A vivid and terrible weather image is presented by the tornado -- a savage funnel of wind, dust, water vapor, and debris that roars across the landscape in the summer months, leaving a track of destruction in its wake; snapped trees, heaped debris, smashed houses, and, on many occasions, several broken corpses as well. Short of hurricanes, no other kind of weather is so immediately dangerous or makes so many headlines. The dramatic image of these whirlwinds has appeared in many films, from the Wizard of Oz to the modern era. Every summer contains its share of tornado warnings on hot, muggy afternoons when the air seems stifling and oppressive, and it seems as though anything could happen.
A tornado is a violent vortex of air, the smallest type of cyclone. It happens mostly when atmospheric instability is at its peak -- when there is hot, wet air at ground level, cooler, drier air aloft, powerful convection, and often the electrifying influence of an approaching frontal system as well. Don`t go Kiteboarding in too strong a wind. Tornados can occur in isolated thunderstorms, or they can crop up along frontal boundaries where a squall line marks the advance of a cold front. Wherever they happen, they are a major event in the local weather, and an interesting demonstration of how the Earth's atmosphere functions as well.
Tornados are whirling columns of air which extend from the base of a cloud to the ground, and can contain rotating winds of anywhere from 80 to more than 300 miles per hour, depending on their size and strength. They rotate counterclockwise in the northern hemisphere and clockwise south of the equator, although around 1 in 100 -- usually very weak tornados -- rotate in the opposite direction.
A tornado develops in the same way as a mesocyclone, and usually forms as part of one. Wind shear causes a long horizontal tube of air to start rotating. This tube then encounters the vast updrafts in the southwestern quadrant of a violent thunderstorm and is tipped up on end until it is completely vertical, and combined with the updraft. This produces a whirling updraft of great strength, a mesocyclone wall cloud, a cloud dome bulging above the top of the thunderstorm, and, about half the time, a tornado or group of tornadoes.
Smaller tornados can also occur as part of frontal squall lines or derechos, and may also be generated by a hurricane. Generally, any area where there are violent weather patterns, strong winds, and a lot of atmospheric moisture and instability can potentially spawn a tornado. A few have even happened in the winter in areas with snow on the ground, although this is extremely rare.
Tornados appear as a funnel of cloud reaching the ground, often surrounded by ragging whirls of dust and debris. Extremely violent tornadoes can manifest as wedge tornados, which are wider than the distance from the ground the base of the cloud. These tornados are among the longest-lasting and can remain on the ground for hours, inflicting incredible damage and potentially killing dozens of people.
Later in their life cycle, tornados can become rope tornados as they lengthen and weaken, stretching out and eventually breaking up as the winds that hold them together become attenuated. A tornado can look light or dark depending on the angle it is seen from, how much debris is in the funnel, and other factors. Slow-moving tornados tend to be darker, since they lift more soil and debris from the surface, while winter tornados are brilliant white from the snow they suck up from the ground. Tornados can be hidden in the darkness at night or in rain and water vapor by day, although fortunately, most occur in the late afternoon when updrafts are strongest because of the maximum solar heat at that time, and are found at a rain-free part of the thunderstorm base, making them visible to anyone nearby.
Tornados are very loud phenomena, producing a rushing or roaring sound. One Civil War commander reported hearing the roar of three thousand cavalry moving at a gallop, and that he first thought that it was an approaching tornado, since the weather was stormy at the time. This sound does not travel far, however, and since any powerful wind can produce a roaring noise, sound is the least reliable way of telling if a tornado is coming. Tornados also cause a vibration in the ground which can be picked up by seismic equipment.
America, tornado capital of the Earth
Tornadoes are common in the American summer, with between 800 and 1,200 being seen annually in the United States, which is the country that witnesses the most of the storms in the world. Tornadoes happen elsewhere in the world, but the United States has the distinction of being the whirlwind capital of the world. For example, in a given year, Europe sees only 25% of the number of tornadoes that the nation across the Atlantic does, and Europe is the area next most prone to these phenomena.
The question naturally arises -- why is the U.S. subject to far more twisters than other regions of the planet? Areas like the steppe regions of Russia and the plains of Africa certainly do not lack for intense thunderstorms, so the answer must lie elsewhere.
The answer lies in the shape of the North American continent itself. Unlike other continents, North America has no mountain ranges that extend from East to West, only North to South. Arctic air and tropical air can therefore clash over the plains of the central U.S. without any natural barrier to mitigate their contact. Frontal boundaries are therefore stronger than in other areas of the world, with arctic dryness and tropical moisture meeting each other head-on all year long.
The Rocky Mountains also play their part in providing the U.S. with a record-breaking supply of tornadoes. The air which passes over them is dried by orographic uplift, with moisture condensing out as rain or snow on their western flanks. This dry air then coasts downwards from the Rockies on the eastern side, pressing down on the hot, wet air flowing in at ground level from the Gulf of Mexico, and creating regular low pressure areas with intense instability. The dry, cool mountain air is positioned aloft and presses down, while the hot, wet air is at ground level but attempts to rise vigorously -- thus setting up the conditions needed for tornadogenesis.
No other continent has free-flowing arctic and tropical air across such a broad area, combined with the low-pressure and instability-building presence of a huge mountain range along its western coast, and thus no other continent is subject to so many tornados as is the North American.
Tornado strength and effects
Tornados can range from fairly weak specimens that can scarcely lift a roof to monstrous, concentrated whirls of wind that can tear down nearly any human structure short of a bunker and present a huge threat to life and property. The winds of a weak tornado are only 80 miles per hour, which is matched by many straight-line winds. However, very powerful tornados can have winds higher than 300 miles per hour, which makes them the strongest winds to be found at the planet's surface. The movement of tornadoes is usually from southwest to northeast, but there are many exceptions. Some have even followed completely circular paths and dissipated at practically the same point where they began.
About four-fifths of tornadoes fall at the weaker end of the scale. As far as there can be said to be an average tornado -- since these storms are extremely variable and unpredictable -- this theoretical average specimen would have winds of 110 miles per hour, a track one mile long, and a width of 250 feet. The most famous tornados much more potent than this, however, and may come in outbreaks or clusters that contain dozens of violent, destructive tornados.
Generally speaking, the stronger a tornado's winds, the longer it will remain intact, the wider its path of destruction will be (up to a mile and a half), and the longer the path it will create across the landscape. The most potent tornadoes are only a tiny fraction of those which occur every year, but are responsible for a large percentage of the deaths and injuries caused by these cyclones. A whirlwind with 300-mile-per-hour winds can rip asphault paving up off the ground and twist a skyscraper into a corkscrew shape, as well as many other strange results.
The majority of people killed in tornados are in mobile homes when the storm strikes. These homes lack a basement, and since they are light and flimsy, they can easily be flipped over or actually picked up and thrown through the air by a tornado. When the mobile home stops abruptly, striking the ground, people inside are hurled against the walls, often with lethal force.
Tornado research continues in the hopes of understanding these violent, dangerous, and fascinatingly bizarre weather events. Tornado chasers follow supercell thunderstorms in the hopes of seeing one, and the rest of humanity listens to the television or radio for updates on the local tornado warning -- one of summer's more dramatic features.