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GEOG 2110L (50)


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GEOG 2110L
David Stanley

THUNDERSTORMS ** a storm characterized by a cumulonimbus (Cn) cloud & accompanied by lightning and thunder * on average of relatively short duration, 1 - 3 hours, but may last for up to 12 hours or more * severe thunderstorms are accompanied by strong winds, heavy rain, and sometimes hail, & tornadoes * they are a good indication of great instability in the atmosphere and show a great deal of vertical development Requirements for Formation: * Warm, moist air: which releases Latent Heat Energy when lifted and condensation occurs; this provides buoyancy, maintains lift & develops updrafts * need continuous supply to overcome dry air & evaporation * High surface temperatures: enhances instability, air parcel warming & uplift Stages of Formation: * Cumulus stage: initial build-up of cumulus clouds fueled by updrafts (up to 160 kph) of warm, moist air cooling adiabatically; droplet formation by Bergeron process & Collision-Coalescence * Mature stage: raindrops start to fall initiating downdrafts; process of entrainment, the influx of cool, dry surrounding air helping to fuel the downdrafts; heavy rains, lightning, thunder are most intense during this stage; It is also during this stage that hail or a tornado may occur. * Dissipating stage: downdrafts dominate; light rain; cooling with loss of warm air source & energy; storm breaks up & clouds evaporate Locations of Occurrence: can occur in many parts of the world: tropical areas, along the ITCZ; in the U.S., form primarily east of Rocky Mts. with Florida being the state with the highest incidence of T-storms and the Great Plains with the greatest number per year LIGHTNING, THUNDER, & DERECHOS LIGHTNING: Flashes of light generated by the flow of tens of millions of volts of electron (electrical charge) between oppositely charges parts of a cloud or between the cloud and the ground What causes lightning? ** charges are separated within the cloud as the T-storm develops * positive charges near the top, & negative charges at the base * lightening is the clouds way of trying to equalize this charge difference or imbalance * after a charge difference builds to millions or hundreds of millions, a lightning stroke occurs to discharge the negative base of the cloud * may do this by: within cloud lightning; cloud-to-cloud lightning; cloud-to- ground lightning The Lightning Stroke: * first, an invisible step leader of electrons is formed from the cloud base to the ground, which usually consists of several pathways * one of these pathways reaches the ground and the connection between the areas of unlike charges is completed * this path is then illuminated as the electrons move back toward the cloud in the return stroke * the first stroke is often followed by 3 or 4 more strokes within .2 seconds making up the single flash you see THUNDER: the sound e
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