Tornadoes, like hurricanes, form in warm climates, but the tornado is the most intense cyclone of all. Its most frequent and violent occurrences are found in the United States, mainly in the Great Plains Region and to a lesser extent in the Central Lowlands. April, May, and June seem to be the favorite times for strikes with an average of more than four hundred in the United States as a whole in these months. The famous “Wizard of Oz” movie was one of the first media events to raise awareness of a tornado’s destructive power. The tornado is a small dark funnel cloud, a few hundred feet in diameter at its base with a cumulonimbus cloud above it. Its dark color comes from the dust it picks up by its powerful inward spiraling winds. These wind speeds can reach 250 mph as the storm twists and turns and races along the surface of the ground. It can devastate almost everything in its path, yet at other times it can rise in the air and leave the ground below completely unscathed.
The National Weather Service maintains a tornado forecasting and warning system similar to the one for hurricanes. Whenever weather conditions seem to favor tornado development, places at risk are warned and arrangements for observing and reporting conditions are set in motion. Western Ohio is not a high risk area compared with states in the Great Plains but, on April 4, 1974, at Xenia, near Dayton, this state was hit with one of the worst tornadoes of the century. Ohio was not alone. Over the two days, April 4 and 5, a rash of 148 tornadoes attacked twelve states, killing 300 people and injuring 6,000. At Xenia some 3,000 structures were demolished. Elsewhere, entire towns were wiped out and $600 million worth of property devastated. The atmospheric conditions were ideal for triggering tornadoes: a cool mass of humid air lay over Chicago while farther west dry air was encountering a cold air mass from the northwest. Against both of these came a moist warm air mass from the south. The combination of all three created an explosive series of thunderstorms extending more than seven hundred miles from Texas to Illinois.
An earlier tornado swept through three states in 1925. It touched down first in Annapolis, Missouri, with a base at times as wide as one mile. Main Street’s buildings were flattened in a few seconds and the twister then swept on into Murphysboro, Illinois, tossing trees, buildings, and even underground pipes as if they were toys. Over 230 people died in Murphysboro. The tornado moved next to DeSoto, a town of about six hundred, where it knocked down every structure more than one story high. Sixty-nine people lost their lives. The twister finally vanished in southern Indiana. Fortunately, storms like this one or those of 1974 are rare but, sadly, not unique. On the evening of May 3, 1999, the worst tornado of the century, as far as costs are concerned, touched down on Oklahoma City. It was the nation’s first billion-dollar tornado. It was not alone. Other parts of Oklahoma, the state that gets more tornadoes per square mile than anywhere else on earth, were hit with sixty of these storms on that same evening, all of them in areas close to Oklahoma City. Within a period of five hours 8,000 buildings were in partial or total ruin as the rash of storms swept from southwest Oklahoma diagonally across the state toward Wichita, Kansas.
The difficulties involved in forecasting were evident on that fateful evening in May. The Storm Prediction Center (SPC) based at Norman, Oklahoma, issues bulletins every day and on that morning’s statement announced it as unlikely that any tornado would appear during the day. By early afternoon SPC raised its estimate to moderate. Not until close to 4:00 in the afternoon did SPC change its prediction to high risk, and then only because a powerful computer had shown that storms were charging across the state. One hour later, across a 150 mile swath that included Oklahoma City, the swarm of storms struck. The greatest damage was caused in Oklahoma City and one or two of its suburbs. On the F-scale of tornado strength, the one that hit the city was at 5, the top of the scale. Any F5 tornado is unusual and one that hits a major city even more rare. Street and after street was devastated. A typical sequence in a single family home would be: windows shattered, roof lifted off, walls caved in. Even homes that were carefully built to withstand 75 mph winds were unable to withstand this tornado.
Mobile homes fared very poorly as they usually do. An F1 tornado is usually enough to knock them over. With an F5 at speeds of 300 mph they were completely shattered. While the overall death toll was low for a storm of this size most of the fatalities occurred in the mobile home areas. Almost every one of the tornadoes that hit over the six hours from 5:00 to 11:00 in the evening was an F5 or close to that strength. They were super cells, sustained severe thunderstorms, and experts were left with the problem of how such a powerful series of storms could be sustained at that level of strength for so long. Most tornadoes develop within very large storms called super cells. These storms are found in unstable environments in which wind speeds vary with height and where cool, dry air rests on top of warm, moist air with a thin stable layer separating the two air masses, a condition similar to temperature inversion in other settings. If a weather system reaches this unstable mass, the status quo is disrupted, the low level air is forced upward and a vertical vortex gradually takes shape as the warm air ascends, cools to the point of condensation, and then is triggered into faster ascent as the latest heat of condensation warms the surroundings.