find main idea Tornadoes have the most violent winds of any kind of weather. They can flip trains upside down or tear the roofs off of buildings. They can also pull huge trees out of the ground. Tornadoes are narrow columns of strong, spinning air that can form during thunderstorms. They extend from the bottom of thunderstorms down to the ground. Tomadoes usually form from very large thunderstorms called supercells. Most tornadoes are less than 1/4 of a mile wide. They are rarely on the ground for more than half an hour. However, their winds can spin at 250 miles per hour or more and can cause terrible damage. Tornadoes can occur almost anywhere on the earth. However, more form in the United States than in any other place. The conditions that cause tornadoes to form are most common in the flat area in the middle of the country. This area has been given the nickname of Tornado Alley. Cool, dry air from Canada meets warm, moist air from the Gulf of Mexico. Warm, dry air also blows in from the Southwest. Each year, as many as 1,200 tornadoes form in the United States. Other countries get tornadoes, too. However, they occur much less often than the United States. Canada is second with about 120 each year. England is third with about 34 each year. New Zealand and Japan are fourth and fifth. They each get about 20 per year. Most people hope to never see a tornado. However, some people get close to them on purpose to study them. These people are known as storm chasers. They take equipment and jump in trucks whenever conditions are right for tornadoes to form. They take pictures and record information with special equipment. Their work helps weather scientists learn more so that they can keep people safe from the earth's most violent weather.
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