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Flash Flooding ChapterChapter 88 continued…..continued….. ThreeThree typestypes ofof cyclones:cyclones: 1. Wave cyclones: cyclone/low 2. Tropical cyclone: hurricane/typhoon 3. Tornado LocalLocal SevereSevere WeatherWeather ` Flash Flooding ` Lightning ` High Winds ` Hail ` Tornadoes AirAir MassMass ThunderstormThunderstorm LifeLife CycleCycle AirAir MassMass ThunderstormThunderstorm LifeLife CycleCycle AirAir MassMass ThunderstormThunderstorm LifeLife CycleCycle •Downdrafts spread outward when they hit the ground and converge with the warmer surrounding air to form the outflow boundary. LifeLife--CycleCycle ofof aa ThunderstormThunderstorm TotalTotal timetime cancan bebe lessless thanthan oneone hourhour SupercellSupercell ThunderstormsThunderstorms SupercellSupercell ThunderstormThunderstorm overover LaPlataLaPlata,, MDMD AprilApril 20022002 InsideInside aa SupercellSupercell RadarRadar ImageImage ofof aa SupercellSupercell –– HookHook EchoEcho ThunderstormsThunderstorms maymay groupgroup togethertogether asas aa MesoscaleMesoscale ConvectiveConvective ComplexComplex (MCC)(MCC) DownburstsDownbursts DeadlyDeadly DowndraftsDowndrafts ` Strong downdrafts may create downbursts ` Wind speeds can exceed 270 km/hr (170 mph). ` When strong downdrafts reach the surface, they can spread outward in all directions to form intense horizontal winds, capable of causing severe damage. ` Downbursts with diameters of less than 4 km are called microbursts Dangerous problem when they occur near airports. Geographic Distribution of Thunderstorms ` Thunderstorms develop where moist air is forced aloft ` Occurs frequently in the tropics, nearly daily in some locations ` In the United States, most frequent region is the Gulf South ` Highest incidence in Florida because it is almost completely surrounded by warm waters AverageAverage annualannual numbernumber ofof daysdays withwith thunderstormsthunderstorms Flash Flooding FLASHFLASH FLOODINGFLOODING More persons are killed by flash flooding than any other form of severe weather. An average of 135 persons die each year. LightningLightning OnOn average,average, 44,00044,000 thunderstormsthunderstorms occuroccur aroundaround thethe worldworld everyevery day.day. AtAt anyany givengiven time,time, anan averageaverage ofof nearlynearly 20002000 thunderstormsthunderstorms exist.exist. SinceSince eacheach thunderstormthunderstorm willwill generategenerate anan averageaverage ofof 100100 cloudcloud--toto--groundground lightninglightning strokes,strokes, thethe totaltotal dailydaily numbernumber ofof lightninglightning dischargesdischarges aroundaround thethe worldworld isis approximatelyapproximately 44 –– 55 MILLION!!MILLION!! MuchMuch ofof thethe energyenergy ofof aa lightninglightning dischargedischarge isis dissipateddissipated throughthrough thethe airair asas shockshock waveswaves thatthat wewe callcall thunderthunder Air heated to a temperature of 50,000°F in the conducting channel LightLight travelstravels atat 300,000300,000 km/seckm/sec butbut soundsound waveswaves traveltravel atat onlyonly 0.330.33 km/seckm/sec ThusThus thethe thunderthunder isis heardheard afterafter thethe lightninglightning strokestroke ThunderThunder ` Caused by the rapid expansion of air from tremendous increase in temperature during a lightning stroke ` Lightning without thunder being heard called heat lightning Thunder is produced but stroke is too far away to reach an observer ` Rumbling thunder is typically caused by sound echoing off topographic features and buildings ` Slower speed of sound (vs. speed of light) causes a lag between the stroke and the resulting thunder ` To determine the distance in km, count the seconds between the stroke and thunder and divide by 3 (divide by 5 to determine distance in miles) ChargeCharge SeparationSeparation –– InitiatingInitiating LightningLightning Leaders, Strokes, and Flashes Development of Cloud-to-Ground Lightning Processes of Lightning Formation (Source: http://thunder.msfc.nasa.gov/primer) TypesTypes ofof LightningLightning –– ManyMany typestypes existexist ` St. Elmo’s Fire tall objects (e.g., church steeples) glow as ionization occurs in