KK

If there is a uniform lift distribution, and the thermal large enough, there
would be little advantage to one direction of turn over another. However,
if there is a small, strong core then turning against the rotation would
reduce the turn radius and make it easier to stay in the core. At least
that's how it seems to work for me.

Bill Daniels

"Ken Kochanski" wrote in message
om...
Bill,

The Weatherwise article also supports the view that thermals (if you
consider them weaker cousins of dust devils) have equal probability of
left or right spin.

"To summarize, the Coriolis force has little bearing on the sense of
rotation in dust devils--about half of them spin one way and half the
other. By contrast, the large-scale rotation in the vicinity of
tornadoes and the storms that spawn them is usually cyclonic,
influenced by the Coriolis force."

But, how much advantage will you get choosing the correct thermalling
direction ? Let's assume the thermal is 500' in diameter with a
uniform lift of 5 knots and the rotation speed at the 250' radius is
10 MPH.

KK


"Bill Daniels" wrote in message
hlink.net...
"Ken Kochanski" wrote in message
om...
This article from Weatherwise looks at the mechanisms that cause spin
in storms, dust devils, etc. The thermals we fly in typically form in
the high following a frontal passage ... the flow in a high is
clockwise ... could it cause most thermals to have a clockwise
rotations ?

http://www.weatherwise.org/qr/qry.02coriolistorn.html


Alas, there have been studies that have found an almost even population
of
left and right hand rotation with, perhaps, a small edge to the left
hand
rotation in the northern hemisphere. Coriolis effects are more likely
to be
seen on large scales - much larger than dust devils.

On one occasion I observed a very large dust devil over a dry lake in
California. The central thermal was rotating counter-clockwise but
ringed
by a dozen or more dust devils rotating clockwise in the shear layer at
the
edge of the large one - somewhat like planet gears around a sun gear.
The
smaller dust devils were more obvious than the large central one so a
casual
ground observer would think that the majority of dust devils that day
were
clockwise. You had to be airborne to see the larger pattern. It pays
to be
careful with observations.

When you can determine the direction of rotation from airborne trash or
dust, it pays to turn against it.

Bill Daniels