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Tornadoes: Terminology Tornado: a violently rotating column of air in contact with the ground. Also called “twister” and “cyclone” Waterspout: a tornado over water, generally much weaker than a tornado Funnel Cloud: a rapidly rotating column of air not in contact with the ground, often the precursor to a tornado, extending from the base of a cumulonimbus cloud Tornado Safety Avoid blowing debris (don’t become a part of it, either) – Get underground, if possible (wind speed = 0) – Find a ditch or low spot and cover your head – Interior, windowless room of a sturdy building, preferably under something that won’t fall on you but will protect you from falling/flying debris – Avoid cars and mobile homes, highway overpasses Tornado Intensity The Fujita Scale: based upon damage surveys and not direct measurement of winds – Similar to Saffir-Simpson scale for hurricanes – Ranges from F0 (EF0) for very weak tornadoes to F5 (EF5) for incredibly strong tornadoes – Scale goes higher (F12) but it’s unlikely we’ll see anything stronger than an F5- how can damage be any worse than “incredible?” – The strongest tornadoes develop within supercells – Enhanced Fujita Scale Supercell Thunderstorms Produce the most extreme severe weather – Large, strong tornadoes Wind shear- direction, speed Rotating thunderstorms – Long (~6 hour) lifespan Wind shear- direction, speed Tilted updraft – Large hail and damaging winds Extreme instability Extremely strong updrafts and downdrafts Supercells: Extreme Vertical Motion The Lid (capping inversion) – Warm, dry air overlies warm, moist air – Result of low-level S,SE winds from Gulf of Mexico and SW winds above this layer blowing from the Mexican plateau – Limits development of “wimpy” thunderstorms: more energy available for a few supercells versus lots of run-of-the-mill thunderstorms Supercells: Extreme Vertical Motion – Breaking the Lid Daytime heating weakens it Rising air will cause a more rapid cooling aloft than near surface: result is explosive thunderstorm development Supercells: Long-Lived Monsters Directional and speed shear tilts the updraft Separates inflow and outflow – Constant supply of warm, moist air Supercells: Tornado Development Directional shear induces thunderstorm rotation Speed shear creates horizontal roll Roll tilts vertically in updraft Supercells: Tornado Development Strong updrafts and downdrafts, especially the rear flank downdraft, appear to be important in tornadogenesis – From Markowski et al, 2002, Direct Surface Thermodynamic Observations within the Rear-Flank Downdrafts of Nontornadic and Tornadic Supercells, Monthly Weather Review Tornadoes Suction Vortices: think of them as mini tornadoes within the primary funnel Image from www.atmo.arizona.edu/.../apr2609.jpg Are tornadoes becoming more frequent? Key Figures Tables 14.8, 14.9, 15.8, 15.10, 15.12 Interesting Time-Lapse Movie of Thunderstorm Development Time-Lapse 2 / Time-Lapse 3 Multi-Vortex Tornado M-V Tornado from Helicopter Rotating Supercells in Wyoming/Texas What you normally should NOT do