Windswept_ The Story of Wind and Weather - Marq de Villiers [48]
Tornadoes can form very quickly, and are very hard to predict. Warm air rising rapidly into colder air above is a necessary precondition, but if the warm air rises steadily and smoothly, tornadoes are actually unlikely. A much more likely result would be another series of rather weak thunderstorms. But if a shallow layer of just-warm-enough air hovers above the surface—warm enough to prevent the ground-level air from rising—the potential is much greater for serious damage. Because if that cap is somehow moved or damaged, say by an incoming cold front, the pent-up warm air on the ground can burst through very rapidly. Then, watch out. Tornadoes can appear in less than an hour.
Fortunately, only one in a thousand thunderstorms becomes a supercell, and only one in about ten supercells causes tornadoes. The exact mechanism for tornado formation is obscure. They are more likely when surface winds blow in a direction other than high-altitude winds and the stronger the winds and the greater the height of the storm, the more intense the results. But as with hurricane beginnings, the actual tipping point is not understood. What is understood is that the Great Plains are the perfect kitchen for cooking tornadoes. This is because the eastern half of the continent is overlain in summer by warm moist air coming in from the Gulf of Mexico, and the western states, where the prevailing winds are westerlies, are very dry—they are in the rain shadow of the Sierras and the Rockies. Thus, Tornado Alley.
In the peak season, hundreds of tornado chasers (known, bizarrely, as "the chase community") spread out across Tornado Alley, usually in Kansas but also anywhere from Texas to South Dakota. Guessing tornado touchdowns is a sophisticated, if hazardous, game, and the Internet is full of more or less fanciful boasts from people who claim predictive powers that range from implausible to deranged. In high season tour buses packed with gawkers who want to experience nature's ferocity for themselves barrel their way down rural highways, hoping to get lucky. Some of these come closer and get luckier than they would have liked, and the occasional bus lurches out from under a supercell with windows shattered by flying debris or side panels dented by furious hailstones.
Some of these chasers are like the ham radio operators who bombard the National Hurricane Center with their track predictions, "useful fools," as they are often described by the professionals. With tornadoes, because of their elusiveness and short duration, a curious symbiosis has developed between the chasers and professionals from places like Oklahoma's National Severe Storms Laboratory, who themselves fan out across Tornado Alley hoping to plant instrument packages directly into a twister's path. In practice, both "communities" keep in touch via cell phones and radio; they are occasionally plugged into emergency services and police bands when tornados are thought to be imminent. Tornado watches, which are little more than a guess at probabilities, are released to the public and the media several hours before tornadoes are expected, but warnings of actual twisters are released in a much shorter time frame. A network of Doppler radar units covers much of Tornado Alley, but the rupture in the cap that can produce tornadoes can be too small for the radar to