"Toto, I've a feeling we're not in Kansas anymore" marveled Dorothy after a fierce tornado swept her and her little dog to Munchkinland in The Wizard of Oz. Contrary to their Hollywood image, most tornadoes are not giant monsters. The tornadoes that command attention on the evening news because they cut swaths of total destruction re the exception rather than the rule. Indeed, many "twisters" (as tornadoes are sometimes called) are not powerful enough to completely destroy a house nor big or long-lived enough to cause a major disaster. Some weak tornadoes have even been known to figuratively "hit a brick wall" and dissipate after running into structures. However, the most prudent attitude to take if you ever come face to face with a tornado is to assume that you are in dire straits.

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). As we will discover later in the chapter, meteorologists chasing the F5-twister that sideswiped Oklahoma City on May 3, 1999, used Doppler radar to measure winds of 512 km/hr (318 mph), a world record for highest wind speed ever measured on earth.

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 14.1). 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 14.A). 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 m (330 ft), so there's no mistaking these "twisters" for hurricanes, which possess diameters on the order of several hundred kilometers (see Chapter 10). 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 even worse, suppose that the tornado's damage path is two kilometers wide. If you think that trying to outrun 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, nooks and crannies behind concrete barriers underneath overpasses on major highways are probably the next best safe shelter, but as we saw in Chapter 13, these do not provide blanket safety. The absolute best course of action, though often inaccessible or infeasible, is to take refuge underground.

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.