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Name: Vince
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Question:
I have a question related to the starting of tornadoes that I cannot find an answer to. Maybe one of your physics wizards can point me. First, I have researched tornadoes pretty thoroughly, and how they start is not well understood in detail. Unlike hurricanes in the northern hemisphere that are always counter clockwise, sometimes tornadoes are clockwise. But that is not my question. I have observed that if you take a basin of water and time how long it takes to drain, you get a certain drainage time. Now if you repeat the same "experiment" but this time start the water rotating (in either direction), the basin empties much faster. I cannot find any references using a "Google" search that sheds light on why the rotational motion should cause the basin to empty faster when it is spinning in a direction perpendicular to the direction of flow. I fear it has something to do with some incomprehensible solution to the Navier-Stokes equation, but I would really like to have an explanation


Replies:
The reason the swirling sink empties is lots simpler. The swirl allows air to go down the drain at the same time as water. Otherwise, air trapped in the pipe interferes with flow. You get the same effect with a pair of two-liter bottles (this works best with rigid glass). Fill two bottles, swirl the fluid in one and invert both bottles. The swirling fluid drains much faster because air enters the bottle easily.

As for the tornadoes, they form along a "dry line" where winds travel in opposite direction on either side of the line. For more details, contact the Severe Storms people at the US Weather bureau in Oklahoma.

R. Avakian


Cole,

Tornados form as a result of rotation of air within a thunderstorm. That rotation must first be present or a tornado can not form.

Strong vertical motions in the thunderstorm are translated into horizontal rotational motions as the storm intensifies. This begins up in the thunderstorm a ways and begins to move downwards until a funnel cloud can be seen protruding from the bottom of the cloud. When it extends all the way down to the ground, it is called a tornado. There are many funnel clouds that do not become full-blown tornados.

It is common for tornados to form from thunderstorms that are themselves rotating, a clear indication of strong rotational forces at work with the thunderstorm which could result in a tornado.

David R. Cook
Meteorologist
Climate Research Section
Environmental Science Division
Argonne National Laboratory


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