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updated 3/22/2005 7:38:48 PM ET 2005-03-23T00:38:48

Spring is the time for birds and flowers … and tornadoes. The season’s warming trend facilitates unstable temperature contrasts that spawn big storms.

Steamy air moving into the South will cause severe thunderstorms that could spawn tornadoes in Alabama, Arkansas, Georgia, Mississippi and Tennessee on Tuesday, meteorologists said. It's the sort of forecast that will be repeated dozens of times in coming weeks.

In the United States, May has historically had the most tornadoes: 176 on average between 1950 and 1999. But April is the deadliest month: an average of 26 tornado-related deaths over the same time period.

Twisters often start to become a minor menace in March, depending on climate conditions and where you live.

Tornadoes form where warm moist air is trapped underneath a layer of cold, dry air. This instability is upset when the warm bottom layer gets pushed up — either by heating near the ground, or by an influx of cold air.

Birth of a tornadoAs the moist air rises — sometimes 50,000 feet into the air — it cools, forming clouds and thunderstorms. If the conditions are right, the rapidly rising air will spin around a central funnel — at speeds sometimes exceeding 250 mph. A tornado technically is born when this funnel cloud touches down on the ground.

Although tornadoes are more frequent in the afternoon, they can happen anytime, even at night.

Tornado alley
Tornadoes can occur almost anywhere in the world, but the United States is the country with the highest frequency of tornadoes. Each year there are about 1,200 tornadoes in the United States, causing about 65 fatalities and 1,500 injuries nationwide.

Twisters strike predominantly along Tornado Alley — a flat stretch of land from west Texas to North Dakota. The region is ideal for tornadoes, as dry polar air from Canada meets warm moist tropical air from the Gulf of Mexico.

Texas gets hit the most, with an average of about 110 tornadoes a year. But every state in the union has recorded at least one tornado in its past. In fact, Florida has the most tornadoes per area, but they are typically weaker.

In southern states like Arkansas and Missouri, the peak of tornado season is March through May, while in the northern states, like Iowa and Illinois, more tornadoes occur in the late spring and summer.

F-scale
In the early 1970s, T. Theodore Fujita developed a damage scale for high-wind events including tornadoes. The F-scale, which goes from F0 to F5, is the only widely used tornado rating method. Although wind speeds are given for different F-scale ratings, these are only estimates, as it is very hard to get reliable measurements near a twister.

Violent tornadoes – F4 and above — are less than one percent of all tornadoes, but account for 70 percent of tornado-related deaths. Some of these twisters can last more than an hour and travel hundreds of miles.

Almost 90 percent of tornadoes are weak — F0 or F1 — lasting usually less than 10 minutes and causing less than five percent of tornado-related deaths.

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