In message <48583104$0$857$ba4acef3@news.orange.fr>, John of Aix 
 writes
>JF wrote:
>> In message , Hot Badger
>> Deluxe  writes
>>
>>> Are you suggesting that such things should not be discussed? Take
>>> Crop Circles. They exist. What causes them is not known.
>>
>> My wife and I once spent a fascinating few minutes on Blackdown's
>> Temple of the Winds watching a mischievous dust devil and its two
>> offspring wreak havoc on a field of barley. Dust devils on still
>> evenings in late summer over the Weald can cause considerable
>> trouble. Even a rabbit running across a field can set them off.
>
>I'm out in Poonah and, thick as I am I don't know what a Dust Devil is.
>Google seems to tell me it is some sort of motor vehicle but I can't see
>how rabbits would 'set them off' unless they're running on rabbit blood
>or something. Could you explain.

I've found the following paper. I'm sorry but I don't have a URL for it.

Dust Devils, Alaska Science Forum Alaska Science Forum
June 2, 1978

Dust Devils
Article #227
by T. Neil Davis

This column is provided as a public service by the Geophysical 
Institute, University of Alaska Fairbanks, in cooperation with the UAF 
research community.

T. Neil Davis is a seismologist at the institute.

Dust devils form most frequently in desert areas where the sun beats 
down on the poorly conducting sand surface. As Meg Hayes of Fairbanks 
has noted, they also occur in non-vegetated areas such as parking lots, 
especially if there are nearby large buildings to help cause irregular 
air currents. The hot desert or parking lot surface heats a thin layer 
of air just above--setting the stage for
the dance of the sun devils.

Even though the hot near-surface air wants to rise up through the 
cooler, and therefore heavier, air above, quiet air tends to retain a 
degree of stability -- a resistance to new air motion. One can visualize 
that resistance as something like the surface tension on water--it is 
strong but it does have a breaking point.

As the stability limit is approached, almost any sort of irregularity 
can cause the limit to be exceeded so that a bubble of hot surface air 
bursts upward. A place on the ground that is hotter than elsewhere will 
cause the limit to be surpassed there first. Or an irregularity caused 
by a minor gust of wind, the nearby motion of a car, and even the 
passage of a rabbit can initiate a dust devil.

The sudden uprush of hot air causes air to speed horizontally inward to 
the bottom of the newly-forming funnel. One of the rules of the physics 
of moving
air is that its vorticity is preserved. The requirement that vorticity 
be preserved leads to a large speed-up in the circular motion of air 
spiraling inward to the bottom of the new funnel.

And so a dust devil is formed; it is an almost self-sustaining whirlwind 
that maintains a funnel-like chimney through which hot air moves both 
upward and circularly.
A dust devil may last less than a minute up to several hours. Air speeds 
up to 70 mph have been measured in vigorous dust devils. This enables 
them to pick up
dust, leaves or, sometimes, rather large objects. Dust devils tend to 
form in groups; a large dust devil is sometimes seen to have little, 
shorter-lived
devils traveling along behind it.

-- 
James Follett