Chasing Tornadoes in Tornado Alley

...Tornadoes are among Earth's most violent natural acts. About a thousand of them touch down in the United States each year, more than in any other country in the world. Some are wispy and last only seconds, others rampage across the landscape for more than an hour, but few are as destructive as the one that obliterated Manchester.

By definition tornadoes are rotating columns of air that extend from swelling cumulonimbus clouds to the ground. No one fully understands tornado dynamics, but certain ingredients seem essential to the witches' brew from which twisters emerge: warm, humid air near the ground, colder air aloft, and shearing winds that change direction and speed with height. The most destructive and deadly tornadoes form under the bellies of supercells, large long-lived thunderstorms whose winds are already in rotation. It was a supercell that gave birth to the Manchester tornado.

Forty percent of all U.S. tornadoes occur in the central plains states from March through July, when cool, dry air from the Rocky Mountains clashes with warm, moist undercurrents from the Gulf of Mexico. In such open country you can see entire supercells, some 30 miles (48 kilometers) wide, bulling over the land, spitting rain and hail, their cauliflower tops bursting into the stratosphere. But only one in a thousand thunderstorms becomes a supercell, and only one in five or six supercells spawns a tornado.

Because it's so difficult to measure tornado winds and power, scientists measure tornadoes by the damage they cause. On the Fujita scale, developed by Ted Fujita of the University of Chicago, an F1 storm does moderate damage with hundred-mile-an-hour (161-kilometer-an-hour) winds. An F5 is horrific. The Manchester tornado was an F4.

Today's warning time for tornadoes—the time a family faced with catastrophe has to gather essentials and bolt for the basement or nearest storm shelter—averages 13 minutes. Most warnings rely on the 121 radar stations of the National Weather Service, but conventional weather radar can miss the birth of a tornado in the five to six minutes it takes a unit's single beam to cover its range. Now scientists at the National Severe Storms Laboratory (NSSL) are working to adapt a shipboard system from the U.S. Navy—the Spy-1 phased array radar—for meteorological use. Spy-1 sends out multiple beams in continuous rotation and is five times faster than conventional radar....