Weather topics > tornadoes |
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AN INTRODUCTION TO TORNADOES A tornado is defined as a violently rotating column of air in contact with the ground and extending from the base of a thunderstorm. In their early and mature stages, all thunderstorms are characterized by rising air, called updrafts. These updrafts supply the warm, humid air that fuels thunderstorms. But, in some cases, the column of rising air becomes a vortex - a funnel cloud or tornado. In a few cases, the vortex becomes a strong tornado with wind whirling around at speeds over 300 mph. Often, a tornado is located on the edge of the updraft, next to air that's coming down from the thunderstorm with falling rain or hail. This is why a burst of heavy rain or hail sometimes announces a tornado's arrival. One of the key questions that scientists are trying to answer about tornadoes is why updrafts in some thunderstorms become twisting whirlwinds while those in apparently similar thunderstorms don't. This was one of the key questions that scientists taking part in the Vortex Project in 1994 and 1995 were trying to answer. Air rising from the ground in the tornado vortex creates low air pressure near the ground which air rushes inward to fill. Such inflow winds can be damaging. In other words, a house or auto doesn't have to be hit directly by the tornado to be damaged. The center of the tornado's vortex is a low-pressure area. As air rushes into the vortex, its pressure lowers, which cools the air. Cooling condenses water vapor in the air into the tornado's familiar funnel-shaped cloud. As the swirling winds pick up dust, dirt, and debris from the ground, the funnel turns even darker. Tornadoes that pick up little dirt can retain their white, cloud coloration. Some tornadoes have taken on a red hue by picking up red dirt. Often tornadoes become wrapped in rainshowers, which can make it difficult to see. Although the air is rising in a tornado, the funnel itself grows from the cloud toward the ground as the tornado is forming. The term "funnel cloud'' refers to a tornado-like vortex that doesn't reach the ground. When a funnel cloud touches the ground, it becomes a tornado. Often, however, apparent funnel clouds are already tornadoes if debris is visible or if damage is occurring. The part nearest the ground is still invisible because cloud hasn't formed there and little dirt is being picked up. As a general rule, a funnel becomes a tornado when it reaches more than halfway to the ground from the cloud base, as it is likely to be causing damage on the ground by that point. The lesson: Don't think you're safe near or under a funnel cloud. Experts once thought tornado winds exceeded 500 mph. But research in recent years, including detailed analysis of movies and video tapes, shows that winds rarely exceed 250 mph and most tornadoes have winds of less than 112 mph. Winds in the May 3, 1999 tornado in Oklahoma City, however, were measured at 318 mph! An average tornado will be 400 to 500 feet wide and travel four or five miles on the ground, lasting only a few minutes. A mile-wide tornado is an extremely large one and tornadoes this strong are rare. Many tornadoes are smaller, less than 100 feet wide, and last only a few minutes. A few monster tornadoes are a mile or more wide and can last for an hour or more. As the parent thunderstorm travels along, tornadoes can develop from the cloud, run along the ground and lift back up to be followed by other tornadoes. Generally, tornadoes move along the ground at around 20 to 50 mph, but some race along faster than 70 mph. Often, the most destructive tornadoes have smaller vortices, known as suction vortices, rotating around the main vortex. These show up in some photos and leave distinctive, looped patterns in fields of corn or other crops knocked over by the winds. These are sometimes referred to as multi-vortex tornadoes.
The most violent tornadoes that pound the Plains of the USA often have more than one vortex rotating around the tornado center. As you can see in the graphic above, the vortices are like small tornadoes moving around the parent vortex. These subsidiary vortices form and die out as the center of the tornado moves along. They are called suction vortexes and are responsible for the strongest winds at the surface. Dust, debris and clouds make it very difficult to see the individual vortices when viewing the tornado from the ground. Multiple vortices are most common in F4 or F5 twisters using the Fujita wind damage scale.
Tornado Information
The "Percentage of Tornado-Related Deaths 1950-1994" pie chart shows that while violent tornadoes are few in number, they cause a very high percentage of tornado-related deaths. The Tornado Project has analyzed data prior to 1950, and found that the percentage of deaths from violent tornadoes was even greater in the past. This is because the death tolls prior to the introduction of the forecasting/awareness programs were enormous: 695 dead(Missouri-Illinois-Indiana, March 18, 1925); 317 dead(Natchez, Mississippi, May 7, 1840);.255 dead(St. Louis, Missouri and East St. Louis, Illinois, May 27, 1896); 216 dead(Tupelo, Mississippi, April 5, 1936); 203 dead(Gainesville, GA, April 6, 1936). In more recent times, no single tornado has killed more than 50 people since 1971.
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The Fujita Scale
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A key point to remember is this: the size of a tornado is not necessarily an indication of its intensity. Large tornadoes can be weak, and small tornadoes can be violent. Large tornadoes can also be strong and small tornadoes can be weak. The Fujita Scale is based on damage, not the appearance of the funnel. Weather observers often try to judge the intensity of a tornado when they are in the field, but the official F-Scale estimate is made after the tornado has passed and the damage has been surveyed. See Videos:
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The Fujita Scale(also known as the Fujita-Pearson Scale) may not be a perfect system for linking damage to wind speed, but it had distinct advantages over what had gone on before its inception. And it was simple enough to use in daily practice without involving much additional expenditure of time or money. From a practical point of view, it is doubtful that any other system would have found its way into widespread accepted use, even to this day. The entire premise of estimating wind speeds from damage to non-engineered structures is very subjective and is difficult to defend from various meteorological perspectives. Nothing less than the combined influence and prestige of Professor Fujita and Allen Pearson, director of NSSFC(National Severe Storm Forecast Center) in 1971 could have brought this much needed system into widespread use. The currently used Fujita Scale standards are as follows:
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