Tornadoes are rapidly rotating columns of wind that extend to the surface of the earth. Sometimes tornadoes look more like plumes of smoke, and other times they are not visible at all, because of heavy rain or thick clouds. During spring and summer storms, tornadoes are usually formed when warm, humid air collides with cold, dry air masses. The warm air streams upward while the cold air rushes in underneath. The in-rushing air swirls around a center of low pressure, forming a funnel.
Tornadoes can also emerge during tropical storms or hurricanes that make landfall.
Meteorologists measure tornado strength using the Fujita Scale, which ranks the storms based on wind speed. According to The Weather Channel, the average tornado is 75 yards wide, has a track length of one to four miles, is on the ground for about 5 minutes and moves at an average of 25 miles per hour. Last year's tornado season was relatively quiet compared to 1998 and 1999, which saw high numbers of tornadoes. Still, the storms that hit in 2000 were powerful and deadly. On March 28, a tornado touched down in Fort Worth, Texas, leaving 5 people dead.