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.

As of Friday, there had been 445 so far this year.

This is the fastest start for the first three months of the year since 1999, and it is in sharp contrast to last year when only 96 tornadoes had formed by April 3. Yet last year ended with exactly 1,200 twisters, according to NOAA. June was the busiest month in 2005.

Expect more: According to NOAA, "Previous years with a busy start have produced high numbers of tornadoes throughout the year." Abnormally warm temperatures and dry conditions during winter kept water temperatures warm in the Gulf of Mexico, the scientists say.

"Once a spring pattern developed in early March bringing weather systems eastward into the central United States, it combined with warm, moist air moving north from the Gulf, and created all the right ingredients for severe thunderstorms and tornadoes," Dan McCarthy, warning coordination meteorologist with the NOAA Storm Prediction Center in Norman, Okla. "If the pattern holds, the season could continue to be busy."

When and how they strike

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. The worst outbreak in history occurred on April 11, 1965.

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

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. As the moist air rises - sometimes 50,000 feet into the air - it cools, forming clouds and thunderstorms [Graphic].

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 gallery].

Tornado Alley

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 [Map]. In fact, Florida has the most tornadoes per area, but they are typically weaker.

In 2005, scientists explained fresh twists on when and where tornadoes can strike, including in November in Ohio or in Michigan on an October night.

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, with winds exceeding 207 mph - 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 with winds below 113 mph - lasting usually less than 10 minutes and causing less than five percent of tornado-related deaths. The scale will be revised in 2007.

Memorable tornado events

  • April 3-4, 1974: In 16 hours, 148 tornadoes were recorded across 13 states. With ratings of F0 to F5, this tornado outbreak killed 330 people and injured almost 5,500.
  • The most deadly single tornado in history was the Tri-state twister of March 18, 1925. Moving across Missouri, Illinois and Indiana at speeds greater than 60 mph, this F5 tornado covered 219 miles and killed 695 people.
  • A mile-wide tornado touched down near the town of Natchez, Mississippi, in 1840. It was estimated that 48 people died on land, while 269 drowned in the Mississippi River in sinking boats and steamships.
  • The biggest recorded tornado was nearly two and a half miles wide. It occurred near Hallam, Neb., on May 22, 2004. It is important, however, to realize that size does not necessarily imply strength. Large tornadoes can have meager wind speeds.
  • By virtue of its land area and location, Oklahoma City has been hit by more tornadoes than any other city. The worst of these was an F5 that struck on May 3, 1999, causing 36 deaths and a billion dollars worth of damage.

Staying safe

If a tornado strikes, the safest place is in a strong building - preferably in a basement or a small interior room. The important thing is to get away from windows and put as many walls as possible between you and the outside.

Mobile homes do not provide adequate protection from a tornado.

If there are no secure buildings nearby, lie flat with your hands over your head in a ditch or depressed area. Do not try to outrun a tornado in your car, experts say.