of theunderlying surface. Air masses are identified by theirtemperature (polar/tropical) and the character of theunderlying surface (continental/maritime). The latter propertyis a proxy for moisture content. Air masses that developabove oceans contain much more moisture than those formedover land.

The distribution of air masses is relatively intuitive. Arctic andpolar air masses are located at high latitudes (+50o) in theNorthern Hemisphere and tropical air masses are located closerto the equator (Fig. 7). Continental air masses are found overland, maritime air masses over ocean. The boundaries betweenindividual air masses vary with seasons. Polar air creeps furthersouth during winter and retreats northward during summer.

Figure 7.Approximatelocations of airmasses developedover the NorthernHemisphere (left)and SouthernHemisphere (right)in July. cA -continentalArctic/Antarctic; cP- continental polar;cT - continentaltropical; mP -maritime polar; mT- maritimetropical. Originalglobes courtesy ofNOAA's NationalGeophysical DataCenter.

8

Heavy rains in the Midwest can result from the interactionbetween the continental polar air mass and the maritimetropical air mass that pushes northward across much of theeastern U.S. during summer.

Meteorologists use a form of scientific shorthand to label themost common types of air masses (Fig. 7). For example amaritime tropical air mass (warm, moist air formed over theocean) is identified by mT (m = maritime; T = tropical). Thecharacteristics of five types of air masses are summarizedbelow.

• cA - continental Arctic/Antarctic air forms at highlatitudes around the poles above permanently snow-coveredground (or pack ice). These air masses are characterized byextremely cold, dry air that may sweep south across Canadaand produce days of bone-chilling cold temperatures overmuch of the central and eastern U.S. during winter.

• cP - continental polar air forms over the northernmostportions of North America, Europe, and Asia. It shares itsbasic characteristics (cold, dry) with cA air, without theexceptional cold temperatures. High "lake effect" snowfallsin the upper Midwest are the result of the dry cP air massespicking up moisture as they cross the warmer waters of theGreat Lakes (Fig. 8).

• cT - hot, dry continental tropical air forms overcontinental interiors such as the dry lands of northernMexico and southwestern U.S. (Fig. 9). This air massdisappears from North America in winter but bringsscorching summers to the southwest. It will decay as itmoves east, absorbing moisture and cooling as it goes.

• mP - maritime polar air masses form in the northernAtlantic and Pacific Oceans and are characterized by cool,moist air that affects states bordering the Atlantic shore of

Figure 8. Lakeeffect snows inMichigan andnorthern Illinoisand Indiana,January 1997.Image courtesy ofNASA GOES.

9

the Northeast and most of the Pacific coastline (Fig. 9).Temperatures at the ocean surface are less extreme than onland (less cold) so mP air is warmer than cP air.

• mT - high temperatures and high humidity distinguish themaritime tropical air masses that move inland from thetropical Pacific, Gulf of Mexico, and tropical AtlanticOcean (Fig. 9). The mT air brings hot, humid summers tosoutheastern states and can form at any time during theyear.

Modification of Air MassesThe initial characteristics of air masses will inevitably changeas the mass of air moves out of its source area and passes overregions with contrasting attributes. The principal factors thatwill cause modification are the temperature and topographyof the underlying surface. Air masses will be heated or cooledfrom below depending upon the relative temperatures of theoriginal air mass and underlying surface. Heating (cP airmoving south) will lead to instability as air near the groundsurface rises, mixing the air column. Cooling (mT air movingnorth) has the opposite effect, because cold air cannot rise butremains in a stable configuration near the land surface.Orographic lifting forces maritime air upward over mountainranges in the western U.S., leading to condensation andprecipitation that converts the formerly humid air to a muchdryer air mass.

Figure 9. Principalsource areas forair masses thatinfluence weatherpatterns acrossNorth America.Original globecourtesy of NOAA'sNationalGeophysical DataCenter.

Think about it . . .Create a concept map that summarizes the characteristicsof the principal air masses and their influence on weatherpatterns in North America.

10

Frontal Systems• Frontal systems form along the boundaries between

colliding air masses of contrasting properties.• Cold fronts and warm fronts are associated with the

interaction of continental polar and maritime tropical airmasses over the central U.S.

• Heavy rainfall, decreasing temperatures, decreasinghumidity, and changing wind directions are associated withpassage of a cold front.

• Light to moderate rain, warmer temperatures, increasinghumidity, and changing wind directions follow passage of awarm front.

• An occluded front forms when a cold front overtakes awarm front.

Frontal systems represent the meteorological battle thatensues when air masses of contrasting properties clash alongtheir boundaries. As air masses move across Earth's surfacethey inevitably interact to create relatively narrow, curvilinear

Figure 10. Weatherpatterns typicallyencountered with coldand warm frontsassociated with acyclone (low-pressuresystem) over thecentral U.S. Theoccluded front formedwhere cold and warmfronts coalesced overthe northern plains.Warm maritime tropicalair from the Gulf ofMexico lies betweenthe two fronts. Notethat cloud cover occursin advance of the coldfront, adjacent to thewarm front, and aroundthe occluded front.Lines A-B and C-Drepresent sectionsthrough the frontalsystem (see Figs. 11and 14).

11

zones that mark a front, a transition from one air mass toanother (Fig. 10). Advancing frontal systems bring clouds andprecipitation and are accompanied by changes in moisture,temperature, pressure, and wind direction.

The clash between cP and mT air masses over the Great Plainsand Midwest is the most common source of frontal systems inthe U.S. (For more on the causes of this phenomenon, see thesection, Mid-latitude Cyclones.) Weather conditions change ina predictable sequence as warm and cold fronts pass over anarea.

