Re: New Mexico Fire-Devils

I can think of the following reasons why a visibly rotating dust devil =
can end in an apparently spin-less cloud:

1. The angular momentum carried in a dust devil is partially dissipated =
to the neighbouring air through the normal momentum transfer mechanisms =
that occur in all fluid flows, and very strongly in turbulent flows.

2. A rising thermal collects a large amount of surrounding air, =
especially in its final (or higher) stages. This incorporated air likely =
has statistically random angular momentum, thus diluting the specific =
angular momentum in the whole thermal.

3. The thermal widens and the rotation speed drops by the inverse square =
of the radius.

4. The angular velocity drops by the inverse cube of the radius.

So a ten metre wide dust devil rotating once every two seconds can turn =
into a two hundred wide cloud rotating once every sixty hours, without =
violating any physical laws.

Regards





.

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