Tornadoes in the United States

Tornadoes are more common in the United States than in any other country. The United States receives more than 1,200 tornadoes annually—four times the amount seen in Europe. Violent tornadoes—those rated EF4 or EF5 on the Enhanced Fujita Scale—occur more often in the United States than in any other country.

Most tornadoes in the United States occur east of the Rocky Mountains. The Great Plains, the Midwest, the Mississippi Valley and the southern United States are all areas that are vulnerable to tornadoes. They are relatively rare west of the Rockies and are also less frequent in the northeastern states. Tornado Alley is a colloquial term for an area particularly prone to tornadoes. There is no officially defined 'Tornado Alley' - at its broadest this area stretches from North Texas to Canada with its core centered on Oklahoma, Kansas and northern Texas. Another highly significant region - colloquially known as Dixie Alley - is the southern United States and particularly the northern and central parts of Alabama and Mississippi. Florida is one of the most tornado prone states. However, Florida tornadoes only rarely approach the strength of those that occur elsewhere.

Although favorable conditions for tornadoes in the United States can occur at any time, they are most common in spring and least common in winter. Because spring is a transitional period for the climate, there are more chances of cooler air meeting with warmer air, resulting in more thunderstorms. Tornadoes can also be spawned by landfalling tropical cyclones, which usually occur in late summer and autumn. In the United States, thunderstorms capable of producing tornadoes usually form when the temperature is at its highest, typically from 4:00 p.m. to 7:00 p.m.

Although the period in which most tornadoes strike ("tornado season") is March through June, tornadoes - including violent tornadoes and major tornado outbreaks - have been documented in the United States during every month of the year. Two examples of this are when a series of tornadoes hit the state of Indiana on November 22, 1992, and injured at least nine people. Another notable non-season tornado was where a tornado struck the area of McLean County, Illinois. Even though the tornado was during a winter month, it blew 20 railroad cars off their tracks, and hauled a camper over 100 yards (91 m).

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