chapter 10

chapter 10 - CHAPTER 10: Thunderstorms and Tornadoes...

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CHAPTER 10: Thunderstorms and Tornadoes Thunderstorms Storms comprised of cumulonimbus clouds that contain: - lightning and thunder They can also produce gusty winds, torrential rains, hail and tornadoes. They form when warm, moist air ( mT ) rises in an unstable environment. Various mechanisms can trigger the upward movement: - convection (unequal heating of earths surface) - cold fronts - topographic lifting - sea-breeze induced lifting The globe experiences 45,000 thunderstorms / day (Fig. 10.2), most in: - lower (more tropical) latitudes - over land masses The US experiences 100,000 thunderstorms every year (Fig 10.3) - more in southeast (mT air) - max. in FL (sea breeze convergence Fig. 4.21) - less on west coast, New England (mP air) - North Carolina averages 40-50
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- Raleigh: 42 Air-Mass Thunderstorms Scattered, individual storms that develop within unstable mT air masses . Characteristics: - small (10s of kms in diameter) - short lived (1- 2 hours) - non-severe - most occur in spring and summer - most occurin the afternoon Stages of Development (Fig 10.4) A three-stage model was developed in the 1940’s after numerous thunderstorm related airplane crashes prompted field studies in Florida. Cumulus Stage (Fig. 10.4a) Warm, moist air rises, expands, cools and condenses, forming the cumulus cloud. Condensation releases Latent Heat Energy, warming the cloud making it less dense than surrounding air - Allows continue rising and growth Characteristics: - 5-10 km in height - updrafts only - no precipitaion - no lightening thunder Mature Stage (Fig. 10.4b) Cumulonimbus cloud grows well above freezing level, allowing cloud droplets to grow larger thru both Bergeron and Collision/Coalescence
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processes, until too large to be supported by updrafts. - rain starts to fall-creating a cool dense downdraft Characteristics: - 10-15 km height - avil cloud spreads out at tropopause - updrafts and downdrafts - intense percipitation - frequent lightening/thunder Dissipating Stage (Fig. 10.4c) Begins when downdrafts begin to dominate updrafts, cutting off the release of Latent Heat Energy, which is the “fuel” of the thunderstorm. - The storm literally collapses on itself Characteristics: - still 15 km height - down drafts only - precipitation decreases, stops - lightening/thunder decreases, stops A single Air Mass Thunderstorm generally goes thru all three stages within several hours. Surprisingly, less than 20% of the water vapor that is condensed in a thunderstorm actually reaches the ground as rain. The remaining 80% Is carried off as cloud that eventually evaporates.
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Hail (Figs. 5.21, 5.22, 5.23) produced by very strong up/down drafts (up to 100 mph) within Cb clouds where hail stones grow by colliding and coalescing with: super cooled liquid water droplets in the upper portions of the Cb cloud that freeze instantaneously
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chapter 10 - CHAPTER 10: Thunderstorms and Tornadoes...

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