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WHAT CAUSES A TORNADO? Tornadoes are whirling, vertical air columns that form from thunderstorms and stretch to the ground. They travel with ferocious speed and lay waste to everything in their path. Thunderstorms occur when denser, dryer cold air is pushed over warmer, humid air, conditions scientists call atmospheric instability. As that happens, an updraft is created when the warm air rises. When winds vary in speed or direction at different altitudes a condition known as wind shear the updraft will start to spin. These changes in winds produce the spin necessary for a tornado. For especially strong tornadoes, changes are needed in both the wind's speed and direction. \"When considerable variation in wind is found over the lowest few thousand feet of the atmosphere, tornado-producing 'supercell thunderstorms' are possible,\" said Paul Markowski, professor of meteorology at Pennsylvania State University. \"That's what we had yesterday." There's usually a lot of wind shear in the winter because of the big difference in temperature and air pressure between the equator and the Arctic, Gensini said. But usually, there's not a lot of instability in the winter that's needed for tornadoes because the air isn't as warm and humid, Gensini said. This time there was. SUMMARY