Hail, Lightning, And Tornadoes
Strong updrafts in a thunderstorm support the growth of large rain drops and ice crystals. In a severe storm some of the ice crystals may be dragged down by the downdrafts then swept up again by updrafts. Ice particles may be circulated several times through the storm cloud in this manner picking up water with each cycle. In a process called riming, rain water freezes onto the ice particles and eventually grows to be large hailstones. Hailstones continue to be recirculated through the cloud until they grow large enough to fall out under their own weight, falling against the strong updrafts. If located in the right part of the storm, hailstones can grow to impressive sizes. Hail as large as 5.5 in (14 cm) in diameter has been recorded.
Another product of the vigorous up and down drafts in the storm cloud is lightning. Lightning is a giant spark caused by a buildup of static electrical charges, a larger version of the spark one gets by touching a metal doorknob after walking across a cloth carpet. By processes that still are not understood fully, thunderstorm clouds build up a large separation of electric charge with positive charges located near the top of the cloud and negative charges concentrated near the middle. Usually the cloud base has a smaller pocket of positive charge. Separating charges results in huge voltage differences within the cloud and between the cloud base and the ground. The voltage difference is equalized suddenly by a bolt of lightning between these areas. The spark heats the air in the lightning channel to over 54,000°F (30,000°C) causing a rapid expansion. The resulting sound is heard as thunder.
Severe thunderstorms also may form tornadoes, columns of air spinning at extremely high wind speeds. Tornadoes pack wind speeds of 220 mph (over 100 m/second) in a small area, making them capable of great destruction.
See also Air masses and fronts; Atmospheric circulation; Tornado.
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