Chapter 11 Thunderstorms and Tornadoes Lifted Index Lightning Mature Stage Mesocylone Mesoscale Convective Complex MCC Mesoscale Convective system MCS Microburst Multicell Multiple vortex tornado Nocturnal low level jet Ordinary cell Overshooting top Pilot leader Red sprites Return stroke Right Mover Severe thunderstorms Shelf cloud Squall line Stepped leader Supercell Thunderstorm Tornado Tornado alley Tornado vortex signature TVS Vertical wind shear Wall cloud Waterspot Wet growth Key Terms Air mass thunderstorm Blue jets Capping inversion Cumulus stage Dart leader Dissipating stage Dry growth Elves Enhance Fujita Scale Entrainment Flash Flood Flood Funnel Cloud Gust Front Hailshaft Hailswath Hook Echo Summary Thunderstorms produce lightning thunder tornadoes hail and other severe weather All thunderstorms form in unstable air masses Indices such as the Lifting Index allow meteorologists to assess quickly the stability of the atmosphere by using observations of the atmosphere above a particular location Severe thunderstorms grow in unstable environments with lift that also possess vertical wind shear and a change in wind speed or direction with increasing altitude Environmental vertical wind shear is important for severe thunderstorms because it helps to separate the updraft from the downdraft This enables the storm to last longer and grow more severe The basic building block of a thunderstorm is the cell A thunderstorm can be composed of a single cell or multiple cells Air mass thunderstorms are single ordinary cells that are not associated with severe weather Single cell thunderstorms follow a predictable life cycle from cumulus and mature stages to a dissipating stage in an hour or less Squall lines and circular MCCs are examples of multicellular thunderstorms both of which can last for hours and produce severe weather Supercells are single cell thunderstorms but they are larger last longer than ordinary thunderstorms and often produce severe weather Supercell thunderstorms develop when the environmental wind changes in both speed and direction with increasing altitude causing the supercell to rotate This rotation promotes the development of tornadoes Tornadoes and large hail are produced by thunderstorms that grow in unstable atmospheres with vertical wind shear Tornadoes are violently rotating columns of air that stretch from the cloud base to the ground in the updraft region of the storm A sign of tornadic conditions is a rotating wall cloud or a funnel cloud extending downward from the base of the thunderstorm Tornadoes appear on radar as hook echo reflections or mesocyclones with particles rushing toward and away from the radar beam forming a tornado vortex signature Once formed the tornado usually grows in size and strength and then narrows to a thin rope before dissipating Tornadoes are usually less than 1 mile wide and only on rare occasions last for an hour Tornado winds can be as high as 200 mph and are estimated using the EF scale The EF scale is damage based and runs from 0 light damage to 5 incredible damage Only about 1 of US tornadoes are EF4 or EF5 tornadoes but they cause 67 of tornado deaths Many US tornadoes occur in the Great Plains in spring but they can occur over nearly the entire country in every month of the year Lightning is a huge electrical discharge that results from rapid rising and sinking air motions within the thunderstorm that cause collisions of ice particles with graupel A typical lightning flash is a composite flash composed of several lightening strikes The stepped leader with subsequent dart leaders leads each stroke The return stroke is the part of the lightning flash we usually observe The most life threatening thunderstorm weather is flooding particularly flash flooding A flash flood occurs suddenly and floods a region with a great volume of water in a short time span Rainfall intensity and duration are two key elements of a flash flood Topography soil conditions and ground cover also play important roles Severe long term flooding can occur when thunderstorms repeatedly drench a region Hail occurs when the strong updrafts and downdrafts in a thunderstorm cause repeated freezing of supercooled water on small ice particles The curtain of hail that falls near the updraft the hailshaft can cause severe damage to cars and agriculture as the thunderstorm passes over urban and rural areas Although thunderstorms and lightning are most common in the southeast United States the Frontal Range of the Rocky Mountains is a preferred area for hailstorms Thunderstorms and their attendant behavior severe weather can threaten lives Although there are many different dangers associated with severe thunderstorms we can summarize who is most at risk from severe weather People who are outdoors in the open under trees or on water are at most risk from lightning Mobile homes and automobiles are at a higher risk when tornadoes are nearby People in automobiles are also at risk in flash flooding conditions as are people in low lying areas and canyons
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