Tornadoes
(Excerpted from UXL Encyclopedia of Science, available online in Discovering Collection)
A tornado is a rapidly spinning column of air formed in severe thunderstorms. The rotating column, or vortex, forms inside the storm cloud (cumulonimbus), then grows downward until it touches the ground. When a tornado is visible but does not touch the ground, it is properly called a funnel cloud. A tornado in contact with a body of water is called a waterspout.
A tornado is capable of extreme damage because it packs very high wind speeds into a compact area. Tornadoes have been known to shatter buildings, drive straws through solid wood, lift locomotives from their tracks, and pull the water out of small streams.
The United States experiences most of the world’s tornadoes, averaging about 800 each year. Most of these tornadoes arise in the states of Texas, Oklahoma, and Kansas. On average, tornadoes are responsible for 80 deaths, 1,500 injuries, and millions of dollars of damage annually in the United States.
Tornado formation
Although tornadoes can occur at any time of the year, most form during the months of March through June, when conditions are right for the development of severe thunderstorms.
In a severe storm, rain that falls from a cloud causes downdrafts (sinking air) along the trailing edge of the cloud. Meanwhile, the advancing edge of the storm has strong updrafts and humid air is pulled into the storm. As this humid air rises and cools, the water vapor it contains condenses to form more water droplets, releasing heat into the surrounding air. This heat, in turn, causes the air mass to rise ever more quickly, strengthening the storm.
As updrafts in a severe thunderstorm cloud get stronger, more air is pulled into the base of the cloud to replace the rising air. Some of this air may be rotating slightly since the air around the base of a thunderstorm always has a certain amount of vorticity or "spin." As the air converges into a smaller area it begins to rotate faster due to a law of physics known as the conservation of angular momentum. This effect can be seen when an ice skater begins slowly spinning with arms outstretched. As the skater brings his or her arms inward, the skater's rate of rotation increases dramatically. In the same way, as air converges into the strong updraft of an intense thunderstorm, its rate of spin increases. Meteorologists are still unsure whether tornadoes form deep within clouds and extend downward or form underneath the cloud and extend upward. It is possible that both situations occur.
Tornado characteristics
Tornadoes move with the thunderstorm to which they are attached at an average speed of 35 miles (55 kilometers) per hour. They have an average path length of about 5 miles (8 kilometers). The diameter of a tornado can vary from 300 feet to 1 mile (90 meters to 1.6 kilometers). Tornadoes come in a variety of shapes and sizes, and often have an ominous dark color due to the soil and other debris they pick up as they move along.
Tornado strength is classified by the Fujita Tornado Scale, or F-scale. Developed by T. Theodore Fujita (1920-1998) of the University of Chicago, the scale measures the power and destructiveness of tornadoes. The six categories of the scale (F0 through F5) classify a tornado by the amount of damage it causes—from light to incredible—and its wind speed—from 40 to more than 300 miles (65 to more than 480 kilometers) per hour. It is estimated that 90 percent of all tornadoes have wind speeds below 115 miles (185 kilometers) per hour.
Tornado prediction and tracking
The precise tracking and prediction of tornadoes is not yet a reality. Meteorologists can identify conditions that are likely to lead to severe storms and can issue warnings when atmospheric conditions are right for the development of tornadoes. They can use radar to track the path of thunderstorms that might produce tornadoes. Yet it is still not possible to detect a funnel cloud by radar and predict its path, touchdown point, and other important details. Scientific research in this area continues.