How tornadoes formThe U.S. experiences the most of any country. They occur on average 1,000 times and kill 60 people per year, mostly in two regions -- Florida and the so-called "Tornado Alley," the area between the Rocky and Appalachian Mountains. But they've been recorded in all 50 states, usually between late winter and mid-summer.
No one knows what forms a tornado, but scientists agree on a few general points. The most common tornadoes come from rotating thunderstorms, called supercells.
In a supercell, the updraft of warm air rotates counter-clockwise in the northern hemisphere, reaching speeds of up to 175 mph and forcing cool air to descend. The updraft can pull cool moist air -- which would ordinarily fall from clouds -- back up into the sky. The saturated air then condenses to form a rotating wall of clouds, typically toward the back of a cloud in rain-free zones.
With enough strength, the spinning effect creates a funnel, hits the ground and creates a tornado that can last several seconds to more than an hour. Damage can range from superficial, such as roof damage from a falling tree, to completely sweeping away a structure, leaving only the foundation.
Read more: http://www.foxnews.com/science/2013/05/21/tornado-primer-earths-violent-storms/#ixzz2TyuoDpbb
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