Weather Conditions Associated with a Passing Frontal System

Conditions Before Warm Front Between Warm andCold Fronts

After Cold Front

Pressure Decreasing Small decrease, thensmall increase

Increasing

Winds South, southeast Southwest West, northwestTemperature Cool Warm ColdClouds Cirrus, cirrostratus,

altostratus,nimbostratus

Cumulus,cumulonimbus

Cumulus, altostratus

Precipitation Light-moderate,increasing

None, then heavy rainsprior to cold front

Moderate-light,decreasing

Cold FrontCold, dense continental polar air replaces moist, warmmaritime tropical air across the cold front (Fig. 10). Peopleliving downwind from the front experience decreasingtemperature and humidity and increasing atmospheric pressurewith the passage of the cold front. The cold front is pictured assteep in Figure 11 but its actual inclination is ~1 degree toward

Figure 11. Weatherconditions associatedwith cross section A-B on Figure 10. Warmair (mT) lies betweenthe cold front andwarm front. The coldfront advances morerapidly than the warmfront, forcing warm airto rise, formingthunderclouds andheavy rains. Warm airis forced to rise abovethe more gentlysloping warm front,resulting in theformation of a seriesof low to high clouds.

12

the warm air (not much, but twice as steep as a warm front).Warm air will always rise over cooler air so both the cold andwarm fronts are inclined toward the warm air mass. Warm airis pushed up and over the advancing cold front, causingrelatively rapid cooling and condensation that results in thedevelopment of tall cumulonimbus clouds that host heavy butrelatively short-lived precipitation (Fig. 11). Rapidly advancingcold fronts may be marked by the growth of a squall line ofthunderclouds (Fig. 12).

Warm FrontChanges following the passage of the warm front (Fig. 10) aremore benign than the storms that travel with the cold front.Friction at Earth's surface causes the warm front to slopegently (~½ degree inclination) toward the warm air mass (Fig.11). Warm, humid air is transported upward over a distance ofapproximately 1,000 km (625 miles). The first signal of anapproaching warm front is the appearance of light, upper-levelclouds (cirrus, cirrostratus). Up to 12 hours later, the highclouds will be replaced by lower nimbostratus with associatedlight to moderate precipitation. Rain associated with a warmfront may last longer than precipitation that accompanies a coldfront because the warm front typically moves more slowly andextends over a larger area. Temperatures and humidity rise andwinds typically shift direction (from south to southwest) withthe passage of the warm front.

Occluded FrontThe cold front moves more rapidly than the warm front (~ 10km per hour faster) and will eventually close the gap betweenthe fronts, forcing the intervening warm air upward generatingadditional precipitation (Figs. 13, 14). An occluded front isrepresented by a combination of warm and cold front symbols

Figure 12. A squallline highlighted byintensethunderstormsassociated with arapidly advancingcold front, Gulf ofMexico. Imagecourtesy of NASA'sJohnson SpaceCenter ImageServices.

Figure 13. Anoccluded front formswhen a cold airmass overtakes awarmer air mass.

on weather maps (Fig. 10). The occluded front juxtaposes twobodies of cold air; the warmer of the two masses is forced upand over the other. Occluded fronts may be marked by theoccurrence of nimbostratus clouds.

Figure 14.Nimbostratusclouds generateprecipitationalong anoccluded front(see section C-Don Figure 10).

Think about it . . .Examine the map located at the end of the chapter andanswer the conceptest questions about frontal systemsbased on the locations featured on the map. One or two ofthe questions may require you to read the section thatfollows on mid-latitude cyclones.

13

Mid-latitude Cyclones• Weather in the eastern U.S. is mainly the result of the

migration of regional-scale low-pressure systems, termedmid-latitude cyclones.

• Mid-latitude cyclones develop where continental polar andmaritime tropical air masses collide over the U.S. along thepolar front.

• Converging surface winds associated with low-pressuresystems must be matched with divergent flow in the upperatmosphere.

• Cyclones develop from a waveform that originates whereirregularities at the surface cause local shearing that distortsthe polar front.

Much of the weather experienced over the eastern U.S. is theresult of the west-to-east migration of regional-scale low-pressure systems known as mid-latitude cyclones (Fig. 15).

14

These weather patterns are differentiated from tropicalcyclones formed over the warm tropical ocean waters that maybuild into hurricanes. Mid-latitude cyclones (also called wavecyclones) may be 1,000 to 2,000 km (625-1,250 miles) acrossand can affect much of the continental land mass for periods ofthree days to as much as a week.

Mid-latitude cyclones develop where continental polar andmaritime tropical air masses collide over the U.S. along thepolar front. The position of the collision zone migrates southduring winter and moves north with summer. The boundarybetween air masses is initially a stationary front, with airflowin opposite directions on either side. Perturbations in the upperairflow of the jet stream are necessary to promote the growth ofa surface low-pressure system. The converging surface windsassociated with low-pressure systems must be matched withdivergent flow aloft to maintain the cyclone (Fig. 16). (Formore on airflow within a cyclone, see Cyclones andAnticyclones in the chapter, The Atmosphere.)

Cyclones develop from a waveform that originates whereirregularities at the surface cause local shearing that distortsthe polar front. Features that may induce shearing include

Figure 15. A classiccomma-shapedcloud pattern isassociated with amid-latitude cyclonein the central UnitedStates, ChristmasEve, 1997. A low-pressure center islocated over thelower MississippiValley and a warmfront spirals over theGulf of Mexico.Image courtesy ofNASA-Goddard SpaceFlight Center, NOAAGOES.

Figure 16. Low-pressure systems(cyclones) formbelow regions ofdivergent flow in thejet stream.

15

mountains and contrasting atmospheric properties at land/waterboundaries. The waveform becomes exaggerated as warm airpushes northward and cold air moves south, generatingcounterclockwise rotation typical of cyclones and forming thepairing of warm and cold fronts discussed in the previoussection.

Warm air advances along the warm front, at rates of 15 to 20km per hour (9-13 mph), moving over ground previouslycovered by colder air. The cold front lies to the west and movesabout twice as fast as the warm front (Fig. 17). Warm air isforced aloft as the cold front sweeps across the previouslywarm sector. The cold front and warm front eventually mergeto form an occluded front, merging cold air masses andproducing a more stable, stratified atmosphere where cool airlies below warmer air and resulting in the decay of the cyclone.

