What causes tornadoes?

What causes tornadoes to form in tornado alley?

• Answer:

  The formation of tornadoes is complicated. First, 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. The thunderstorms in Tornado Alley typically form when warm moist air from the Gulf of Mexico collides with cool air from Canada and dry air from the Rockies. This forces the warm moist air, which is less dense, upwards, causing the moisture in it to condense and form thunderstorms.