How Tornadoes Work

Learn how the combination of thunderstorms and instability of air masses combine to create a tornado, Mother Nature's most violent meteorological event.

When weather conditions are ripe, a thunderstorm can very rapidly turn into a tornado. Tornadoes are violent, rotating columns of air that causes massive destruction when they touch the ground. Tornadoes can hit at any time of the day and anywhere, but frequently occur during the afternoons or early evenings due to the confluence of rising heat with low-pressure instability. Changes in the wind speed and direction of air masses contribute to the birth of the tornado.

Tornadoes may strike either land or sea. On land, tornadoes are most intense in the United States, particularly in a region known as the Tornado Alley, which covers the states of Texas north to Oklahoma, Missouri, Kansas, Nebraska, and Ohio. When a tornado strikes over water, the result is a water spout. In the Northern Hemisphere, tornadoes spin in a counter-clockwise direction while it spins clockwise in the Southern Hemisphere.

Changes in wind speed and air pressures provide the impetus for the birth of the tornado. Tornadoes arise from a supercell, which is a large, powerful thunderstorm. In the Midwestern United States, thunderstorms develop ahead of the eastward flowing storm front, fueled by the warm moisture drawn in from the Gulf Coast. The result is an upper layer of cold, dry air and a lower layer of warm, humid air. Air rises as it expands and cools when it condenses, releasing heat. Air masses spreads out to form an anvil-like storm cloud when it stops rising. This may produce rainfall. An upward draft is generated at the boundary between the cold dry air and warm moist air, mixing the two layers, creating further instability. Add in the differences in wind speed (known as wind shear) and height, and a rotating column of air under this thunderstorm becomes a mesocyclone. A wall cloud forms beneath the mesocyclone, which brings heavy rain and hail to the north and east faces of the storm. When the base of the cloud descends, it gives birth to the funnel cloud. The funnel cloud expands to a billow of smoke or dust swirl as it touches the ground. When the base of the wall cloud descends to where it appears as a hanging cloud, this usually indicates that a particular region is one hour away from a tornado touchdown. There may be a green tinge to the sky. This is usually indicative of tornado-like conditions.



A mesocyclone can break into a series of smaller cyclones, called suction vortices. These smaller funnel clouds may seem deceiving less evil, but they move forward at the same speed as the parent cyclone. Wind conditions during a tornado touchdown can reach as high as 300 miles per hour on the Doppler radar. The strongest winds or vortices occur about 30 feet above the ground. A tornado can have a path length of approximately 5 miles, and span 160 to 170 yards in diameter. The typical area a tornado covers is one square mile. This is the reason why sometimes a building may be bypassed by the parent tornado, but is subsequently destroyed by the smaller funnel clouds that sweep past the area as it breaks away from the parent cyclone.

Tornadoes are not isolated to a specific season, although weather conditions in the summer and fall favor instability that fuels the birth of the tornado. The peak tornado months occur between March and August. April is typically the month that records the highest number of deaths, while May is the month that has the greatest number of tornadoes. The majority of the tornadoes strike between the hours of 2 to 6 P.M. Only 2% of all tornadoes will reach an intensity that is violent enough to cause damage. The 2%, however, is responsible for causing 70% of the fatalities that occur during a tornado.

As with the hurricane, varying weather conditions during a tornado storm can lull people into a false sense of security. In addition to the lightning that comes with the thunderstorm, as warm air rises and cools, light rain can appear. This is followed by heavy rain as conditions develop the wall cloud. Hail may also appear. Because the center of the tornado is calm, no rain or hail falls. During this �calm�, people may be led to think the tornado is all over when the worst is yet to begin.

Because it is Mother Nature�s most deadly and violent event, tornado damage causes millions of dollars of damage and can result in severe loss of life. The National Weather Service issues warnings and alerts to areas lying in the path of an oncoming tornado so that people can take the appropriate action.

Avoid potential losses of life by paying attention to the National Weather Service warnings, as well as looking out for the telltale signs of a tornado (dark, green-tinged sky, large hail, low hanging clouds, and a windy roar) and making the right preparations.

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