Why does a tornado happen?


Quick Answer

When wind shear and unusually humid and warm conditions in the lower atmosphere meet with cool conditions in the upper atmosphere during a violent thunderstorm, tornadoes can form. Wind shear refers to wind that changes directions and increases speed with height.

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Full Answer

Generally, the type of atmospheric instability and wind shear needed to foster the formation of tornadoes is found ahead of low pressure systems and cold fronts. The downdrafts and updrafts caused by unstable air aloft in thunderstorms can cause the telltale spinning of the tornado as it interacts with wind shear to form the vortex of the tornado.

Dry air found in the middle of the atmosphere can sometimes help the formation of a tornado as it is rapidly cooled by precipitation inside a thunderstorm. This rapid cooling strengthens the downdrafts that allow the tornado to form. For this reason, most tornadoes form along the boundary between the storm downdraft and updraft regions.

Weaker tornadoes often occur when strong wind shear conditions exist without the instability in the atmosphere that causes stronger storms. Still, the most violent and damaging tornadoes tend to form when strong wind shear and instability meet; these conditions are often found in the midsection of the United States during spring and sometimes fall.

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