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  1. 1. Thunderstorms
  2. 2. Thunderstorms• What is a thunderstorm?– A thunderstorm is simply a storm that generateslightning and thunder.– They frequently produce gusty winds, heavy rain andhail.– They may be produced by a single cumulonimbus cloudand influence a very small geographic area.– Or they may be formed from clusters of cumulonimbusclouds and affect large geographic areas.
  3. 3. Different Types ofThunderstorms• Air-Mass Thunderstorms– Frequently occur in maritime tropical (mT) air thatmoves northward from the Gulf of Mexico.– These warm, humid air masses contain abundantmoisture in their lower levels and can be renderedunstable when heated from below or lifted along a front.– mT air most often becomes unstable in spring andsummer, and form mostly in the midafternoon whensurface temperatures are highest.– Generally occurred as scattered, isolated cells.
  4. 4. Stages of ThunderstormDevelopment• Important field studies conducted in the 1940sprobed the dynamics of air-massthunderstorms.– Cumulus Stage• Buoyant thermals produce fair weather clouds.• This is important because this moves water vaporfrom the ground to greater heights.• The development of the cumulonimbus towerrequires a constant supply of moist air.• Release of latent heat allows each new surge of warmhumid air to rise higher than the last, adding to theheight of the cloud.
  5. 5. Stages of ThunderstormDevelopmentCumulus Stage (cont’d)Dominated by updrafts.Precipitation within the cloud begins.Eventually the precipitation in the cloud becomes toogreat for the updrafts to support.Falling precipitation causes drag on the air and initiatesdowndraft.Heavy cool air surrounding the cloud causes some ofthe precipitation to evaporate (cooling).This cools the air within the downdraft.
  6. 6. Stages of ThunderstormDevelopmentMature StageAs the downdraft leaves the cloud, precipitation isreleased.The sharp, cool gusts of wind that you feel before athunderstorms are indicative of the updrafts higher inthe thundercloud.When the cloud grows the top of the unstable region(often at the base of the stratosphere) the updraftsspread laterally and produce the characteristic anviltop.
  7. 7. Stages of ThunderstormDevelopment
  8. 8. Stages of ThunderstormDevelopment– Mature Stage (cont’d)• Generally, ice-laden cirrus clouds make up the top.• Most active period of the thunderstorm.– Dissipation Stage• Once the downdrafts begin, the vacating air andprecipitation encourage more cooler and drier air tosurround the cell.• Eventually, downdrafts dominate.• Without a supply of warm most air, the cloud begins toevaporate.–
  9. 9. Severe Thunderstorms• Capable of producing heavy downpours, flashfloods, strong, gusty straight-line winds, largehail, frequent lightning, and if this all wasn’t badenough, possibly tornadoes.• What is a “severe” thunderstorm?– Winds in excess of 93 km/h (58 mp/h)– Or produce hailstones with diameters larger than1.9 centimeters (0.75 inches)– Or generate a tornado.
  10. 10. Thunderstorm Safety• Lightning impacts peoples lives–• As the summer approaches, here is a publicservice announcement from NOAA.–• Safety Tips–
  11. 11. Thunderstorms
  12. 12. ThunderstormsDid you know that a storm is classified as athunderstorm only after thunder is heard?But, because thunder occurs as a result oflightning, lightning must be present.
  13. 13. Where Does LightningCome FromHow does lightning form?During the formation of a cumulonimbus cloud, aseparation of charge occurs, meaning that part ofthe cloud itself develops a more negative chargethan the rest of the cloud.The whole object of lightning is to equalize thesedifferences by producing a negative flow to theregion of positive charge, and vice versa.
  14. 14. The Making of LightningBecause air is a poor conductor of electricity (it is agreat insulator), the electrical potential (chargedifference) must be very high for lightning to occur.The most common type of lightning occurs betweenoppositely charged zones within a cloud. (80 % of alllightning is of this type).
  15. 15. The Making of LightningLightning within a cloud is often referred to as sheetlightning because it produces a bright but diffuseillumination of those parts of the cloud in which theflash occurred.The second type of lightning is cloud-to-ground, andrepresents 20% of all lightning.It is the most damaging and most dangerous form.
  16. 16. What Causes Lightning?The origin charge separation in the clouds is not fullyunderstood.What is strongly suspected is that it hinges on rapidvertical movements in the clouds.This is because lightning occurs mainly in the violentmature stage of a cumulonimbus cloud. (In mid-latitudes[that’s us], this is a function of summertime heat).Lightning rarely occurs before the growing cloudpenetrates the 5km level. (≈ 3 miles)
  17. 17. What Causes Lightning ?Some physicists speculate that charge separationoccurs during the formation of ice pellets.This is because, through experimentation, asdroplets begin to freeze, positively charged ions areconcentrated in the warmer regions.Thus, as droplets freeze from the outside in, theydevelop a positively charged ice shell and anegatively charged interior.
