If you've seen the old movie "Twister" on TV, then you know that a tornado — a violently rotating column of air extending from a thunderstorm to the ground — can be a pretty scary sight. While about 70 percent of tornadoes are fairly weak and don't cause serious damage, the remaining 30 percent can be frighteningly powerful and intense. These violent tornadoes have wind speeds of 250 miles per hour or more, and they have the potential to cause tremendous destruction. When a tornado touches ground, it can knock down trees and houses in a mile-wide swath that stretches for as far as 50 miles. Once, a tornado in Broken Bow, Okla., picked up a big motel sign, carried it for 30 miles, and dropped it in the state of Arkansas! The U.S. National Oceanic and Atmospheric Administration (NOAA) says that in an average year, 800 tornadoes are reported nationwide, resulting in 80 deaths and more than 1,500 injuries.

What is a tornado?

Tornadoes are a byproduct of thunderstorms, which develop in warm, moist air in advance of moving cold fronts. Before a big storm actually develops, the wind begins to change direction and increase speed. That, in turn, causes a horizontal spinning effect in the lower atmosphere. The rising warm, moist air within the developing thunderstorm changes the tilt of the rotating column of air from horizontal to vertical. This spinning, rising column is called a vortex. By this point, the rotating column of air is 2 to 6 miles wide and extends through much of the thunderstorm. It quickly forms into a tornado.