The devastating outbreak of tornadoes that raked Oklahoma and Kansas on Monday night is expected to rank among the worst in history.
One monstrous Oklahoma twister -- among the 76 in one unofficial count -- was up to a mile wide. It churned across some of the state's most densely populated communities for four hours.
But despite dramatic TV images, meteorologists say there is no evidence that such deadly outbreaks are becoming more severe or more frequent as a result of global warming or some other factor unique to our times.
"There is no existing basis for saying we have an increasing trend of severe tornadoes," said Robert Livezey, of the National Centers for Environmental Prediction.
What has changed is what people have put in the tornadoes' path.
Inflation and development across "tornado alley" -- a wide swath from western Texas to Iowa -- have begun to produce outbreaks with damage above $1 billion. In January, 106 tornadoes killed 17 people over three days in Arkansas and Tennessee, causing $1.7 billion in damage.
Population increases, the ubiquity of video cameras and the interest of news media have also placed more observers in the field, given us higher counts and a more acute awareness of the storms and their impact.
"If you look at the number of tornadoes counted by month, or by year from 1950, there has been a gradual increase. But there are also more people paying attention," said Joseph S. D'Aleo, chief meteorologist at Weather Services International, a private weather services firm in Billerica, Mass.
In earlier times, he said, "a tornado crossed the prairie and nobody heard about it." Today, even if no one is around, tornadoes are spotted on the new Doppler weather radar systems that blanket the country.
Tornado counts may also be skewed by new counting methods. A twister used to be counted each time someone saw it touch the ground. Today they're counted each time they enter a new county, triggering a new warning.
"If it goes through three counties, with three warnings, it might be counted as three tornadoes," D'Aleo said.
Despite rising populations, however, tornado-related deaths are declining. Thanks to radio, TV, public education campaigns and technologies such as Doppler radar, "people are better warned, and also better prepared. That, no doubt, has saved a lot of lives," he said.
