How Kansas Tornado Became a Monster
Richard A. Lovett
for National Geographic News
|May 8, 2007|
The tornado that leveled Greensburg, Kansas, Friday night was born of the same forces that generate about a thousand tornadoes a year in the United States.
(See pictures of the Greensburg, Kansas, tornado aftermath.)
But it took a rare collision of conditions to spawn such a monster. And tornadoes like the Kansas twister occur only every few years.
Friday's tornado peaked the scale with a designation of F5, which means that it packed winds in excess of 200 miles (321 kilometers) an hour.
"Ted Fujita [who devised the tornado-measurement scale] used the word 'incredible' for the F5," said John Harrington, Jr., a climate scientist at Kansas State University.
Based on the Greensburg disaster, he adds, "it's hard to imagine anything worse."
The tornado, Harrington said, might have been up to 2 miles (3.2 kilometers) across and may have remained on the ground for 100 miles (160 kilometers)—although scientists are awaiting word on the precise storm track.
Most tornados that touch down measure only a few hundred yards across and remain on the ground for only a few miles.
The last F5 tornado to strike the U.S. hit Moore, Oklahoma on May 3, 1999, killing 36 people and doing 1.1 billion U.S. dollars in damage.
Colliding Air Masses
One of the factors that produce deadly storms is the collision of warm, humid air from the Gulf of Mexico and drier air flowing eastward from the U.S. Southwest deserts.
When this happens warm air rises above the dry air, creating unstable, thunderstorm-spawning conditions.
(See a video about how and why tornadoes form.)
The strength of the collision affects the strength of the storm.
Also, the upper atmosphere affects the degree to which the warm air rises. If the upper winds are diverging, for example, they can pull air up more efficiently, Harrington said.
And curvature in the wind pattern or interactions with the jet stream can greatly magnify the effect.
On Friday night there was an unusually strong boundary between humid air and dry air. In addition, Harrington said, "all three of the upper-air mechanisms were there."
The result, he said, is that by 3 p.m. forecasters were predicting major storm activity in the region where the tornado later struck.
"They definitely had a handle on what was going to happen," he said.
But, he added, "It's one thing to say it's going to happen. It's another to have to deal with the consequences."
U.S. Leads in Tornadoes
Tornadoes, Harrington adds, have occurred on every continent except Antarctica. But they are most common in the U.S.
Tornado reports have also been increasing.
(Learn about tornado safety tips.)
But Harrington believes that this is simply a factor of more people living in twister-prone areas and reporting more accurately what they see.
For these reasons, he said, the increase in sightings has been limited to small tornadoes.
"We're now seeing things that may have occurred before in open country and not been counted," he said.
"We're seeing more of the ones that may not have done much damage but were pretty to photograph."
(See a photo gallery of tornadoes.)
Overall, he added, global warming seems to have little impact.
"The processes involved [in tornado formation]," he said, "are not necessarily related to slight changes in planetary temperature."
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