Figure 17. Threestages incyclogenesis, thedevelopment of amid-latitudecyclone over theU.S. Note west toeast track of thecyclone and themerging of warmand cold fronts toform an occludedfront in the finalimage shortlybefore thecyclone decays.

16

T•

•

•

•

ThupthaiThevco

Think about it . . .Review the Frontal Systems and Mid-latitude Cyclonesexercise (see end of chapter) referred to following theprevious section. Would you change any of the answersafter reading the section above?

hunderstormsThunderstorms form where warm, humid air is forcedupward at cold fronts or as a result of differential heating atEarth's surface.Latent heat, released during condensation, generatesupdrafts that maintain upward movement.Thunderstorms are most frequent over the southeasternU.S.The three stages (cumulus, mature, dissipating) in the lifecycle of a thunderstorm occur over approximately twohours.

understorms form where warm, humid air is forcedward to altitudes of up to 15 km. Condensation occurs as

e air cools, releasing latent heat and ensuring that the risingr remains unstable (warmer than surrounding air).understorms may occur as relatively isolated, short-livedents or as longer-duration severe storms depending upon thenditions that cause the air to rise.

Figure 18. Afternoonthunderstorms formover the Floridapeninsula as humidmaritime tropical airmoves over thewarmer landmass.

17

Isolated afternoon thunderstorms, or cells, are commonplace inwarm summer months where moist maritime tropical airmasses move over land (Fig. 18). The temperature of the landsurface rises to a maximum during the mid-afternoon, warmingoverlying air parcels and causing them to become unstableenough to rise, generating scattered thunderstorms. (For moreon this process, see Clouds and Cloud Formation in thechapter, The Atmosphere).

Severe storms, or supercells, are associated with frontal liftingalong the cold front between the continental polar and maritimetropical air masses in mid-latitude cyclones. These storms aremost common during spring and early summer, when thecontrast in temperatures between air masses is greatest.Because thunderstorms are associated with mid-latitudecyclones it should come as no surprise that storms are mostcommon over the Great Plains and southeastern U.S. Statessuch as Florida may experience over 100 days a year withthunderstorms whereas such storms are rare (less than 20 peryear) in Pacific Coast states (Fig. 19).

Figure 19. Map ofthe averagenumber of daysper year withthunderstormsfeaturingdamaging windsor wind speeds ofmore than 50knots/hour, 1980-1994.

18

There are three stages during the life of a typical thunderstormthat rarely lasts for more than two hours:

1. Cumulus stage - early cloud development when a cumuluscloud expands laterally and vertically as air enters the cloudmass at all levels. Cloud formation is rapid, requiringapproximately 15 minutes to grow vertically to heights of10 km (6 miles; Fig. 20a). Updrafts (~ 4 m/sec near groundsurface to ~ 10 m/sec at high levels) within the cloud carryhumid air to higher, colder levels where condensationoccurs.

2. Mature stage - top of cloud cell may be at altitudes up to15 km (9 miles; Figs. 20b, 21). Rain and ice formed bycondensation become too large to be supported by updraftsand fall to ground. Falling precipitation generates frictionwithin rising air, creating a zone of downdrafts. Descendingair warms up, resulting in evaporation that absorbs latentheat and cools the cloud. This stage lasts for 15 to 30minutes generating rainfall or hail (Fig. 22) at the surfaceaccompanied by gusty winds (downdraft).

Figure 20a.Cumulus stage inthe life cycle of athunderstorm ischaracterized byrapid verticalgrowth of the cloudand condensation.

Figure 20b.Downdrafts andprecipitationcharacterize themature stage inthe life cycle of athunderstorm.

19

3. Dissipating stage - cloud formation processes end asmoisture is expended and descending air cools the cloudmass, returning stability to air. The final stage ischaracterized by diminishing precipitation (light rain) asthe cell dissolves (Fig. 20).

Tornadoes• Tornadoes are narrow, funnel-shaped spirals of wind that

rotate at speeds of up to 500 km/hr because of extremepressure gradients.

• Tornadoes are ranked from F0 (weakest) to F5 (strongest)using the Fujita Intensity scale.

• Most tornadoes move to the east or northeast at an averagespeed of approximately 50 km/hr.

• Tornadoes are associated with thunderstorms and developin association with mesocyclones within the thunderstormcell.

Figure 21.Developingthunderstormcloud (top) andmature supercell(below). Imagescourtesy of NOAAPhotolibrary.

Figure 22. Hail coatsthe ground after astorm (above right).Thunderstorms cangenerate individualhailstones up to 15cm (6 inches) indiameter (above).The hail above is ~6cm across. Imagescourtesy of NOAAphotolibrary.

20

• The U.S. experiences more tornadoes than any other nationand most occur in tornado alley (Texas, Oklahoma, Kansas,Nebraska, Iowa, Illinois, and Indiana).

Tornadoes are narrow, funnel-shaped spirals of rapidlyrotating air (Fig. 23) that form in association withthunderstorms. Like hurricanes and mid-latitude cyclones,tornadoes are near-circular low-pressure systems. However,the pressure gradient is much more intense for tornadoes.Pressure differences across mid-latitude cyclones are in therange of 20 to 30 mb (millibars) over hundreds of kilometers(pressure gradient, 0.02-0.03 mb/km). Hurricanes mayexperience pressure gradients of more than 100 mb overshorter distances (~0.2-2 mb/km) but large pressure differencesin tornadoes occur over distances measured in hundreds ofmeters. Extreme pressure gradients of up to ~0.1 to 1 mb/mare possible for tornadoes, generating the strongest naturalwinds on Earth with wind velocities of up to 500 km/hr.

