Twisters are usually accompanied or preceded by severe thunderstorms and hlgh wlnds. Hail is also common.

Once a tornado hits the ground, it may live for as little as a few seconds or as long as three hours.

The average twister is about 660 feet (200 meters) wide and moves about 30 miles (50 kilometers) an hour. Most don't travel more than six miles (ten kilometers) before dying out.

Massive tornadoes, however—the ones capable of widespread destruction and many deaths—can roar along as fast as 300 miles (480 kilometers) an hour.

These measurements are scientists' best estimations. Anemometers, which measure wind speed, cannot withstand the enormous force of tornadoes to record them.

Using units F0 to F5, the Fujita scale measures a tornado's intensity by analyzing the damage a tornado has done and then matching that to the wind speeds estimated to produce comparable damage.