Tornado

Most tornadoes form in the northern hemisphere during the months of March through June. These are months when conditions are right for the development of severe thunderstorms. To understand why tornadoes form, consider the formation and growth of a thunderstorm. Thunderstorms are most likely to develop when the atmosphere is unstable; that is when atmospheric temperature drops rapidly with height. Under unstable conditions, air near the surface that begins rising will expand and cool, but remains warmer (and less dense) than its surroundings. The rising air acts like a hot air balloon; since it is less dense than the surrounding air it continues to rise. At some point the rising air cools to the dew point where the water vapor in the air condenses to form liquid water droplets. The rising column of air is now a visible cloud. If the rising air, or updraft, is sustained long enough water droplets will begin to fall out of the rising air column, making it a rain cloud.

This cloud will become a severe storm capable of producing tornadoes only under certain circumstances. Severe storms are often associated with a very unstable atmosphere and moving low pressure systems that bring cold air into contact with warmer, more humid air masses. Such weather situations commonly occurs in the eastern and Midwestern United States during the spring and summer months. Large scale weather systems often sweep moist warm air from the Gulf of Mexico over these regions in a layer 1.2-1.9 mi (2-3 km) deep. At the same time winds aloft (above about 2.5 mi [4 km] in altitude) from the southwest bring cool dry air over the region. Cool air overlying humid air creates very unstable atmospheric conditions and sets the stage for the growth of strong thunderstorms.

The warm surface air is separated from colder air lying farther north by a fairly sharp temperature boundary called a front. A low pressure center near the earth's surface causes the cold air to advance into the warmer air. The edge of the advancing cold air, called a cold front, forces the warmer air ahead of the front to rise and cool. Since the atmosphere is so unstable the displaced air keeps rising and a cloud quickly forms. Rain that begins to fall from the cloud causes downdrafts (sinking air) in the rear of the cloud. Meanwhile the advancing edge of the storm has strong updrafts and humid air is pulled into the storm. The water vapor in this air condenses to form more water droplets as it rises and cools. When water vapor condenses it releases latent heat. This warms the air and forces it to rise more vigorously, strengthening the storm.

The exact mechanism of tornado formation inside severe thunderstorms is still a matter of dispute but it appears that tornadoes grow in a similar fashion to the small vortices that form in draining bathtubs. When the plug is pulled in a bathtub, water from other parts of the tub rushes in to replace that going down the drain. If the water has any swirl in it, the drain soon has a little vortex.

Tornadoes appear to be upside down versions of this phenomenon. 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 usually contains some rotation, or vorticity. 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 spinning with arms outstretched. As the skater brings his or her arms inward, his or her rotational speed increases. In the same way air moving into a severe storm begins in a tighter column and increases its rotational speed. A wide vortex is created, called the mesocyclone. The mesocyclone begins to build vertically, extending itself upward throughout the entire height of the cloud. The rapid air movement causes the surrounding air pressure to drop, pulling more air into growing vortex. The lowered pressure causes the incoming air to cool quickly and form cloud droplets before they rise to the cloud base. This forms the wall cloud, a curtain-shaped cloud that is often seen before a tornado forms. The mesocyclone continues to contract while growing from the base of the storm cloud all the way up to 6.2 mi (10 km) above the surface. When the mesocyclone dips below the wall cloud it is called a funnel cloud because of its distinctive funnel shape. This storm is on its way to producing a tornado.