How do tornadoes and waterspouts form?

Answer 1

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.

Answer 2

Most "water tornadoes" or, more properly, waterspouts, form when a cool air mass moves over a relatively warm body of water. The water creates a ayer of warm, moist air just above the surface. However, such a low-level layer of wamer air is unstable and will tend to rise. This leads to convection and the formation of clouds, which may lead to rain showers and thunderstorms. If conditions are right, convection beneath the clouds may intensify. If there is any sort of twist to the wind, which can occur as the resulf of simple chaotic movements, it may be caught up in the rising air and pulled inward. As the twisting motion narrows it intensifies, eventually forming a waterspout. These fair-weather waterspouts as they are called are weaker than ordinary tornadoes. When they hit land they quickly die and rarely cause more than minor damage. However, they can be a hazard to small boats.

In less common cases a waterspout may be an ordinary tornado that just happens to have touched down on water rather than land. This is called a tornadic waterspout. In such cases the development usually involces a larger scale storm system. Instability, with warm air beneath much cooler air, occurs and a much larger scale, so that once a system such as a cold front triggers convection, it leads to the formation of violent thunderstorms. Wind shear, differences in wind speed and direction ad different heights, starts these thunderstorms rotating, turning them into supercells, the most powerful thunderstorms on earth. The rotation is focused around an area of low pressure within the storm called a mesocyclones. As the storm intesifies a downdraft, or downward rush of air, may wrap around the mesocyclone, tightening and stretching the rotation. As this happens it intensifies and eventually touches the ground, forming a tornado. If it forms on water or moves on to water, then it becomes a tornadic waterspout.