How are Tornadoes Formed

The United States, parts of Europe and South America, Southern Africa, Australasia and far eastern Asia are the tornado prone regions of the world. The most severely affected area is the United States which accounts for 1,200 tornadoes every year. The absence of an east-west mountain range in the North American continent makes it vulnerable to the strong north-south winds. The Netherlands is hit by more than 20 tornadoes per square mile area. Bangladesh in the Indian subcontinent registers about 179 deaths per year because of tornadoes. Wind speeds in a tornado range from 40mph to 110mph, on an average have a 250 feet wide area and travel for a few miles before finally dissipating. However, in some cases, wind speeds of 300mph have been recorded by these twisters with a range greater than one mile.

Supercell tornadoes

The causes of tornado formation are still a matter of scrutiny. A thunderstorm called a supercell, which can last for several hours, mostly go on to form tornadoes. The warm and moist southern winds meet the cold and dry polar winds on the continental plates. The warm moist air, on account of its lighter weight, tends to move upward. In a thunderstorm, it manifests into a strong air current moving upward. During its upward journey, it meets varying winds at different altitudes. These winds have high speeds and when their direction of flow is synchronized, they act vigorously on the warm air, pushing it upward in a spinning motion. The storms which constitute these spinning winds, ultimately form a tornado. About 30 percent of mesocyclones (areas which consist of organized rotating winds a few miles up in the air) and contained in a supercell, go on to produce a tornado.

Two tornadoes can never be the same, although, they require the same set of conditions to form. The warm air is also due to the high temperatures at the ground level. As the altitude rises, the temperature of the surrounding air decreases. If the difference between the temperatures at different heights is significantly large, accompanied by a raging thunderstorm, a tornado is likely to form. The warm air explodes in the upward direction, as though puncturing the cool air above. There is a probability of thunder cloud formation, resulting in rain, thunder and lightening. The upward movement of air is so fast, that it develops into a rotating mesocyclone, high up in the atmosphere. As the air pressure increases with the amount of rain, it 'pulls' down a layer of descending air. This phenomenon is called as 'rear flank downdraft'.

This downdraft accelerates as it descends towards the surface of the earth. The high velocity of this formation also drags along the high speed rotating winds. On nearing the surface, this downdraft is visible in the form of a funnel. The gust of wind accompanying this drop, can spread the area of influence beyond the actual limits of the tornado. The influx of warm, moist air serves as a catalyst to the strength of the twister. It grows in intensity, when these warm winds continue to mix with the cold upper atmospheric air. Thus, a tornado reaches maturity when the content of the warm air is maximum. It is most destructive in this phase of its formation. The rear downward draft, over a course of time, lasting from a few minutes to more than an hour, envelop the spiraling tornado. This chokes up the air supply and the tornado subsides.

The mesocyclone accompanying the spiraling winds subsides, the storm gets concentrated in the center of the storm. If this concentration is again followed by another surging cyclone, the entire cycle repeats, giving rise to yet another tornado. Therefore, in some rare instances, more than one funnel might be visible, connecting the earth's surface to the clouds. A tornado is not necessarily accompanied by a visible funnel. Sometimes, though in extremely rare cases, there are spiraling winds and heavy thunderstorms but no visible funnel.

The low-pressure systems and storms consisting of thunder, lightening and hail are the perfect 'recipe' or the most likely cause of a tornado.