A tornado is a column of air that rotates violently (and in a few cases, not so violently). By definition, the circulation of a tornado must touch the ground. It was once thought that the whirling winds of a tornado blew at speeds comparable to the speed of sound (1200 km/hr, or about 750 mph!), but we now know that top wind speeds in even the most violent tornadoes seldom exceed 480 km/hr (about 300 mph).
Most tornadoes have a common physical feature: a whirling, funnel-shaped cloud, appropriately called a funnel cloud, that lowers toward the ground from the base of a severe thunderstorm. Near the ground, the profile of a relatively weak tornado is typically outlined by the dirt and debris that is whipped up into the rotating column of air (see Figure 10.1). Depending on the strength of the tornado and the availability of low-level moisture, the funnel cloud can lower all the way to the ground (see Color Plate 30). Sometimes the funnel cloud associated with a weak tornado disappears altogether, leaving only airborne dirt and debris to give away its position. Over grassy fields, where there is little exposed dirt to be picked up, tornadoes can become invisible, moving like whirling phantoms across the countryside. Caution: these tornadic ghosts are still very dangerous.
Tornadoes have an average diameter of about 100 meters (330 ft), so there's no mistaking these "twisters" for hurricanes, which possess diameters on the order of several hundred kilometers (see Chapter 11). The width of the damage path of some powerful tornadoes can sometimes reach 2 km (about 1.3 mi), though the strongest wind speeds typically lie just inside of the edge of the dust cloud that often outlines the lower part of the twister. More than 99% of all tornadoes that spin up in the Northern Hemisphere rotate in a counterclockwise sense. Of the outlaw tornadoes that rotate clockwise, most are formed along gust fronts of powerful thunderstorms. We will discuss these "gustnadoes" later in the chapter. For now, it suffices to say that gustnadoes are like switch-hitters in baseball: they can "swing" clockwise or counterclockwise.
Like the tornado in The Wizard of Oz, many twisters, when shown on video, appear as still life against the backdrop of a distant horizon, seemingly motionless as they corkscrew violent winds into the ground. In reality, tornadoes are often streakers, racing southwest to northeast (the typical direction of motion) at forward speeds as high as 96 km/hr (60 mph).
Now imagine you're driving along a picturesque highway. Out of nowhere, a tornado approaches your car at breakneck speed. To make this nightmare seem even worse, suppose that the tornado's damage path is two kilometers wide. If you think that trying to outrace a speedy tornado with such a gaping jaw of destruction is a good idea, think again. The most judicious action to take is to leave your car and seek shelter in a ditch, preferably to the west or southwest, where the chances that your car will be thrown on top of you are reduced (because most tornadoes approach from the southwest). If a ditch isn't nearby, the nooks and crannies beneath an overpass on major highways can provide safe shelter.
The traits of tornadoes mentioned briefly in this introduction are merely a sampling of the discussions to come. And tornadoes are not the only whirlwinds in nature. There are suction vortices, dust devils and fair-weather waterspouts. Let us now depart on a whirlwind tour to the Land of Whirlwinds. Like Dorothy and her companions Toto, the Tin Man, the Lion and the Scarecrow, all we have to do is follow the yellow brick road.