|Not all tornadoes are alike. At left is an example of the tornadoes often called "landspouts," which tend to be relatively weak. This one was photographed near Denver on 30 June 1987. Below is a powerful tornado that struck in the Texas Panhandle on 2 June 1995. Such violent twisters originate from supercell thunderstorms and occur most often in the central and eastern United States.|
|Photo by Harald Richter.|
As springtime unfolds, the clashing of warm and cold air results in frequent tornado activity in the Gulf Coast states in March, with the zone moving northward and westward to the upper Midwest by June and July. Tornadoes are rare west of the Continental Divide, where moisture for strong storms is usually lacking. However, every state in the union, including Alaska and Hawaii, has reported tornadoes. On the High Plains of eastern Colorado, the most frequent type of tornado is a ''landspout''--a relatively weak vortex somewhat like the waterspouts that occur at sea.
It takes more than rising air in a thunderstorm to create a tornado. Somehow, the air must be given a spin. Thunderstorms often develop weak rotation as strong winds aloft, sometimes racing eastward at 160 kilometers (100 miles) per hour or more, impart a spin to the column of rising air. On one side of a severe storm, you can sometimes see clouds moving in a circular fashion. A "wall cloud" may hang from a larger rain-free cloud base. Large hail and heavy rain may occur near the wall cloud, and winds can be blowing upward at 160 kilometers per hour.
What finally produces a tornado? By some mechanism not fully understood, the circulation in the storm's lower levels may tighten into a narrow cylinder, elongate, and speed up, much as figure skaters spin faster by pulling in their arms. Computers can simulate the large-scale rotation of a thunderstorm, and a newer NCAR computer model examines the motions in an area as small as a half kilometer (a quarter of a mile) across, providing insight on the preexisting circulations that can tighten into a tornado. Doppler radar, able to sense the winds in the vicinity of a tornado, is another valuable tool. One portable Doppler radar trained on tornadoes in Oklahoma has measured winds of over 450 kilometers (280 miles) per hour--the fastest winds ever measured near the earth's surface.
Researchers at NCAR and elsewhere collect many kinds of information on tornadoes by going into the field. On a storm "chase," scientists take instruments and cameras into the countryside and attempt to document tornadoes as thoroughly as possible. Wall clouds and other visual precursors of tornadoes discovered by chasers are now used by law enforcement and civil defense agencies as clues to issue warnings. The most damaging twisters are often spotted well before they strike populated areas, enabling residents to take cover. The homes of some 20,000 people lay in the path of the 1979 Wichita Falls, Texas, tornado, but extensive warnings kept the death toll to fewer than 50. Simple construction techniques, such as bolting roofs to walls, can also reduce tornado damage and injury.
The outward appearance of tornadoes can vary tremendously, even for the same storm viewed at different times or from different angles. A complete funnel need not be visible from cloud to ground for a tornado to be capable of damage. Large tornadoes may resemble a cloud on the ground rather than a narrow funnel. Especially in the eastern United States, rain may wrap around a tornado's circulation and cut off visibility.