The tornadoes that tore across the south-eastern US on Thursday, killing at least 19 people, were devastating but not unprecedented, say tornado experts. However, the twisters did strike unusually early in the year.

The tornado season in the US normally reaches its peak between mid-April and June. The tornadoes tend to get stronger as the year progresses because warming temperatures increase the amount of energy in the atmosphere.

"Early March is a bit early for a severe tornado," says Nigel Bolton, national forecaster at the UK Met Office and member of the UK-based Tornado and Storm Research Organisation (TORRO). But he notes that such events have happened before this early in the year, when the key atmospheric circumstances combine.

In this case, the extremely cold US was probably a contributing factor. Tornadoes in the US form when a front of dry, cold air descending from the north meets warm, moist air coming up from the south. Sometimes, a body of the cold air slides over the top of the warm air, trapping it underneath.

This creates unstable atmospheric conditions, and if there is enough energy in the system, the warm air will punch through the denser cold air above, triggering thunderstorms. The next step required is for the winds to be changing their speed and direction with altitude. "If there's enough rotation you get a tornado," says Bolton.

Perfect conditions

Bolton says a depression has indeed been drawing the cold northern air down across the southern US. At the same time, the anticlockwise motion of the depression drew warm, moist air in from the Florida coast.

Furthermore, the temperatures in Alabama on Thursday were above 20ยฐC, with high levels of humidity. All this will have created the right conditions for the tornado, Bolton says.

There have already been several killer tornadoes in the US in 2007, though none as strong as the F4-level tornado that hit Enterprise, Alabama on Thursday. According to the US National Oceanic and Atmospheric Administration, two F3 tornadoes hit Florida in February, killing 20 people between them.
Climate change

There is no evidence that the world's warming climate is leading to an increase in tornadoes, says Terence Meaden, deputy head of TORRO.

"Higher average temperatures mean the air is able to contain more water vapour, which increases the tendency towards more severe thunderstorms, and more severe thunderstorms tend to produce tornadoes," he says. "But are the small average temperature increases that we're seeing across the planet having any influence on tornadoes? I don't know."

More tornadoes are being reported each year, but both Meaden and Bolton say it is impossible to say whether that is simply because people are more aware of tornadoes and have more cameras to document them.

According to Bolton, an early severe tornado also does not bear any relationship as to how many there will be later in the season. Some years, a very active start is followed by a quiet period, while in other years the pattern is reversed.