Tornadoes form inside thunderstorms, frequently when a cold front or a line of hot, dry air collides with warm, humid air and winds create a horizontal spinning effect. Strong updrafts then pull the warm, moist air upward and create a giant vertical chimney of rotating air. Some facts about tornadoes:

• The peak season for tornadoes in southern states is March through May; in northern states it's the summer months.
• Tornadoes are most likely to occur between 3 p.m. and 9 p.m., but they have been known to happen at all times of the day.
• Some tornadoes stay put, moving only a mile or two, while others travel more than 100 miles. 
• Some tornadoes are almost stationary, but most move at an average speed of 30 mph while others hit 70 mph.
• Tornadoes can strike in batches. In one 24-hour period in 1974, a remarkable 148 tornadoes struck in 13 states and Canada.   

All Strengths and Sizes
Tornadoes are classified three ways: weak, strong, and violent.

• The majority (69%) of all tornadoes are considered weak, meaning that they last only one to 10 minutes, have winds of less than 110 mph and cause only 5% of all tornado-related deaths.
• Strong tornadoes make up 29% of all tornadoes. They may last up to 20 minutes or more, have winds of 110 to 205 mph and account for nearly 30% of all tornado-related deaths. 
• Violent tornadoes can last longer than an hour and have winds greater than 205 mph. While only 2% of all tornadoes are considered violent, they account for 70% of all tornado-related deaths.