the air around them Blue-green tint, hissing sound ` Ball lightning may be the most unusual type A round mass of electrified air about as large as a basketball Rolls through the air or along a surface for 15 seconds or so before either dissipating or exploding St. Elmo’s Fire (Source: http://www.physics.northwestern.edu/classes/2001Fall/Phyx135-2/17/relatedequations.html) Ball Lightning (Source: http://www.pharmachip.hu/zyx/images/ball.jpg) A strong electrical field occurs prior to lightning WindsWinds Roll cloud with gust front MicroburstMicroburst andand WindWind ShearShear HAILHAIL Tornadoes Tornadoes Around The World. TheThe LocationLocation andand TimingTiming ofof TornadoesTornadoes ` The United States is the world leader in tornadoes! ` Results from the regular interaction between extremely unlike air masses which originate in very high latitudes and over the Gulf of Mexico ` Absence of topographic barriers (especially East-West barriers) ensures regular mixing and the production of violent storm systems Average Tornado Incidence per 10,000 mi2 Tornado Alley TheThe LocationLocation andand TimingTiming ofof TornadoesTornadoes ` Texas has the highest annual tornado frequency of any state ` Oklahoma and Florida have the highest concentrations (annually) of tornadoes (and Delaware! ☺) ` Tornadoes can happen in any month of the year in the United States ` Late spring is the time of greatest overall activity in the United States May is the month of highest tornado frequency June is the month of next highest tornado frequency ` Many states show tornado peaks during different months TheThe distributiondistribution ofof tornadoestornadoes duringduring thethe yearyear byby state.state. MonthlyMonthly TornadoTornado FrequenciesFrequencies forfor thethe U.S.U.S. AA PerfectPerfect Geography!!Geography!! TornadoTornado--producingproducing SupercellSupercell Tornado Development along a Convergence Boundary where strong convection exists A wall cloud may descend from the rotating mesocyclone Wall cloud Wall cloud TheThe lifelife cyclecycle ofof aa tornado….tornado…. TheThe CollegeCollege Park,Park, MDMD TornadoTornado AA hugehuge GreatGreat PlainsPlains SupercellSupercell andand TornadoTornado TowardToward thethe endend ofof theirtheir lifelife cycle,cycle, manymany tornadoestornadoes becomebecome “rope“rope tornadoes”tornadoes” MammatusMammatus CloudsClouds OverOver MexicoMexico Credit & Copyright: Raymundo Aguirre (2004) TornadoTornado DamageDamage ` Winds, not pressure change, cause the greatest amount of damage ` Flying debris causes the greatest amount of injuries ` Although most tornadoes rotate around a single, central core, some of the most violent ones have small zones of intense rotations called suction vortices ` The existence of multiple suction vortices (≈10 m in diameter) within a single tornado account for selective damage patterns TornadoTornado DamageDamage ` Tornadoes are classified using the Fujita Scale which ranks tornadoes based on damage ` Roughly 69% of all tornadoes fall into the weak categories (0-1) while 29% are classified as being strong (2-3), and only 2% are deemed violent (4-5) ` Violent tornadoes are capable of nearly catastrophic damage The Fujita (F) Scale TornadoTornado FatalitiesFatalities ` Due to their small spatial scales, tornadoes kill few people ` On average 760 tornadoes occur in the United States annually Only 91 people are killed annually 88% of all tornadoes kill no one Most fatalities occur in associated with a few large tornadoes rather than with many smaller ones Only about 1% of all tornadoes are responsible for over 2/3 of all deaths ` Mobile homes and cars are the sites of many deaths ` Safest place to be during a tornado is in a building basement CarsCars areare thethe worstworst placeplace toto bebe duringduring aa tornado!tornado! Over 600 fatalities and thousands of injuries! TheThe “super“super outbreak”outbreak” ofof tornadoestornadoes onon AprilApril 33--4,4, 19741974 spawnedspawned 148148 tornadoestornadoes inin 2424-- hours!hours! Aerial View of Damage from Moore, Oklahoma Tornado (May 3, 1999) (Source: http://www.k5kj.net/news_archives.htm).
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