Scientists hope the deadly storm front that has struck twice this week in the United States' Tornado Alley, will reveal more about the nature of twisters and how to protect people and property from the wrath of Mother Nature.

It's been a deadly start to tornado season. As Oklahomans spent a third day cleaning up from a monster set of twisters, the same front spawned fresh storms on Thursday, killing four people in Tennessee.

Thursday's fatal tornado brings the death total to 51. Damage from the earlier sweep -- classified as an F5, the most dangerous category of twisters -- included the leveling of 3,000 homes in Oklahoma, 1,500 in Kansas.

The Southwestern Insurance Information Institute of Austin, Texas, says it'll be weeks before all the claims are tallied and the most heavily damaged areas in Oklahoma, Kansas, Texas and now Tennessee are added.

"A billion dollars sounds like a reasonable estimate for all those areas," Jerry Johns, the institute's president, told Reuters Thursday.

"This will probably be one of the worst tornadoes in the history of this country in terms of insured losses."

Scientists hope analyzing the massive storm system will reveal new information about tornados because it produced so many twisters and was tracked for so long on radar.

A group of scientists called "rubble rooters" believe it may be possible to find a way to use radar to predict the path of twisters.

At the moment people have to rely on reports from the ground made by "storm chasers" to find out where tornados are heading. But using radar to forecast where tornadoes are heading would make it easier to get emergency personnel where they are needed as the storm is happening.

Other research teams from Texas Tech are combing through the rubble trying to learn which rooms in the house are most likely to survive twisters.

A tornado is an extremely violent windstorm that is usually created within a thunderstorm or sometimes a hurricane. The recognizable shape of the tornado is the swirling funnel cloud that extends from the base of a storm cloud to the ground.

The funnels are usually found at the trailing edge of a thunderstorm, and because of that, it is not uncommon to see clear, sunny skies behind them.

A tornado is produced when cool air overrides a layer of warm air, forcing the warmer air to rise rapidly. The most violent have speeds of 400 km/h. They can last from a few seconds to more than an hour.

Some twisters will "hop" without spending much time on the ground while some can stay on the ground for distances of more than 150 kilometres.

The United States is the tornado capital of the world with about 800 to 1,000 reports a year. Although they have struck every U.S state, they tend to occur most frequently in the Midwest, Southeast and Southwest.

The states of Alabama, Arkansas, Florida, Georgia, Illinois, Indiana, Iowa, Kansas, Louisiana, Mississippi, Missouri, Nebraska, Oklahoma, South Dakota, and Texas are at greatest risk. Tornados are common in this area because the mountains to the west and the Gulf of Mexico to the south produce the layers of warm, moist air and cool, dry air which cause the twisters.

Although tornados can happen at any time of the year, they generally happen between March and August and tend to occur in afternoons and evenings. In the spring, a strong westerly jet stream flows across the part of the United States known as Tornado Alley drawing in warm, moist air from the Gulf of Mexico.

The Oklahoma storm ranks among of the worst in recent American history. In terms of people killed, the worst on record occurred in 1925 when a tornado system hit three states without warning -- 625 people died. The worst recent tornado system was in 1974 when 315 people were killed in 11 states. Just last year, 36 people were killed in storms in Mississippi, Arkansas and Tennessee.

Scientists rank tornados on the Fujita scale of intensity. The scale was named after Tetsuya Theodore Fujita, a University of Chicago researcher.

The lowest ranking is F0 and the highest is F5. On average, only one F5 tornado hits the United States each year.

  • F0: Gale tornado Wind Speed: 64-116 km-h Damage: Some damage to chimneys; breaks branches off trees; pushes over shallow-rooted trees; damages sign boards.

  • F1: Moderate tornado Wind Speed: 117-180 km-h Damage: The lower limit is the beginning of hurricane wind speed; peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos pushed off the roads; attached garages may be destroyed.

  • F2: Significant tornado Wind Speed: 181-253 km-h Damage: Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars pushed over; large trees snapped or uprooted; light object missiles generated.

  • F3: Severe tornado Wind Speed: 254-332 km-h Damage: Roof and some walls torn off well-constructed houses; trains overturned; most trees in forests uprooted.

  • F-4: Devastating tornado Wind Speed: 333-419 km-h Damage: Well-constructed houses leveled; structures with weak foundations blown off some distance; cars thrown; large missiles generated.

  • F-5: Incredible tornado Wind Speed: 420-512 kph Damage: Strong frame houses lifted off foundations and carried considerable distances to disintegrate; automobile-sized missiles fly through the air in excess of 100 metres; trees debarked; steel-reinforced concrete structures badly damaged.

    Tornados are less frequent in Canada but they do happen and they can be big. There have been two F4 tornadoes, one rank lower than Oklahoma's May 3 twister, in recent memory. The Edmonton tornado of 1987 and the Barrie tornado of 1985 were rated at F4 intensity along parts of their tracks. The Edmonton twister was Canada's second worst causing 27 deaths and $350 million in damage. The worst Canadian tornado was in Regina, Saskatchewan in 1912. There were 28 deaths and $4 million in damage.

    The greatest number of tornados in Canada are in southwestern Ontario, with an average annual incidence peaking at three tornados per 26,000 km-sq. The Red River Valley of Southern Manitoba has reported frequencies about one-third of the highest values in the United States.