Tornado

A funnel cloud may form in a severe storm and never reach the ground. If and when it does, the funnel officially becomes a tornado. The central vortex of a tornado is typically about 328.1 ft (100 m) in diameter. Wind speeds in the vortex have been measured at greater than 220 mph (138 km/h). These high winds make incredible feats of destruction possible. They also cause the air pressure in the tornado to drop below normal atmospheric pressure by over 100 millibars (the normal day-to-day pressure variations we experience are about 15 millibars). The air around the vortex is pulled into this low pressure zone where it expands and cools rapidly. This causes water droplets to condense from the air, making the outlines of the vortex visible as the characteristic funnel shaped cloud. The low pressure inside the vortex picks up debris such as soil particles, which may give the tornado an ominous dark color. A tornado can act as a giant vacuum cleaner sweeping over anything unlucky enough to be in its path. The damage path of a tornado may range from 900 ft (300 m) to over 0.5 mi (1 km) wide.

Tornadoes move with the thunderstorm that they are attached to, traveling at average speeds of about 10-30 mph (15-45 kph), although some tornadoes have been seen to stand still, while other tornadoes have been clocked at 60 mph (90 kph). Since a typical tornado has a lifetime of about five to 10 minutes, it may stay on the ground for 5-10 mi (8-16km). Occasionally, a severe tornado may cut a path of destruction over 200 mi (320 km) long. Witnesses to an approaching tornado often describe a loud roaring noise made by the storm similar to jet engines at takeoff. There is no generally accepted explanation for this phenomenon although it has been suggested that supersonic winds inside the vortex cause it.

The destructive path of tornadoes appears random. One house may be flattened while its neighbor remains untouched. This has been explained by the tornado "skipping"—lifting up off the surface briefly then descending again to resume its destructive path. Studies made of these destructive paths after the storm suggest another possible explanation: tornadoes may have two to three smaller tornado-like vortices circling around the main vortex like horses on a merry-go-round. According to this theory, these "suction vortices" may be responsible for much of the actual damage associated with tornadoes. As they rotate around the main tornado core they may hit or miss objects directly in the tornado's path depending on their position. Thus if two houses were in the tornado path one may be destroyed by a suction vortex but the vortex had moved into a different position (and the next vortex had not yet taken its place) by the time it reached the next house. The tornado's skipping behavior is still not completely understood.

When houses or other structures are destroyed by a tornado, they are not simply blown down by the high winds: they appear to explode. For many years it was believed that the low pressure of the tornado vortex caused such explosions. According to this theory, if the pressure outside a building drops very quickly the air inside may not escape fast enough (through cracks, holes, and the like) to equalize the pressure difference. The higher pressure inside the building then pushes out windows or whole walls, and the structure looks like it had exploded. Studies of tornado damage have shown that buildings do not actually explode in this manner. Instead, high wind passing over a house roof acts like the air moving over an airplane wing: it gives the roof an upward force or lift which tends to raise the roof vertically off the house. Winds also enter the building through broken windows or doors pressurizing the house as one would blow up a balloon. The combination of these forces tends to blow An approaching tornado with its distinctive funnel visible. Photograph by Howard Bluestein. Photo Researchers, Inc. Reproduced by permission.

the walls and roof off the structure from the inside out giving the appearance of an explosion.

Tornado strength is classified by the Fujita scale, which uses a scale of one to six to denote tornado wind speed. Since direct measurements of the vortex are not possible the observed destruction of the storm is used to estimate its "F scale" rating.