Tornadoes are classified using the Fujita Intensity scale whichplaces tornadoes in one of six categories (F0-F5) according tolevel of destruction which is taken as a proxy for wind speed.The scale divides tornadoes into three subgroups: weak (F0,F1); strong (F2, F3), and violent (F4, F5). Wind speed cannot bemeasured directly because the high winds that can blast apartwhole buildings (Fig. 24) would make short work of measuringinstruments. Scientists use the level of destruction to gaugeestimates of wind speed and thus determine the Fujita value fora specific tornado. This makes it difficult to rank winds thattouch down in sparsely populated areas.

Fujita Intensity ScaleScale Wind Speed

km/hr (miles/hr)DamageDescription

Tornado Class % of U.S.Tornadoes

Time onGround

F0 <116 (<72) LightF1 116-180 (72-112) Moderate Weak 69% <10 minutes

F2 181-253 (113-157) ConsiderableF3 254-332 (158-206) Severe Strong 29% ~20 minutes

F4 333-419 (207-260) DevastatingF5 >419 (>260) Incredible Violent 2% > 1 hour

The funnel of the tornado moves more slowly than the windsthat give it shape. Funnels are typically less than 600 m (2,000feet) wide and average funnel velocities are approximately 50km/hr, although velocities as high as 200 km/hr (125 mph)have been recorded. Tornado paths follow the direction ofmovement of their parent thunderstorms that are in turn

Figure 23.Twisting, near-vertical funnel-shaped tornado.Image courtesy ofNSSL's photoalbum.

21

associated with east to northeast-directed mid-latitudecyclones. Scientists have been unable to observe the birth of atornado because of the difficulty in determining exactly wheretornadoes may originate. One hypothesis on tornado formationconsiders tornado development in three stages (Fig. 25):

• Early stage: Friction slows winds at the ground surface,resulting in increasing wind velocity with elevation in thelower troposphere. These contrasting vertical wind speedsgenerate local winds that rotate about a horizontal axis.

• Updraft stage: Updrafts below thunderstorm cell draw thespiraling horizontal winds upward forming a mesocyclone(a small cyclone) within the larger storm cloudcell. Mesocyclones may be up to 10 km in diameter.

• Tornado stage: Rotation within the mesocyclone formssmaller, more intense spiraling winds within a tornado thatextend downward from a cloud base toward the groundsurface (Fig. 26).

Tornado paths are typically 5 to 25 km (3-16 miles) in lengthbut some larger tornadoes may remain on the ground for over100 km (62 miles). Smaller funnels may skip across the surfacelike a fickle avenger, destroying one home while leavingneighboring properties undisturbed. Improvements in

Figure 24.Destructionassociated with aviolent tornado in asuburb of OklahomaCity, May 3, 1999.There were 38deaths from thissingle tornado thatreached F5 strengthalong part of itspath. Image courtesyof FEMA.

Figure 25.Stages in thedevelopmentof a tornado.

22

forecasting methods have reduced the number of fatalitiesassociated with tornadoes (Fig. 27). Approximately two-thirdsof U.S. fatalities occur as a result of tornadoes destroyinghomes; nonpermanent mobile homes are especially susceptible(Fig. 28).

The U.S. is home to the majority of the world's tornadoes,averaging about 1,000 a year. Tornadoes occur whenthunderstorm activity is at an optimum, during the late springand early summer across much of the nation. The highestfrequency of tornadoes per area occurs over the Great Plainsstates (Texas, Oklahoma, Kansas, Nebraska) and parts of theupper Midwest (Iowa, Indiana, Illinois), a region that has cometo be known as tornado alley (Figs. 29, 30).

The timing of tornado activity is tied to seasonal movement ofthe polar front that drives mid-latitude cyclones. Tornadoesmove out of the Gulf Coast and southeastern states into the

Figure 26. Left:Waterspouts aretornado-likephenomena formedover water. Right:The latter stages intornadodevelopment for anexample thattouched down inEnid, Oklahoma.Images courtesy ofNSSL's photo album.

Figure 27. U.S.tornado fatalitieshave declined asforecastingtechnologyimproved to providebetter tornadowarnings.

Figure 28.Proportion of U.S.tornado fatalities bylocation, 1985-1998.

Great Plains as the front retreats northward in late spring.Summer sees tornado activity shifting to the northern Plainsstates and the upper Midwest.

Figure 29.Average annualdistribution ofstrong andviolenttornadoes,1950-1995.

Figure 30.Tornadobearing downacross opencountry. Imagecourtesy ofNSSL's photoalbum.

Think about it . . .Use the Venn diagram located at the end of the chapter tocompare and contrast the characteristics of tornadoes andhurricanes.

23

Hurricanes• Hurricanes are rotating storm systems hundreds of

kilometers across that strike the U.S. during summer andearly fall.

• Hurricanes grow from tropical depressions in regions ofconvergent winds and warm oceans, and are sustained bydivergent airflow in the upper troposphere.

• Destruction from high winds, heavy rains, and coastalflooding occurs when hurricanes make landfall.

24

• Hurricanes are divided into five categories by wind speedusing the Saffir-Simpson hurricane intensity scale.

Hurricanes are cyclonic storm systems that form over tropicaloceans during summer and fall. Also known as typhoons(Pacific Ocean) and cyclones (Indian Ocean), hurricanes arecharacterized by high winds (more than 119 km/hr , 74 mph),heavy rainfall (10-25 cm 4-10 inches), and storm surges(sudden rise in sea level) along coastlines. Hurricanes aresmaller (approximately a third to half the size) and lessfrequent than mid-latitude cyclones that govern most U.S.weather patterns but they have much more powerful winds.Unlike mid-latitude cyclones, hurricanes do not originate as aresult of perturbations at a boundary between contrasting airmasses. Instead they grow from tropical storms generated bydisturbances in the belt of equatorial trade winds.

