I'm pretty sure the hawk was trying to climb - he was working the same
small weak bubble pretty diligently, but was having to flap periodically
to maintain altitude. A heavy rain shower had passed through just a
short while earlier so not much was cooking.

I wonder what percentage of thermals have a defined rotation compared to
bubble type thermals.



Bill Daniels wrote:
I don't think there is anything like a "standard" thermal behavior.

I have seen thin tubular dust devils rotating vigorously all the way to
cloudbase above 18,000 feet. This type of highly organized convection
usually only occurs under very hot and dry desert conditions and then only
late in a day with calm winds. These are surprisingly smooth with strong
lift. I guess that if the air were very turbulent the laminar vortex
couldn't form.

Most weaker thermals are just plumes or bubbles. If there is a whirlwind
associated with them, it is probably an effect of the inflow that fills in
after a bubble leaves the surface.

Most hawks circling low are looking for rodents, not lift.

Bill Daniels


"Mark Zivley" wrote in message
om...

I was killing time before a meeting and I was watching two things.
First was a towering Cu and the second was a hawk down low circling. I
wondered if the hawk was aware of any rotation of the thermal he was in.
Then I started watching the Cu as it continued to billow upwards.
What struck me was that in watching the cloud form I did not see
anything which indicated any sort of rotation. We've all seen dust
devils. They definitely DO rotate, but I wonder what percentage of
thermal drafts actually end up forming a rotation. Think about all the
cu's you've ever seen, have any Cu's formed in a manner that looks like
they are rotating? Is it possible that any vortex type rotation has
disintegrated into just a turbulent bubble type of flow. Just

pondering...

Mark

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