Tornado.

Tornado. I INTRODUCTION Tornado, violently rotating column of air extending from within a thundercloud (see Cloud) down to ground level. The strongest tornadoes may sweep houses from their foundations, destroy brick buildings, toss cars and school buses through the air, and even lift railroad cars from their tracks. Tornadoes vary in diameter from tens of meters to nearly 2 km (1 mi), with an average diameter of about 50 m (160 ft). Most tornadoes in the northern hemisphere create winds that blow counterclockwise around a center of extremely low atmospheric pressure. In the southern hemisphere the winds generally blow clockwise. Peak wind speeds can range from near 120 km/h (75 mph) to almost 500 km/h (300 mph). The forward motion of a tornado can range from a near standstill to almost 110 km/h (70 mph). A tornado becomes visible when a condensation funnel made of water vapor (a funnel cloud) forms in extreme low pressures, or when the tornado lofts dust, dirt, and debris upward from the ground. A mature tornado may be columnar or tilted, narrow or broad--sometimes so broad that it appears as if the parent thundercloud itself had descended to ground level. Some tornadoes resemble a swaying elephant's trunk. Others, especially very violent ones, may break into several intense suction vortices--intense swirling masses of air--each of which rotates near the parent tornado. A suction vortex may be only a few meters in diameter, and thus can destroy one house while leaving a neighboring house relatively unscathed. Scientists study tornadoes to gain a better understanding of their formation, behavior, and structure. Scientists who study tornadoes have a variety of powerful research tools at their disposal. Advances in computer technology make it possible to simulate the thunderstorms that spawn tornadoes using computer models running on desktop computers. Doppler radars, which detect the rain in clouds, allow meteorologists, scientists who study weather, to 'see' the winds inside the storms that spawn tornadoes. Modern video camera footage and reports from trained storm-spotters provide an unprecedented amount of high-quality tornado documentation. These tools all contribute greatly to the scientific understanding of tornadoes. This information may eventually lead to increased tornado warning times, better guidelines for building construction (especially schools), and improved safety tips. See also See also Meteorology. II FORMATION Many tornadoes, including the strongest ones, develop from a special type of thunderstorm known as a supercell. A supercell is a long-lived, rotating thunderstorm 10 to 16 km (6 to 10 mi) in diameter that may last several hours, travel hundreds of miles, and produce several tornadoes. Supercell tornadoes are often produced in sequence, so that what appears to be a very long damage path from one tornado may actually be the result of a new tornado that forms in the area where the previous tornado died. Sometimes, tornado outbreaks occur, and swarms of supercell storms may occur. Each supercell may spawn a tornado or a sequence of tornadoes. The complete process of tornado formation in supercells is still debated among meteorologists. Scientists generally agree that the first stage in tornado formation is an interaction between the storm updraft and the winds. An updraft is a current of warm, moist air that rises upward through the thunderstorm. The updraft interacts with the winds, which must change with height in favorable ways for the interaction to occur. This interaction causes the updraft to rotate at the middle levels of the atmosphere. The rotating updraft, known as a mesocyclone, stabilizes the thunderstorm and gives it its long-lived supercell characteristics. The next stage is the development of a strong downdraft (a current of cooler air that moves in a downward direction) on the backside of the storm, known as a rear-flank downdraft. It is not clear whether the rear-flank downdraft is induced by rainfall or by pressure forces set up in the storm, although it becomes progressively colder as the rain evaporates into it. This cold air moves downward because it is denser than warm air. The speed of the downdraft increases and the air plunges to the ground, where it fans out at speeds that can exceed 160 km/h (100 mph). The favored location for the development of a tornado is at the area between this rear-flank downdraft and the main storm updraft. However, the details of why a tornado should form there are still not clear. The same condensation process that creates tornadoes makes visible the generally weaker sea-going tornadoes, called waterspouts. Waterspouts occur most frequently in tropical waters. III CLASSIFICATION Direct measurements of tornado wind speeds are difficult (and dangerous) to obtain. In 1971 Theodore Fujita, a meteorology professor at the University of Chicago, devised a classification system linking the degree of damage to humanmade structures to possible wind speeds. In 1973 this system, known as the Fujita or F-scale