Hurricanes can result in fatalities and substantial propertydamages as winds and high seas cause structural damage toocean-going vessels and coastal developments, and heavyrainfall leads to flooding as much as 200 km inland from thecoast. An estimated 300,000 people died in Bangladesh (Fig.31) in 1973 when cyclone pushed onshore from the Bay ofBengal, generating a 7 meter (22 foot) storm surge andproducing widespread flooding of the low-lying coastal plain.The government of Bangladesh built nearly a thousandconcrete shelters in coastal communities with sufficient spaceto shelter a million residents and improved communicationslinks to try to reduce the danger from future cyclones.Hurricane Andrew became the costliest natural disaster toaffect North America when it decimated much of southernFlorida in 1992, generating up to $30 billion in damages.

Building a HurricaneHurricanes develop under a specific suite of conditionsincluding warm surface waters, cyclonic circulation, anddivergent flow in the upper troposphere.

• The initial stage in the development of a hurricane is theformation of a tropical depression (low-pressure system)where the trade winds converge near the equator. Thelocation of the convergent winds changes with seasons,lying north of the equator during summer in the NorthernHemisphere and migrating to southern latitudes during ourwinter. The rising air cools and condenses to form cumulus

Figure 31.Location ofBangladesh.

25

clouds that will develop into cumulonimbus cells if therising air is sufficiently warm and humid.

• Water temperatures must be at least 27oC and shouldextend downward for 50 to 65 meters (165-215 feet) toensure that colder water won't be drawn to the surface bythe developing storm. Warm surface waters typicallystraddle the equator but are absent south of the equator inthe eastern Pacific and Atlantic Oceans due to oceaniccirculation patterns (Fig. 32). The North Atlantichurricane season officially lasts from June 1 to November30, and most U.S. hurricanes strike in August andSeptember.

• Earth's rotation as reflected in the Coriolis effect imparts aclockwise (Southern Hemisphere) or counterclockwise(Northern Hemisphere) rotation to the growing storm (Fig.33). The magnitude of the Coriolis effect increases withincreasing latitude and is zero at the equator. Consequently,the necessary rotation is not imparted on storms within 5degrees either side of the equator.

• The inflow of air into the developing low-pressure systemmust be matched with an outflow of air in the uppertroposphere to maintain the pressure gradient in thedeveloping hurricane. If not, the pressure contrast decreasesand wind speed declines.

If all of these conditions are met, a tropical depression (windspeed <37 km/hr) forms and has the potential of growing to atropical storm (wind speed 63-119 km/hr) before developinginto a hurricane with wind speeds of at least 119 km/hr. Windspeed increases in inverse proportion to the decrease inpressure in the eye of the hurricane; the lower the pressure thehigher the wind velocity. Rising air in the deepening low-pressure system cools and condenses, releasing latent heat andgenerating a dense spiral of cumulonimbus clouds punctuated

Figure 32.Hurricanesoriginate inareas of theworld's oceanswhere watertemperaturesare greater than27oC.Hurricanes inthe NorthernHemisphere aremost commonduring summerand early fall.SouthernHemispherehurricanes arefrequent duringour winter (theirsummer).

26

by a central eye characterized by clear skies. Bands of cloudsspiral outward from the vortex that will continue to grow insize and intensity as long as the underlying water temperatureremains above 27oC. Precipitation is concentrated within aradius of approximately 100 km on either side of the eye,releasing up to 20 billion tons of water per day.

Hurricane LandfallAtlantic hurricanes, driven westward by prevailing winds atrates of 10-25 km/hr (6-16 mph), may turn north parallel to theU.S. east coast or pass south of Florida to strike along the GulfCoast or Caribbean islands (Figs. 34, 35). Florida and Texasexperience more hurricane landfalls than any other states (Fig.35). A hurricane will begin to decay when it passes over landas it experiences greater frictional drag and a dramatic decreasein the water supply that is essential for its maintenance.Although wind speeds will be reduced to the level of a tropicalstorm or depression, the storm itself is still capable of dumpinglarge volumes of rain for some distance inland.

Figure 33. Fourhurricanes in theNorth AtlanticOcean, 1998.Image courtesy ofNOAA.

27

Much of the destruction associated with hurricanes isn't causedby high winds or storm surges but is linked to the exceptionalprecipitation events that can unload 60 cm (24 inches) of rainfrom a single storm system in just a few days. Heavy rainsfrom Hurricane Mitch (Fig. 36), the most devastating storm tostrike North America and Central America in the last twocenturies, claimed over 10,000 lives from flooding (both inlandand coastal flooding) and landslides. Honduras, one of thehemisphere's poorest nations, took the brunt of thestorm. Entire villages were demolished, nearly 20% of thepopulation evacuated their homes, a quarter of the schools werewrecked, water supplies were cut off, and almost all majorroads were damaged. The nation's economy was devastated andthe loss to sugar, banana, and sugar crops pushed up prices ofthose commodities worldwide.

Figure 34.Selectedhurricane tracksfor storms thatoriginated in theAtlantic Ocean.

Figure 35.Number ofhurricanelandfalls by state1900-1996. Blue- all hurricanes;red - major(category 3, 4,and 5) storms.OT (other)includesDelaware, Maine,Maryland,Massachusetts,New Hampshire,and New Jersey.

28

As hurricanes approach landfall, winds in the northeastquadrant of the storm, north of the eye, are blowing onshore,piling up water in a storm surge (Fig. 37). However, winds inthe southwest quadrant of the storm, south of the eye, areblowing offshore, and opposite to the direction of the storm'smovement. Offshore winds reduce the local impact of thestorm surge. Emergency workers are especially interested inidentifying where the hurricane eye comes onshore as it allowsthem to better gauge the potential for destruction and todistribute their staff accordingly. The National HurricaneCenter's forecasters add an extra 145 km on either side of theprojected landfall site issued 24 hours ahead of the stormbecause of the capricious nature of hurricane motion. Stormsmay change speeds, remain stationary, or reverse direction,making it difficult to predict exactly where and when toevacuate residents.

