How do tornadoes and waterspouts form?
The formation of ordinary 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.
There are two types of waterspout. First there are tornadic or type 1 waterspouts. These are ordinary tornadoes that happen to be on water, and form by the mechanism detailed above.
More common atre type 2 or fair weather waterspouts. These are weaker than regular tornadoes and can form from an ordinary thunderstorm, or even a towering cumulus cloud. Both thunderstorms and cumulus cloud generate an updraft. If they occur over a relatively warm body of water the updraft at the surface can be strengthened. If there is any hint of rotation in the air, which can occur on its own, it can get caught up in the updraft, tightened, and intensified to produce a waterspout. This mode of formation is more like that of dust devils than it is of tornadoes.