By David A. Fahrenthold
Washington Post Staff Writer
Wednesday, May 21, 2008
Are tornadoes becoming more common? It's a simple question with no simple answer, scientists say.
It certainly might seem as if the country is in a tornado glut. Ninety-nine people have been killed by twisters this year, including 23 earlier this month as a line of storms raged from Oklahoma to Georgia. That's the highest total in a decade, and the summer thunderstorm season is still to come.
The Washington region, far from the country's Great Plains tornado belt, has also been hit by a series of powerful twisters in the past decade. The latest one smashed through Stafford County two weeks ago. Others struck La Plata in 2002 and College Park in 2001, killing a total of seven people.
But, both here and elsewhere, scientists say they don't see any evidence that the storms are becoming more frequent. Weather records do show more tornadoes now than in decades past, but that change probably has more to do with record-keeping than with the weather, they say.
"We're just too good at spotting these things now," said Jerry Stenger of the University of Virginia's Climatology Office. Because of that, he said, it's difficult to compare current statistics with those from decades in which tornadoes were not as carefully tracked.
"So there's just not a good answer to that question" about whether there's been an increase, Stenger said.
Tornadoes are a hard subject to study. There are about 1,200 of them a year in the United States, but they are exceedingly difficult to anticipate, spawned by a confluence of warm, moist air and dry air and a certain pattern of winds. And, even if a scientist could predict one, it's hard to do much research around a 200-mph whirlwind.
So researchers are often confined to looking for answers in the historical record, comparing total tornadoes from one year to the next. At first glance, that method seems to show a significant uptick in tornadoes nationwide: In the 1950s, there were about 550 tornadoes recorded in a year. Now, there are more than twice that, according to the National Oceanic and Atmospheric Administration.
Federal records also seem to show a jump in the Washington area. The states of Virginia and Maryland averaged about 10 tornadoes a year between them from 1961 to 1990. Since then, the average appears to be about 28. Federal records show just two tornadoes that began in the District since 1950, in 1995 and in 2001.
But experts say the apparent increases are probably just on paper. They say improvements in weather radar, better public awareness and the proliferation of video cameras mean that many more small tornadoes are now sighted and officially tracked. So tornado records from the 1950s and today aren't comparable.
Trying to get around that problem, researchers have tried to compare only the numbers of very strong tornadoes -- which probably would have been recorded in any decade. But that comparison, they said, shows no sign that tornadoes are increasing or decreasing.
"There's no trend," said Greg Carbin of NOAA's Storm Prediction Center.
An equally difficult question: Will tornadoes become more common because of climate change? Warming temperatures could increase the amount of water vapor in the lower atmosphere, which could boost tornadoes. But they could also change wind patterns, dampening the forces that make twisters spin.
"We absolutely don't know," said Howard Bluestein, a professor of meteorology at the University of Oklahoma. "And I would not even . . . venture a guess."