Figure 36.Hurricane Mitchapproaching thecoast of CentralAmerica. Imagecourtesy ofNOAA.

Figure 37.Hurricane Floyd(1999)threatenedmuch of thesoutheasternU.S. beforemaking landfallin NorthCarolina. Imagecourtesy ofNOAA.

29

The largest peacetime evacuation in U.S. history occurred inadvance of Hurricane Floyd which endangered Florida andother southeastern states in September 1999 (Fig. 37). Floydwas unusually large and threatened to affect a substantiallength of highly developed coastline. Authorities err on the sideof caution and evacuate large regions in advance of a hurricanebecause of the difficulty in accurately predicting the stormsfuture path. Florida residents breathed a sigh of relief whenFloyd turned north, eventually coming onshore in NorthCarolina. Thousands of people stranded on crowded, slow-moving highways highlighted the difficulty of evacuating largepopulations in advance of an impending storm.

Hurricane MeasurementHurricanes are divided into five categories by wind speed usingthe Saffir-Simpson hurricane intensity scale. Destructionassociated with major (category 3, 4 and 5) hurricanes includesdamage to permanent homes, widespread coastal flooding,uprooting of trees, toppling of power lines. Anticipation ofsuch damages prompts evacuation of residents from the area ofexpected landfall. Hurricane Camille in 1969 was the mostrecent category 5 hurricane to make landfall in the U.S.,coming onshore along the Gulf Coast of Mississippi.

Saffir-Simpson Hurricane Intensity Scale

Category Wind Speedkm/hr (miles/hr)

Pressure(millibars)

Storm Surgemeters (feet)

DamageDescription

1 119-154 (74-95) >980 1.2-1.5 (4-5) Minimal2 155-178 (96-110) 965-979 1.6-2.4 (6-8) Moderate3 179-210 (111-130) 945-964 2.5-3.6 (9-12) Extensive4 211-250 (131-155) 920-944 3.7-5.4 (13-18) Extreme5 >250 (>155) <920 > 5.4 (>18) Catastrophic

Camille caused 256 deaths and over a $1 billion in damages(Fig. 38). The cost of the same storm today would be over $11billion because of the increased development along thecoastline. The most extensive coastal development has takenplace in Florida, greatly increasing the potential damages froma major storm. A recent study suggests that an unnamed 1926category 4 hurricane that smashed into southern Florida beforecrossing the Gulf of Mexico to Alabama, would generatedamages totaling $77 million (twice the cost of Andrew, Fig.

39) if it were to occur today. The convention of naminghurricanes didn't begin until the 1950's.

Figure 38.Before and afterpictures ofdamage to ahotel resultingfrom HurricaneCamille. Thirtypeople at a“hurricaneparty” on thesite died.Images courtesyof NOAA'sphotolibrary.

Figure 39.Destruction ofhomes in a cul-de-sac insouthern Florida,caused byHurricaneAndrew, acategory 4 storm.Image courtesy ofNOAA.

Think about it . . .1. Use the Venn diagram located at the end of the chapter

to compare and contrast the characteristics of tornadoesand hurricanes.

2. You work in a team of disaster specialists for theWeather Channel. The channel wants to create its ownscoring system that better evaluates the potentialdamage from incoming storms. You and your team aregiven the assignment to create an evaluation rubric toassess factors that will influence the risk of potentialdamage from a future hurricane. Go to the end of thechapter to complete the exercise.

30

31

Summary1. Where does the daily weather forecast come from?Despite what they claim about "exclusive" weatherinformation, the original data for all local weather forecastscomes from observations made by the National WeatherService. These observations may be used to generateimpressive graphics by commercial weather companies but thebasic data come from the NWS.

2. What is an air mass?An air mass is a large region of the lower troposphere withrelatively uniform properties (temperature, moisture content).An air mass develops when the atmosphere above a land orwater surface adopts the characteristics of the underlyingsurface. Air masses are modified as their properties changewhen they move over land/water surfaces with contrastingtemperatures and/or moisture content.

3. How are air masses identified?Air masses are differentiated by their temperatures andmoisture content. The most common types are categorized aspolar (cold) or tropical (warm), continental (dry) or maritime(humid). The air masses can be represented by symbols suchas cT (continental tropical), cP (continental polar), mT(maritime tropical), and mP (maritime polar).

4. Which air masses have the greatest impact on U.S.weather?

Continental polar and maritime tropical air masses interact overthe central and eastern U.S. Continental polar air forms overthe northernmost portions of North America. Hightemperatures and high humidity distinguish the maritimetropical air masses that move inland from the tropical PacificOcean, Gulf of Mexico, or tropical Atlantic Ocean.

5. What is a frontal system?Fronts mark the boundaries between air masses of contrastingproperties. Fronts come in two basic varieties. Cold fronts formwhere cold polar air masses move over ground previouslyoccupied by warm air. Warm fronts form where warm airmoves over surfaces previously covered by cold air masses.One consequence of either situation is that warm air risesabove the underlying colder air, forming clouds and releasingprecipitation.

32

6. What is the difference between weather at cold and warmfronts?

Heavy rainfall, decreasing temperatures, decreasing humidityand changing wind directions are associated with passage of acold front. Warm air will always rise above cold air so the frontis inclined toward the warmer air mass. Friction at the groundsurface causes the front to steepen, forcing warm air aloft morerapidly and leading to the development of tall cumulonimbusclouds. The warm front slopes gently toward the warm air massand humid air rises gradually over a distance of hundreds ofkilometers. Warm fronts are characterized by light to moderaterain, warmer temperatures, increasing humidity, and changingwind directions.

7. What is an occluded front?Occluded fronts form when a warm air mass is forced aloftbetween two bodies of cooler air. The front marks the locationon the surface where the colder air masses meet.

