Many tornadoes are small, less than 100 feet wide, and last only a few minutes. A few monster tornadoes are a mile or more wide and can last for an hour or more. As the parent thunderstorm travels along, tornadoes can come down from the cloud, run along the ground and lift back up to be followed by other tornadoes. Generally, tornadoes move along the ground at around 20 to 50 mph, but some race along faster than 70 mph. Tornado debris can be huge.
Debris blown by tornado winds can pound buildings to pieces.
The strongest tornadoes come from the kind of long-lasting, especially fierce thunderstorms known as supercells. As the name implies, these are intense thunderstorms which can produce large hail and downbursts in addition to tornadoes. Some bring heavy rain while others are relatively dry. Supercells are most common on the Plains in the Southeast and across the Midwest, but do occur elsewhere. Storms form when there is unstable air, a source of lifting, and humidity.