National Geographic : 2013 Nov
Cool downdraft Overshooting top Upper-level winds and direction of storm Warm updraft TORNADO TORNADOALLEY Cold, dry air Warm, moist air Gulf of Mexico ROCKYMOUNTAINS Warm, dry air Jetstream UNITED STATES The “cap” between air masses temporarily prevents thunderstorm formation. Less dense air wants to rise. Heaviest air wants to sink. Up to 45,000 feet 6,000 3,000 Wall cloud 2 to 3 miles wide Precipitation Mesocyclone The rising warm, moist air forms clouds as it cools and condenses. Only a fraction of supercells (rotating thunderstorms) produce tornadoes. 2 3 4 1 48 national geographic • november 2013 In Harm’s Way tornadoes have touched down in every state. but big ones happen most regularly each spring in tornado alley, from texas to the northern Great Plains. here warm, moist air from the Gulf of Mexico collides with fast-moving cold, dry air from the rockies, generating strong wind shear and atmospheric instability—key elements of tornadic thunderstorms. winD shear fast-moving winds roll air below into a horizontal vortex—a spinning tube—above opposing surface winds. EL RENO TIME LINE, May 31, 2013 Forecasters note extreme atmo- spheric instability in Oklahoma. uPDraft warmed by the sun, buoyant air near the ground begins to lift a section of the horizontal vortex into a vertical position. The National Weather Service issues a tornado watch for El Reno at 3:30 p.m. storM the stronger of two vortices created by the updraft becomes the heart of a thun- derstorm. the other one dies. The El Reno thunderstorm forms in under an hour, faster than most. MaP: Martin GaMaChe, nGM staff. sourCe: noaa GraPhiC: lawson ParKer, nGM staff. art: niCK KaloteraKis sourCe: Gabe GarfielD, national weather serviCe anD CooPerative institute for MesosCale MeteoroloGiCal stuDies warm, moist air rises until it hits warm, dry air—the cap. if the moist air becomes warmer than the cap, it can punch through it explosively. HOW A TORNADO FORMS *the ef scale is based on wind speed and degree of damage.