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Download Severe Thunderstorms
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Thunderstorms - form in unstable atmosphere - usually warm moist air near surface forced to rise • Ordinary (Air-Mass) Thunderstorms - last relatively short time ~ 1 hour - downdrafts overwhelm updrafts and storm weakens • Severe Thunderstorms - need vertical wind shear to form - updrafts and downdrafts are separate ==> storm can last much longer and intensify SEVERE THUNDERSTORMS Z 0 WIND 3 Storm motion 4 2 "Shelf Cloud" forms on side with updrafts 6 1 5 (0) Presence of vertical wind shear (1) Updrafts on one side of cloud (2) Due to wind shear, updrafts slant into upper cloud (3) Droplets/crystals grow => fall on other side of cloud => drag induces downdraft (4) Entrainment of dry air => evaporation (cooling) => enhances downdraft (5) Downdraft hits surface => spreads => creates gust front (6) Shelf cloud often forms on side with updrafts SUPER-CELL THUNDERSTORMS AND TORNADOES noted for extremely strong updrafts Favorable Conditions: - Vertical wind shear (change in speed with height) => helps produce severe thunderstorm - Change in wind direction with height => helps rotate updrafts Conditions often found in US Midwest during Spring just ahead of cold front TORNADO Rotating updraft can produce Mesocyclone: - 5-10 km system within Severe Thunderstorm - may produce Wall Cloud - Tornadoes often form within Wall Cloud Wall Cloud Wall Cloud TORNADO FREQUENCY F2 strength or greater Thunderstorm Frequency F4 strength or greater © Jim Leonard Clicker Question Set Frequency to "BB" Why doesn't Lightning form in shallow Marine Stratus Clouds? (A) cloud air is too warm (B) cloud air is too cold (C) temperature range of cloud air is too narrow Clicker Question Set Frequency to "BB" Why doesn't Lightning form in shallow Marine Stratus Clouds? (A) cloud air is too warm (B) cloud air is too cold (C) temperature range of cloud air is too narrow Hurricane Katrina on August 28, 2005 HURRICANE CROSS-SECTIONAL VIEW ANTI-CYCLONIC FLOW OUT OF HURRICANE AT TOP OF STORM CLOUDS CYCLONIC FLOW INTO HURRICANE AT SURFACE EYE FASTEST WINDS IN THE EYEWALL - Hurricane consists of many cloud bands - upward moving air inside clouds - downward moving air in between clouds and in eye - Inflow at surface is cyclonic (CCW in NH) - Outflow at top is anticyclonic (CW in NH) - Hurricane is fueled from below (warm moist air) and exhaust is at top.
