Ch14 - Jon Ahlquist Chap 14 Thunderstorms Tornadoes Ordinary thunderstorms Severe thunderstorms Microburst Mesoscale convective systems Squall line

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Jon Ahlquist 12/3/2006 MET1010 Intro to the Atmosphere 1 Chap 14: Thunderstorms Ordinary thunderstorms Severe thunderstorms Microburst Mesoscale convective systems Squall line Mesoscale convective complex Multicell thunderstorms move to right of wind Dryline thunderstorms Thunderstorms and Hail Lightning and lightning safety Tornadoes and water spouts Thunderstorms (p. 368) ± As the name indicates, all thunderstorms produce thunder and hence lightning. They are "convective" storms associated with warm, buoyant rising air. ± Atmosphere is often “conditionally unstable” (p. 145), meaning that air isn’t warm enough to rise on its own. Initial rise to condensation level and instability occurs because: ² Unequal surface heating creates warm bubble and/or ² Air flows over upward-sloping topography and/or ² Air is lifted by converging cooler air, perhaps from another thunderstorm, which wedges underneath ± T’storms most common in summer but can occur in cool season when cold air aloft creates very unstable situation. Florida can have winter thunderstorms. Thunderstorm Structure ± Any thunderstorm contains rising air (updraft) and sinking air (downdraft) ± Updraft: warm air rising in cloud. It cools by work of ± Downdraft : air sinking in cloud. Drier air from around the cloud is entrained (mixed) in at the cloud’s sides, as evidenced by the swirls on sides of cumulus clouds. The entrained air is cooled as rain evaporates into it, and cooled air sinks. Also, falling rain drags down air. ± Cell : updraft and downdraft pair. Thunderstorm may contain one or more cells. ± Supercell : thunderstorm with single very strong cell Ordinary Thunderstorms (pp. 368-371) ± “Ordinary thunderstorm” (or pop-up or air-mass thunderstorm) forms within an air mass having little vertical shear, i.e., change of wind speed and/or direction with increasing height. ± Cumulus (or growth) stage : the updraft fills the cloud. (That’s why there is a cloud.) As condensation forms, latent heat is released, making the air more buoyant. Updraft strong enough to prevent small droplets from raining out. ± Mature stage : cloud becomes tall enough that droplets become large and heavy enough to rain out of the cloud. Downdraft appears. Mature stage may last 15-30 min. ² Updraft is at peak intensity when rain first starts to fall, because once the downdraft gets going, it impedes & ultimately kills the updraft in an ordinary thunderstorm. ± Dissipating stage : updraft weakens, downdraft dominates Stages of an Ordinary Thunderstorm Rain starts, creating Downdraft cools by expansion. downdraft from dominates, Vapor condenses, cooling by evaporation so air can’t rise & thunder- rain. Gust fronts form storm dies| at surface Mature Gust fronts Dissipating Fig 14.1, p. 369 Cumulus Multicell storm complex (fig. 14.4, p. 371) Typical ordinary thunderstorm contains multiple “cells,” each
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This note was uploaded on 07/21/2011 for the course MET 1010 taught by Professor Staff during the Fall '08 term at FSU.

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Ch14 - Jon Ahlquist Chap 14 Thunderstorms Tornadoes Ordinary thunderstorms Severe thunderstorms Microburst Mesoscale convective systems Squall line

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