8. What is a mid-latitude cyclone?Mid-latitude cyclones are low-pressure systems that may beover a thousand kilometers wide and migrate from west to eastacross North America. The position of the cyclone will changewith seasons but typically marks the boundary between coolcontinental polar and warmer maritime tropical air masses.

9. How are frontal systems related to mid-latitude cyclones?Mid-latitude cyclones develop where cold and warm air massescollide along the polar front. The warm air pushes northwardand is surrounded by cooler air. The boundaries between airmasses represent fronts. The initial boundary is a stationaryfront that becomes distorted to form a pairing of cold and warmfronts. These fronts travel eastward with the cyclone but thecold front typically moves more rapidly and overtakes thewarm front to form an occluded front near the east coast.

10. Under what conditions do thunderstorms form?Thunderstorms are characterized by clouds that span the rangefrom low- to high-level clouds and form under unstableatmospheric conditions caused by the rapid rise of warm,humid air. Warm air rises rapidly at cold fronts (associatedwith mid-latitude cyclones) or where differential heatingoccurs during hot weather.

11. What are the stages of thunderstorm development?

33

There are three stages in the life of a thunderstorm. A cumuluscloud grows rapidly during the early (cumulus) stage asupdrafts carry warm, humid air to high elevations.Condensation produces rain or hail during the second (mature)stage, and the falling precipitation helps cool the cloud andstop growth. The third (dissipating) stage is marked by thedecay of the cloud as the supply of moisture is depleted and theair becomes stable.

12. Is there any similarity between mid-latitude cyclones,tornadoes, and hurricanes?

All are examples of low-pressure systems where air convergesinward from areas of higher pressure. The speed of the winds isgreatest for tornadoes where the pressure gradient is greatest(change in pressure over smallest distance) and is least for mid-latitude cyclones that have the smallest pressure gradient.

13. How do scientists classify tornadoes?Tornadoes are classified by their wind speed using the FujitaIntensity scale. The scale divides tornadoes into six categories(F0-F5) based upon level of destruction. Destruction is matchedto wind speed that is too high to measure with conventionalinstruments. F0 and F1 tornadoes are weak tornadoes with windspeeds up to 180 km/hr. F2 and F3 are strong tornadoes withwind speeds from 181 to 332 km/hr. Violent (F4, F5) tornadoesrepresent only 2% of all tornadoes and are characterized byestimated wind speeds of more than 332 km/hr.

14. How do tornadoes form?Scientists can't get too close to tornadoes because of theirintense speeds so direct observations of tornado formation havenot been made but there are thought to be three stages totornado formation. The first stage in tornado formation is thedevelopment of horizontally spinning winds just above theground surface. These horizontal spirals form mesocyclones(small-scale low-pressure systems) that are pulled intothunderstorm clouds by updrafts of warm air and beginspinning about a near-vertical axis. Rotation within themesocyclone generates smaller intense twisters that growdownward to the ground surface as narrow funnels.

15. Why does the U.S. have more tornadoes than any place onEarth?

The location of tornadoes is directly linked to the passage ofthe mid-latitude cyclones across the Great Plains and Midwest.Most tornadoes grow from thunderstorms formed at cold fronts

34

where cold and warm air masses interact. The North Americancontinent at relatively high latitudes and the warm tropicalAtlantic Ocean and Gulf of Mexico provide the ideal breedinggrounds for air masses of contrasting properties needed togenerate the necessary atmospheric conditions to formtornadoes.

16. What are the key conditions needed for hurricaneformation?

Hurricanes form over warm ocean waters with temperatures ofat least 27oC (80oF) extending to depths of ~50 meters. Inaddition, the Coriolis effect must be sufficient to impartrotation on the low-pressure system that will evolve into ahurricane. Combining these two factors requires that stormscannot form at the equator (where the Coriolis effect is zero)and can't form beyond latitudes that are more than 20 degreesnorth or south of the equator (where waters are too cool).

17. What hazards are associated with a hurricane?Hurricanes endanger lives and property because of their highwinds, heavy rainfall (and resulting flooding), and storm surgesthat generate waves of more than 7 meters above normal sealevel. The size of a hurricane means that it will affect a largearea if its eye comes within a few hundred kilometers of thecoastline.

18. Why do hurricanes affect the east coast and not the westcoast?

Hurricanes travel in the direction of the prevailing atmosphericand oceanic circulation systems. Hurricanes move from east towest across the Atlantic and Pacific Oceans following the tradewinds. The prevailing wind direction therefore carries Atlantichurricanes toward a U.S. landfall while transporting Pacificstorms away from the West Coast.

35

Weather Hazards

Examine the maps of extreme weather events for 2000.

1. What patterns can you identify in the weathercharacteristics displayed on the maps?

2. Identify three states that are relatively free of weatherhazards.

3. Identify three states that have the highest risk of weatherhazards.

36

Frontal Systems and Mid-latitude Cyclones

Use the map to answer the questions that follow.

1. The map illustrates the relative positions of a warm front

and a cold front associated with a mid-latitude cyclone.Where is the warm front located?

a) between A and B b) between C and D c) at E

2. Where is it raining?a) A and B b) B and C c) C and D

d) B and D e) A and C

3. Which location is in a maritime tropical air mass?a) A b) G c) E d) H

4. What direction is the mid-latitude cyclone travelingtoward?

a) F b) G c) H

5. Winds at C would be ________________________.a) southwesterly b) southeasterlyc) northeasterly d) northwesterly

6. Which location will become warmer in the next 12 hours?a) A b) B c) C d) D e) E

7. Which of the images below best represents conditionsbetween A and D on the map?

Venn Diagram: Tornadoes vs. Hurricanes

Use the Venn diagram, below, to compare and contrast thesimilarities and differences between tornadoes and hurricanes.Print this page and write features unique to either group in thelarger areas of the left and right circles; note features that theyshare in the overlap area in the center of the image.

