They came in a swarm: A raging storm of tornadoes ran across Tornado Alley on Sunday, May 4, 2003. One of the worst tornado outbreaks in recent memory killed at least 39 people, and obliterated towns in Missouri, Tennessee and Kansas.
Darlene Young reacts to the devastation around her house in Pierce City, Mo., Monday, May 5, 2003, the day after the town was hit by a tornado. Young says her house was spared destruction by a church next to it that took the brunt of the winds. (AP Photo/John S. Stewart)
As the hard-hit towns start to clean up, The Why Files is wondering about tornadoes, nature's most horrific storm. Where do these spinning winds originate? What drives tornadoes, and how can we protect ourselves from them?
Although tornadoes occur throughout the world, including India and Bangladesh, they are most intense and devastating in the United States. Tornadoes can strike at any time of day, but are much more frequent in the afternoon and evening, after the heat of the day has produced the hot air that powers a "tornadic thunderstorm" -- a t-storm that produces a tornado.
Tornado headquarters
Tornadoes are common in Tornado Alley because of the Rocky Mountains to the west and the Gulf of Mexico to the south, explains Howard Bluestein, a veteran storm chaser and professor of meteorology at the University of Oklahoma. In spring, he says, a strong westerly jet stream flows across the Alley, creating instability and a trough of low pressure that draws warm, moist air in from the Gulf. "Conditions for the supercells [large, powerful thunderstorms] that spawn tornadoes require strong vertical wind shear [changes in wind speed and direction with height] and lots of instability," he says. And that's exactly what happens in Tornado Alley.
The fact that supercells occur where warm, moist air meets cold, dry air suggests the source of energy for both t-storm and tornado: latent heat in the warm, moist air. Latent heat is heat you can't detect with a thermometer. An air-conditioner struggles to remove humidity from Houston's soggy July air because the air holds so much latent heat.
A tornadic thunderstorm can form where moist, warm air gets trapped beneath warm, dry air under a stable layer of cold, dry air. This air sandwich is called an inversion.
If the cap is disturbed by a front or movement in the upper atmosphere, the warm, moist air can punch through the stable air above it. The condensing of water vapor releases latent heat, and the warm air starts to spiral upward. Aided by different winds at different levels of the atmosphere, the rotating updraft gains velocity.
The Nuts and Bolts of a Whirlwind
1. A large, layered sandwich develops in the atmosphere. Hot, humid air is trapped beneath cold, dry air.
2. The "cap" (a layer of stable air between the hot and cold air) is disturbed by winds in the upper atmosphere or by the arrival of a weather front.
3. Lower-level air rises and expands in the reduced air pressure aloft.
4. As the air cools, moisture condenses, releasing latent heat which warms the air, making it buoyant, and causing it to rise at speeds up to 150 mph. By now, the cloud is a thunderstorm. Upper-level winds tilt the thunderhead, creating the anvil shape.
5. The thunderstorm may die out in intense rain and/or hail. Or it may spawn a tornado.
6. Interactions between air at various altitudes, humidities and temperatures cause rain, lightning, air circulation and strengthening of the rotating updraft, now called a "mesocyclone." Low-level wind helps cause this rotation, which is almost always counter-clockwise (seen from above) in the Northern Hemisphere.
7. A tornado may form below the mesocyclone. As the spinning air column narrows, it rotates faster and extends higher into the storm.
Wind shear -- where the wind changes direction with height -- precedes many tornadoes. If a south wind is near the ground (orange arrows) and a west wind blows above it (blue arrow), wind shear may cause a column of air to rotate near the ground (pink arrows). Eventually, physical forces move the vortex into a vertical position, and you have a tornado. From original graphics by NOAA.
That, much simplified, is one theory of the origin of tornadoes. Scientists do not fully understand the first stage of formation, however. Some suspect that wind shear -- differences in wind speed at different altitudes - may start the vortex. Just as, when you roll a wad of clay in your hands, it eventually becomes a rotating column of clay, when the different bodies of air move against each other, they create wind shear and then a tornado.