  18. 18. What Causes Lightning?As the interior begins to freeze, it shatters theoutside shell.The smaller, positively charged fragments arecarried upward by the turbulence, and thenegatively charged, heavier particles traveltowards the base of the cloud.
  19. 19. What Causes Lightning?
  20. 20. What Causes Thunder? electrical discharge of lightning superheats theair surrounding the lightning channel.In less than a second, the temperature rises by asmuch as 33,0000C (59,4320F).When air is heated this quickly, it expands explosivelyand produces the sound waves that we hear asthunder.
  21. 21. What Causes Thunder?Lightning is seen almost instantaneouslyBut sound waves, which are relatively slower,travel at approximately 330 meters (1000 feet)per second.If thunder is heard 5 seconds after the lightningis seen, the lightning occurred about 1650meters (approximately 1 mile) away.
  22. 22. What Causes Thunder?The lightning that we hear as a rumble is producedalong a long lightning path located at some distancefrom the observer.When lightning occurs 12 miles away, thunder israrely heard.We often heard this referred to as heat lightning.This is no different than lightning that we normallyassociate with thunder.
  23. 23. Supercell ThunderstormsConsists of a single, very powerful cell.It can extend to heights of 20 km (65,000 ft)They can persist for many hours.Their diameters may range between 20 and 50km (12 to 30 miles).
  24. 24. Supercell ThunderstormHere is a supercell that was spotted in Charlton,Massachusetts.
  25. 25. MicroburstsWhat is a microburst?Beneath some thunderstorms, strong localizeddowndrafts known as microbursts can occur.When they are small, they are less than 4 km (2.5miles) across.These straight-line concentrated bursts of wind areproduced when downdrafts are accelerated by agreat deal of evaporative cooling.
  26. 26. Microbursts(Remember, the colder the air, the denser it is).Typically last just two to five minutes.Despite size and duration, they represent asignificant atmospheric hazard.1993: Millions of trees were uprooted by amicroburst near Pak Wash, Ontario.
  27. 27. MicroburstsJuly, 1984: 11 people drowned when amicroburst caused a 28 meter (90 foot)sternwheeler boat to capsize on the TennesseeRiver.Sometimes damage from these storms ismistaken for tornado damage.
  28. 28. MicroburstsWind shear associated with microbursts have been associated withplane crashes. Hampshire microburst is suspected in damage to camperdealership
  29. 29. LightningNo place is absolutely safe from the threat oflightning, but some places are safer than others.Large enclosed structures (substantially constructedbuildings) tend to be much safer than smaller or openstructures. (Does the structure incorporate lightningprotection?)Fully enclosed metal vehicles such as cars, trucks,busses, vans, fully enclosed farm vehicles with thewindows rolled up provide good shelter. Avoid contactwith metal or insides and outside conducting surfaces.
  30. 30. Lightning (cont’d)Avoid being in or near high places, open fields,isolated trees, unprotected gazebos, rain orpicnic shelters, baseball dugouts,communication towers light poles, flagpoles,bleachers (wood or metal) , metal fences,convertibles, golf carts and water (yes, evenswimming pools).
  31. 31. Lightning (cont’d)When inside a building, avoid:Using the telephoneTaking a showerWashing your handsDoing dishesAny contact with conductive surfaces withexposure to the outside. (Metal doorframes,electrical/telephone/cable tv wiring, plumbing, etc.
  32. 32. Lightning (cont’d)In general, if you can see lightning or hearthunder, you’re already in trouble. The morefrequent that it is seen or heard, the storm isapproaching.If the time difference between the lighting andthe thunder is less than 30 seconds, you hadbetter be seeking a safe location.
  33. 33. Lightning (cont’d)Ranging is not perfect due to difficulty with associatingthe thunder with the proper lightning flash.High winds, rainfall and cloud cover can serve aswarnings.Many lightning casualties occur in the beginning of thestorm as it approaches OR as the perceived threat haspassed.Pay attention to weather warnings (TV, NOAA,Weatherbug)
  34. 34. Lightning (cont’d)About how many people who are struck bylightning are actually killed?According to the National Weather Service, about10% of lightning strike victims are killed; 90%survive.However, many survivors suffer from long lastingdisabilities.