A tornado is a violently rotating column of air that descends from a thunderstorm to the ground. Tornadoes have been reported in Great Britain, India, Argentina, and other countries, but most tornadoes occur in the United States. Tornadoes form when warm, humid air collides with cold, dry air.
The most violent tornadoes come from supercells, large thunderstorms that have winds already in rotation. Tornado season begins in early spring for the states along the Gulf of Mexico. The season follows the jet stream-as it swings farther north, so does tornado activity. Twisters are usually accompanied or preceded by severe thunderstorms, high winds, or hail.
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) per hour. Meteorologists use Doppler radar, satellites, weather balloons, and computer modeling to watch the skies for severe storms and tornadic activity.