system, was adopted as the official tornado classification system of the National Weather Service (NWS). Using the F-scale system, the NWS ranks tornado damage as light to moderate (F0 and Fl), considerable to severe (F2 and F3), or devastating to incredible (F4 and F5). The weakest tornadoes (F0) may damage chimneys and signs, whereas the most violent tornadoes (F5) can blow houses completely off their foundations. Although media reports may sometimes refer to a tornado as an F6, this ranking is not recognized by the NWS. However, the F-scale system applies only in regions where humanmade structures exist. Also, scientists are able to correlate F-scale values only roughly with wind speeds. For instance, a wind speed of 145 km/h (90 mph) might do minor F0 damage to a well-constructed building but significant F2 damage to a poorly constructed building. Scientists estimate that F0 tornadoes may have wind speeds up to 110 km/h (70 mph), while F5 tornadoes may have wind speeds somewhere in the range of 420 to 480 km/h (260 to 300 mph). While the F-scale system remains a convenient means for scientists to classify and discuss the intensity of tornadoes, the limitations of the scale have inspired attempts to design a more sophisticated modeling system. In the United States, 75 percent of the tornadoes rate F0 or F1 in strength. Most remaining tornadoes rate F2 or F3, with only 1 percent rating F4 or F5. Usually no more than one or two tornadoes per year reach F5 strength. On the other hand, the few F4 and F5 tornadoes account for 67 percent of the fatalities caused by tornadoes. IV OCCURRENCE The United States has the highest average annual number of tornadoes in the world, about 800 per year. Outside the United States, Australia ranks second in tornado frequency. Tornadoes also occur in many other countries, including China, India, Russia, England, and Germany. Bangladesh has been struck several times by devastating killer tornadoes. In the United States, tornadoes occur in all 50 states. However, the region with the most tornadoes is "Tornado Alley," a swath of the Midwest extending from the Texas Gulf Coastal Plain northward through eastern South Dakota. Another area of high concentration is "Dixie Alley," which extends across the Gulf Coastal Plain from south Texas eastward to Florida. Tornadoes are most frequent in the Midwest, where conditions are most favorable for the development of the severe thunderstorms that produce tornadoes. The Gulf of Mexico ensures a supply of moist, warm air that enables the storms to survive. Weather conditions that trigger severe thunderstorms are frequently in place here: convergence (flowing together) of air along boundaries between dry and moist air masses, convergence of air along the boundaries between warm and cold air masses, and low pressure systems in the upper atmosphere traveling eastward across the plains. In winter, tornado activity is usually confined to the Gulf Coastal Plain. In spring, the most active tornado season, tornadoes typically occur in central Tornado Alley and eastward into the Ohio Valley. In summer, most tornadoes occur in a northern band stretching from the Dakotas eastward into Pennsylvania and southern New York State. The worst tornado disasters in the United States have claimed hundreds of lives. The Tri-State Outbreak of March 18, 1925, had the highest death toll: 740 people died in 7 tornadoes that struck Illinois, Missouri, and Indiana. The Super Outbreak of April 3-4, 1974, spawned 148 tornadoes (the most in any known outbreak) and killed 315 people from Alabama north to Ohio. V PROTECTION AGAINST TORNADOES The National Weather Service alerts the public to severe weather hazards by issuing watches and warnings that are broadcast on National Oceanic and Atmospheric Administration (NOAA) weather radio, television, and commercial radio. Meteorologists issue a tornado watch when weather conditions are favorable for the development of tornadoes and severe thunderstorms. Watches are often issued hours before severe weather develops and generally cover many counties or even several states. A tornado warning means that a tornado is occurring or is imminent. A warning is issued if a tornado has touched down, if a funnel cloud is present, or if Doppler radar indicates the presence of strong rotation in a thunderstorm updraft. The area covered by a warning is much smaller than a watch, usually only a county or two, or a portion of a county. During a tornado warning, people should seek shelter immediately in a basement or in the interior portion of a building (a closet, interior hallway, or bathroom). Mobile homes and cars have a tendency to roll in high winds and should therefore be abandoned. Structures with large, free-span roofs, such as auditoriums, gymnasiums, and supermarkets, are subject to collapse and should also be avoided. If caught outside, a person should lie flat in a ditch and cover his or her head for protection from flying debris. Contributed By: Alan Shapiro Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.