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Tornadoes

What causes tornadoes?

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September 26, 2017 11:25AM

Tornadoes are caused by the interaction of weather systems, which create large thunderstorms (the most powerful and long-lived type is called a supercell). The winds created by the growth and motion of these storms may create vortices (funnel clouds) that can descend to reach the ground, and move along the ground as tornadoes.

The formation of tornadoes is complicated.

First, you need thunderstorms, then you need a condition called wind shear, in which the speed or direction of the wind changes with altitude. If the shear is strong enough it can essentially tilt a thunderstorm. This separates the updraft and downdraft of the thunderstorm, preventing them from interfering with one another. This allows the storm to become stronger and last longer.

Additionally, if the wind shear is strong enough it can start the air rolling in what is called horizontal vorticity. This horizontal vorticity can then be turned vertical by a thunderstorm's updraft. When this happens, the thunderstorm may start rotating. The rotation is especially strong in an updraft called a mesocyclone. If the storm intensifies rapidly enough, a relatively warm downdraft called a rear-flank downdraft or RFD can wrap around the bottom part of the mesocyclone. This can then tighten and intensify its rotation and bring it down to the ground to produce a tornado.