Twister!

It was a dark and stormy afternoon. Outside, the rain was lashing the windowpanes, and thunder was barely an instant behind the lightning. Your children were huddled on the floor of a downstairs closet, wearing bicycle helmets and looking like an ad for a very bad summer movie. You were sitting on the sofa down the hall, peering at a TV screen, admiring the multicolored and three-dimensional graphics that showed a storm front from every angle. A team of weather forecasters, who kept repeating the phrase "tornadic activity," had just informed you that a tornado was working its way down West End and would be stomping around your neighborhood in precisely two-and-a-half minutes.

Dorothy in The Wizard of Oz is lucky. Even after falling out of the sky and braining a witch, her tornado-whirled home remains impressively solid. She herself is unhurt. As Middle Tennesseans know all too well, a real tornado will land its victims not in Oz but in building supply centers, insurance offices, hospitals and even funeral homes. Before the middle of the 20th century, tornadoes in the U.S. killed more than 100 people annually. Recent technological developments and ongoing education have greatly reduced yearly fatalities, but scores still die in these horrible storms. If a tornado comes after us, we have only two choices. We can hide or we can run.

So how does a tornado work?

To understand this question, we need to ask an even more basic one: What makes the wind blow? If the Earth's temperature were uniform across its surface, there would be no wind to whip itself into a tornado. Instead, as Earth turns its face toward and then away from the warming rays of the sun, the amount of solar radiation varies on different regions. This disparity causes air movement. Wind blows from cooler regions to warmer; warm air rises because it is lighter and less dense, and cooler air moves in to replace it. Meanwhile the Earth's spin on its axis causes the air to turn too, creating still more atmospheric currents. Consequently, high pressure in the Northern Hemisphere moves in a clockwise pattern and low pressure counterclockwise.

As a severe thunderstorm develops, fast-moving wind higher up can meet slower wind near the ground and start the air spinning. Some of these whirling proto-tornadoes sputter out immediately. Under the right (or wrong) conditions, others keep going and grow stronger—and meaner. Scientists who analyze tornadoes employ such terms as positive vorticity, the Coriolis effect and the conservation of angular momentum.

But most of us aren't scientists, so here's another way of thinking about it: A tornado works like a drinking straw. Inside a container, atmospheric pressure is the same on every part of the surface of a drink until you draw air upward through the straw. As you suck on the straw, you reduce the air pressure inside it. The pressure on the rest of the surface remains the same, so the liquid rises upward in the area of reduced pressure. You have created a vortex, like an upside-down bathtub drain, pulling air and liquid upward rather than downward. A close-up view of the bottom of your drink would show everything rushing toward the submerged end of the straw, just the way that a tornado pulls everything into its low-pressure end on the ground.

A tornado's color changes as soon as it drops out of the sky and touches the ground because it instantly picks up debris: soil, plants, mobile homes, cows, witches. As it moves along, a tornado draws in a huge volume of air. The air moves so quickly that the mouth of the tornado maintains amazingly low pressure—which pulls in still more air. And this action becomes the very definition of a vicious circle.

It is important to remember that estimated wind speeds are after-the-fact guesses. No instrument for measuring wind speed has yet survived a full attack by a tornado. But if it makes you feel safer, 69 percent of tornadoes in the U.S. are considered weak, 29 percent strong and only 2 percent violent. These tidbits of information remind us of a key aspect of averages: Although they can help us predict likelihood, they aren't worth a thing in the individual circumstance. When a tornado is coming toward us, we don't care what the odds were before it all started going crazy.