for National Geographic News
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.
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.
SOURCES AND RELATED WEB SITES