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Getting Tornadoes Started

Both hurricanes and tornadoes require a very specific set of conditions to get started. Tornadoes form when a front of cold, dry air advances over warm, moist air near the ground (a fairly common occurrence in the U.S. plains as cold fronts advance over the Rockies and warm air comes up from the Gulf). Generally, an intervening layer of warm yet dry and stable air keeps the two air masses separate--one over the other, but separate. Yet if this "cap" of air gets disturbed, the warm moist air below can punch its way through. And now you've got big trouble.

               Twisting terror takes Midwest toll

The air near the ground, because it is warmer, begins to rise and spiral up through the disturbance. As this warm air rises and cools, the moisture inside it condenses to form large clouds, and a nasty "supercell" thunderstorm takes shape. The process of condensation releases latent heat energy too, which makes the rising air even warmer and causes it to spiral up even higher and more quickly, often at an astonishing pace (at speeds up to 150 miles, or 241 kilometers, per hour).

In some cases, particularly strong rotating updrafts can pull horizontally rotating winds in the warm, moist air below the storm (think of the wind spiraling like a football as it advances) into a spinning vertical vortex far tighter and more compact than the rotating column of rising air that spawned the storm in the first place. If this vortex--an infamous "funnel cloud"--is strong enough to reach the ground, look out, because you're looking at a tornado. Hurricanes require a slightly different recipe, but one ingredient is the same: warm, rising air near the surface. For hurricanes, that means ocean water above 80 degrees Fahrenheit (27 degrees Celsius). This water creates warm, extremely humid air that can rise and create an area of disturbance--typically when warm ocean winds coming from different directions converge and force the air upward. As the air rises, it cools and condenses into a storm and releases the latent heat that powers the hurricane engine.

And it is an engine. As the warm air rises, it leaves behind a low-pressure area (the lowest barometric pressures ever recorded occur inside hurricanes) that literally sucks in more warm humid air--fuel for the storm to grow and intensify. If the rising and condensing air encounters even more humid air as it rises, and if preexisting winds higher up in the atmosphere don't shear the growing storm apart, and if there's a high-pressure area above it, the nascent storm may become a hurricane.

The high-pressure area above the storm is the engine's "exhaust pipe," halting the rising air and pushing it out and away, so that even more warm air can get sucked into the space below. In fact, a hurricane's gargantuan winds result from warm air rushing inward to replace the air that's rising up in the center of the storm and, eventually, being pushed out and away high in the atmosphere.

If this whole engine--sucking in humid tropical ocean air, condensing it into a massive storm, and pushing the air away so that more can take its place--becomes large and strong enough, the rotation of the Earth takes the disturbed area and begins to spin it around, creating that classic, spiral hurricane shape. Once created, the hurricane engine can draw life from the warm ocean for days. If, however, the hurricane moves over land or into colder areas, removing the warm water that fuels its great storm engine, it will quickly dissipate.