Tornadoes

Hurricanes

37

38

Hurricane Evaluation Rubric

You work in a team of disaster specialists for the WeatherChannel. During discussions about coverage of the upcominghurricane season your boss states that she doesn't believe thatthe Saffir-Simpson scale sufficiently reflects the risksassociated with hurricanes because it emphasizes one factor(wind speed). The channel wants to create its own scoringsystem that better evaluates the potential damage fromincoming storms.

You and your team are given the assignment to create aevaluation rubric to assess factors that will influence the risk ofpotential damage from a future hurricane. One factor isincluded as an example in the table below, identify five more.Consider both physical and cultural factors when developingyour rubric.

Factors Low Risk(1 point)

Moderate Risk(2 points)

High Risk(3 points)

Wind speed Low(category 1, 2)

Intermediate(category 3)

High(category 4, 5)

39

Reviewing your evaluation rubric you realize that some factorsare more significant than others. Your team decides to doublethe score of the most important factor. Which do they choose?Why?

Read the descriptions of Hurricanes Floyd, Dennis, and Mitchthat follow. Would you change any categories in your scoringrubric? Rank these storms using your modified rubric.

40

Hurricane Floyd, September 1999Floyd brought flooding rains, high winds, and rough seas alonga good portion of the Atlantic seaboard from the 14th throughthe 18th of September. The greatest damages were along theeastern Carolinas northeast into New Jersey, and adjacent areasnortheastward along the east coast into Maine. Several stateshad numerous counties declared disaster areas. Floodingcaused major problems across the region, and at least 77 deathshave been reported. Damages are estimated to be $1.6 billion inPitt County, North Carolina alone, and total storm damagesmay surpass the $6 billion caused by Hurricane Fran in 1996.

Although Hurricane Floyd reached category 4 intensity in theBahamas, it weakened to category 2 intensity at landfall inNorth Carolina. Floyd's large size was a greater problem thanits winds, because the heavy rainfall covered a larger area andlasted longer than with a typical category 2 hurricane.Approximately 2.6 million people evacuated their homes inFlorida, Georgia, and the Carolinas—the largest peacetimeevacuation in U.S. history. Ten states were declared majordisaster areas as a result of Floyd, including Connecticut,Delaware, Florida, Maryland, New Jersey, New York, NorthCarolina, Pennsylvania, South Carolina and Virginia. Therewere several reports from the Bahamas area northward of waveheights exceeding 50 feet. The maximum storm surge wasestimated to be 10.3 feet on Masonborough Island in NewHanover County, NC.

Hurricane Dennis, August 1999The coastal areas of North Carolina had their fourth tropicalstorm scare in as many years during August 29th and 30th.Hurricane Dennis developed over the eastern Bahamas on the26th and drifted northward parallel to the southeast U.S. coastfrom the 26th through the 30th. Dennis became an immediatethreat to southeastern North Carolina on the 29th. The centerapproached to within 60 miles of the coast early on the 30th asa strong category 2 hurricane with highest sustained winds of105 miles per hour. Due to the fact that the hurricane nevermade landfall, damage was only moderate. Rainfall amountsapproached 10 inches in coastal southeastern North Carolinaand beach erosion was substantial. This area is no stranger tohurricane activity. Category 2 hurricane Bertha and category 3hurricane Fran hit Brunswick County in 1996 and HurricaneBonnie (category 2) followed nearly the same path in 1998.Prior to 1996, the area had been spared from the direct impact

41

of a hurricane since Charlie (category 1) hit Carteret County in1986.

Dennis made a return visit in September as a tropical storm,moving west-northwest through eastern and central NorthCarolina. The main impact this time was flooding due to heavyrains, with the maximum preliminary report being 13.82 inchesin Allisonia, VA. However, due to the storm lingering off thecoast for several days, beach erosion and damage to coastalhighways was significant. Residents of Hatteras and OcracokeIslands were stranded for several days because of severedamage to Highway 12.

Hurricane Mitch, October/November, 1998Hurricane Mitch will be remembered as the most deadlyhurricane to strike the Western Hemisphere in the last twocenturies! The death toll currently is reported as 11,000 withthousands of others missing. More than three million peoplewere either homeless or severely affected. In this extremelypoor third world region of the globe, estimates of the totaldamage from the storm are at $5 billion and rising. ThePresident of Honduras, Carlos Flores Facusse, claimed thestorm destroyed 50 years of progress.

Within four days of its birth as a tropical depression onOctober 22, Mitch had grown into a category 5 storm on theSaffir-Simpson hurricane scale. On October 26, the monsterstorm had deepened to a pressure of 905 millibars withsustained winds of 155 knots (180 mph) and gusts well over200 mph!

Mitch moved westward and on October 27, the category 5storm was about 60 miles north of Honduras. Preliminarywave-height estimates north of Honduras during this time atthe height of the hurricane are as high as 44 feet, according toone wave model. Although its ferocious winds began to abateslowly, it took Mitch two days to drift southward to makelandfall. Coastal regions and the offshore Honduran island ofGuanaja were devastated. Mitch then began a slow westwarddrift through the mountainous interior of Honduras, finallyreaching the border with Guatemala two days later on October31. Although the ferocity of the winds decreased during thewestward drift, the storm produced enormous amounts ofprecipitation caused in part by the mountains of CentralAmerica. As Mitch's feeder bands swirled into its center from

42

both the Caribbean and the Pacific Ocean to its south, the stagewas set for a disaster of epic proportions. Taking into accountthe orographic effects by the volcanic peaks of CentralAmerica and Mitch's slow movement, rain fell at the rate of afoot or two per day in many of the mountainous regions. Totalrainfall has been reported as high as 75 inches for the entirestorm. The resulting floods and mud slides virtually destroyedthe entire infrastructure of Honduras and devastated parts ofNicaragua, Guatemala, Belize, and El Salvador. Whole villagesand their inhabitants were swept away in the torrents of floodwaters and deep mud that came rushing down themountainsides. Hundreds of thousands of homes weredestroyed.