Thermal Rotation - Revisited....

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

reply to address has duplicate hot

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

reply to address has duplicate hot

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

reply to address has duplicate hot

Ugh! I am sure people are tired of my pointing it out, but they do
rotate. However, it is nearly impossible to see from anywhere except
directly below the cloud. The upward motion of the air is so much
greater than it rotational speed, that you cannot see the slow
turning.

If you lie on your back and watch a cu develop (directly above you),
you will note several things. First, you will see individual vortices
around the edges of the cloud (not because they are limited to this
area, but because the center of the cloud lacks the needed contrast to
reveal them). These turn in both directions. You may see only one, or
as many as four or five distinct votices. They create the familiar
spirals along the edges of the clouds.

As well as localized spirals, if you watch carefully and over a long
period (10 minutes) you will get indications that the cloud as a whole
is turning. This is difficult to see at a glance because the short
term, localized dynamics of the cloud edge overpowers this effect
visually -- in much the same way that vertical development overpowers
visual measure of horizontal circulation. You can cancel out the
vertical development effects by placing yourself under the cloud, but
ignoring edge vortices in order to recognize gross rotation is more
difficult. You must segment the cloud and follow the motion of a
single segment with respect to others. Then the slow rotation of the
whole system becomes more obvious.

Is this useful? I'm not sure. What is useful, however, is a relatively
simple model that I think can help diffentiate thermal "types" and
suggest a climbing strategy for each.

The model is of a "stranded" thermal. Think of thermals as a cable of
varying numbers of strands, twisting about each other. Near the ground
these strands are loosely bound. They can turn in either direction and
have varying upward velocities. At some altitude they become more
tightly bound into a core, then near cloudbase they loosen once again.
At low altitudes and at cloud base, knowing the location and rotation
of these separate strands is especially critical to maximizing climb
rate (and in some cases, even staying in the rising air). At middle
altitudes, the height band, this is less criticial since the
individual sinews form what is effectively a single column of lift.

As these sinews (or strands) draw together, they must begin to twist
about each other (for the sake of the model, we'll assume they don't
subsume one antother) rotating as a whole in order to conserve
momentum (linear is transfered to angular -- think of two skaters
coming together on the ice). This is the impetus for whole system
rotation.

Using the model. Most thermals on a given day within a discreet area
are cousins. If you take the measure of one, you know how to deal with
the majority. But how to measure, and how to recognize a difference
when it appears.

Count the number of strands. A thermal with a strong core from ground
to cloudbase has a single broad strand. The only trick here is
determining which angle of bank nets the best climb. A crescent shaped
thermal has two strands. These are among the most difficult to climb,
since neither strand is large enough to use singly and the interaction
of the two tends to bend the thermal into unusual shapes. Three or
more strands produce thermals with a well-defined height band. Above
or below, it is necessary to employ more subtle tactics to take
advantage of the lift, but within the height band, the thermal is
easily centered. However, such a thermal will tend to be turbulent,
some even having multiple cores. Recognizing this may allow you to
improve your climb by slowing in each of the cores as you circle. I've
seen some pilots do this brilliantly, gaining noticeably over other
pilots flying standard, single speed circles.

So how many strands can a thermal have? Remember, it's just a model.
But I think a reasonable upper limit for the purpose of climbing
tactics is 4.

1 strand -- steady climb form surface to cloud base. Tactics simple.

2 strand -- Half a thermal. Strong lift on one side, weak lift or sink
on other. Often difficult to center even within the height band. Seems
to move around.

3 strand -- Distinct height band. Strong core. Weakening of lift and
difficulty centering above or below. Occasional strong vortices may
tempt pilot to extend climb or glide with relatively low risk.

4 strand -- Distinct height band. Strong core. Climbs outside height
band are low yield, high risk due to difficulty staying within the
bounds of individual strands.

A related tangent, I hope. If anyone chooses to apply this model, let
me know how it works for you. As for cloud rotation, lie down on your
back and observe. There's alot going on in the horizontal. Even if you
don't see gross rotation, you can't miss the strands.