Increasingly sighted in groups (Image: REX/Mike Hollingshead/Solent News)

Tornadoes are aping buses: you wait ages and then a bunch of them come along at once. The twisters are increasingly arriving in clusters, with some days spawning more than 30 across the US, according to an analysis of weather records.

Global warming may be to blame. Climatologists have long suspected that a warmer world would influence tornadoes but the evidence has been scant. In particular, the number of tornadoes has held steady for decades. It’s when they do hit that they seem increasingly to turn up in groups, says James Elsner of Florida State University in Tallahassee, who examined 60 years of tornado statistics.

“Although the climate does not appear to be making tornadoes more frequent, when they come, they come in bunches,” says Elsner. “So you’ll see fewer days in which you’re threatened by tornadoes, but when you are, the threat will be greater.”


“Elsner’s analysis reaches the same conclusion as mine,” says Harold Brooks of the National Severe Storms Laboratory in Norman, Oklahoma.

Arriving in waves

Elsner studied records of major tornadoes in the US between 1954 and 2013. On average the US had 505 tornadoes per year. The number of tornado days, when at least one tornado strikes somewhere in the country, varied from 79 in 2013 to a high of 187 in 1971.

He found there have been fewer “tornado days” per year in recent years, but the tornadoes are increasingly arriving in waves. Before 1980, there were only three or four days per year when 16 or more tornadoes struck. But since 2000, there have been seven such days per year, on average.

Similarly, before 1990 there were no days when 32 or more tornadoes struck. But every year since 2001 has had at least one such day, and in 2011 there were six.

Brooks presented a similar analysis in 2012 at a meeting of the American Meteorological Society. He showed that the average number of tornado days per year has dipped over the past 30 years from 150 to around 100, even though the total number of tornadoes has stayed the same.

Climate change link?

Both Elsner and Brooks say the clustering cannot yet be firmly pinned on global warming. “We’ve seen increasing variability in tornado occurrence over the past 10 to 20 years, but the question is whether it’s associated with warming,” says Brooks.

However, Elsner says the link would make sense. That’s because warming air and oceans can trap more heat and humidity near Earth’s surface. In theory that should lead to more violent weather. But for thunderstorms and tornadoes to form, there must also be relatively cold air high up, which forces the warm air below to rise. Climate change is heating up this high-altitude air, making it harder for air lower down to be drawn upwards and form tornadoes.

This may explain why there are now fewer days when any tornadoes form. But on the rare occasions when there is cold air aloft, th e extra heat trapped below may create exceptionally fertile conditions for tornadoes. “When you do have cold air aloft, the atmosphere goes crazy,” says Elsner.

These shifts in the pattern of atmospheric temperatures may be influencing other weather systems, says Elsner. There is evidence that hurricanes in the US are getting stronger, and summer storms in the UK are predicted to produce more sudden torrential downpours.

Journal reference: Climate Dynamics, DOI: 10.1007/s00382-014-2277-3