"If you look at the past 60 years of data, the number of tornadoes is increasing significantly, but it's agreed upon by the tornado community that it's not a real increase," said Grady Dixon, assistant professor of meteorology and climatology at Mississippi State University.

"It's having to do with better (weather tracking) technology, more population, the fact that the population is better educated and more aware. So we're seeing them more often," Dixon said.

But he said it would be "a terrible mistake" to relate the up-tick to climate change.

The tornadoes that ripped through the US south this week killed more than 300 people, in the worst US weather disaster in years, with residents and emergency workers sifting through the rubble on Thursday.

Violent twisters that famously rip through the US south's "Tornado Alley" are formed when strong jet winds bringing upper-level storms from the north interact with very warm, humid air mass from the Gulf of Mexico, said David Imy from the NOAA Storm Prediction Center in Norman, Oklahoma.

The upper-level winds caused the storm system to rotate, meteorologist Jeff Masters, director of the website Weather Underground, told National Geographic's daily news website. Rotating thunderstorms produce tornadoes.

They said it amounted to a "perfect storm" of conditions.

"This is a history-making tornado outbreak," Masters told the site. "You don't see many like this."

Colorado-based tornado expert Tim Samaras told National Geographic that it's a "very, very rare day for everything to come together for this type of event."

On Wednesday, a particularly potent storm was whipped up around the heart of that tornado-prone corridor where the states of Arkansas, Oklahoma, eastern Texas and northwest Louisiana meet, said Kristina Pydynowski, a senior meteorologist at the AccuWeather.com website.

Sparking the severe thunderstorms from that point was the much warmer air arriving from the south, over the tropical Gulf. The combining winds at differing altitudes, said Pydynowski, created "significant twisting motion in the atmosphere, allowing the strongest thunderstorms to spawn tornadoes."

Craig Fugate, administrator of the Federal Emergency Management Agency (FEMA), on Thursday also dismissed climate change as a factor in the deadly tornadoes: "Actually what we're seeing is springtime," he said.

"Many people think of Oklahoma as 'Tornado Alley' and forget that the southeast United States actually has a history of longer and more powerful tornadoes that stay on the ground longer."

Wednesday's deadly tornadoes, according to Imy of the National Oceanic and Atmospheric Administration (NOAA), were unusual for being "long track," meaning they were on the ground for a longer period of time than usual -- in this case, roiling across the land for 30 miles (48 kilometers) or more.

An average track would be less than five miles, said Imy.

However, the stronger-than-usual tornadoes affecting the southern states were actually predicted from examining the planet's climatological patterns, specifically those related to the La Nina phenomenon.

"We knew it was going to be a big tornado year," he said. But the key to that tip-off was unrelated to climate change: "It is related to the natural fluctuations of the planet."

earlier related report
Deadliest tornado days in US history
Tuscaloosa, Alabama (AFP) April 28, 2011 - More than 280 people have been killed across seven southern states by some of the worst tornadoes ever to strike the United States.

Here is a list of the deadliest tornado days in America's recorded history:

1. 747 deaths - March 18, 1925 - The Tri-State Tornado

The deadliest single tornado in American history claims 695 lives as the monster twister crosses Missouri, southern Illinois and into southwestern Indiana. The wider tornado outbreak leaves 747 people dead.

2. 332 deaths - March 21, 1932 - The Deep South tornado outbreak

Tornadoes are produced from Texas to South Carolina, 270 of the dead are in Alabama alone.

3. 317 deaths - May 17, 1840 - The Great Natchez Tornado

The second deadliest single tornado in US history strikes the Mississippi town of Natchez, killing 317 people. Most of the dead are killed on flatboats on the river.

4. 310 deaths - April 3, 1974 - The "Super Outbreak"

The largest tornado outbreak on record claims 310 lives as 148 confirmed tornadoes over a 24-hour period rage across 13 US states.

5. 305 deaths - May 27, 1896 - St. Louis-East St. Louis tornado

Rounding off a deadly two-week storm period, 305 people are killed by tornadoes in Missouri, Illinois and Kentucky.

6. 260 deaths - April 11, 1965 - The Palm Sunday outbreak

Some 50 twisters hit the Midwest, killing 260 people and leaving more than 1,000 injured in Illinois, Indiana and Michigan.

7. 249 deaths - April 5, 1936 - Tupelo-Gainesville tornado outbreak

Most of the 249 people killed are in one Mississippi town, Tupelo.

8. 224 deaths - April 20, 1920 - Alabama-Mississippi

The deadly tornado-packed spring of 1920 ends with 224 people killed in an unusual early morning outbreak in Alabama and Mississippi.

9. 220 deaths - April 24, 1908 - Dixie tornado outbreak

Dozens of twisters stretching from South Dakota to Texas leave 220 dead.

10. 205 deaths - April 6, 1936 - Gainesville tornado

The day after 249 people are killed in Tupelo, Mississippi (see 7. above), another 205 people are killed by a single tornado, nearly all of them in the city of Gainesville, Georgia.

SOURCE: The Severe Storms Laboratory of the National Oceanic and Atmospheric Administration (NOAA).

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