Thermals and Circling Direction

[Gary Osoba]

There have been recent discussions in the past few weeks on both the
Hang Gliding and Sailplane internet forums about the above subject. Here
are some additional observations:

Contrary to oft-quoted Coriolis effects, etc., which are not applicable
due to scale and other factors, dust devils and thermals rotate either
cyclonically or anticyclonically with equal probability irrespective of
what hemisphere they occur in (Renno' 1997; Sinclair 1966; Williams
1948). They have been traditionally defined with surface diameters
between 1 and 50 meters, while larger systems have been identified as
strong convective plumes which may carry detrital matter. So to answer
one poster's question, the technical dividing line between a dust devil
and a thermal would be 50 meters. In practice, most pilots I know who
witness some of the 100 meter plus monsters which can occur in the
desert regions of the US call them all dust devils when they are
visible.

There is wider acceptance of the notion now that dust devils inductively
form at the bottom of convective plumes, representing the surface
extensions of normal thermal systems (Renno' 1996, 1997; Sinclair 1966).
I subscribe to this theory in most cases, and Renno' has sucessfully
modeled both systems discreetly with the same methods as simple
convective heat engines. His methods and formulae have proven
"universal" in application, such that he accurately predicted the
strengths, pressure sinks, and other characterisitcs of convective
plumes on the Martian surface as measured by the recent Pathfinder
mission.

It has been my observation that rotational velocities which are begun by
mechanical turbulence and/or local wind shears will determine the
ultimate direction of rotation. Whereas circling diameters for a typical
modern sailplane are significantly larger than those of very light or
Ultralight Sailplanes, hang gliders, and even more so for paragliders,
circling direction has little effect on the achieved climb of such
sailplanes. At a given bank angle, these diameters will vary as a
function of the square of the circling velocity. Rotational velocities
of the thermal become quite low at the diameters utilized by the heavier
ships. The lighter designs may be circling at diameters in the 50 to 100
foot range. Circling directions which are oriented against the rotation
of the thermal can make a significant difference in these cases,
something I have experienced many times in the prototype Carbon Dragon.

Here in the Great Plains, where treelines have been planted on east-
west orientations nearly every mile or so since the dustbowl of the
1930's, thermals will most often exhibit a counterclockwise rotation
during the warm months due to the fact that the southerly winds
experienced then generally have a westerly component. As the surface
winds rush over an east- west treeline, which forms a wind shadow and
provides for a reduced horizontal heat transport over a plowed field,
thermals are often triggered in this fashion. The mechanical turbulence
in these instances usually induces the final direction of rotation. In
areas with very little topographical variation or wind shadowing, any
existing wind shears will usually be the determining factor.

Best Regards,
Gary Osoba

[Kurt Sermeus]

Gary Osoba wrote in message <35DAD6...@fn.net>...

>Contrary to oft-quoted Coriolis effects, etc., which are not applicable
>due to scale and other factors, dust devils and thermals rotate either
>cyclonically or anticyclonically with equal probability irrespective of
>what hemisphere they occur in (Renno' 1997; Sinclair 1966; Williams
>1948).

> ...........

>There is wider acceptance of the notion now that dust devils inductively
>form at the bottom of convective plumes, representing the surface
>extensions of normal thermal systems (Renno' 1996, 1997; Sinclair 1966).
>I subscribe to this theory in most cases, and Renno' has sucessfully
>modeled both systems discreetly with the same methods as simple
>convective heat engines.

> ..........

Hello Gary,

Thanks for your interesting observations about dust devils.
Could you give us the references you mentioned (Renno, Sinclair and
Williams) ?
Personally I'm most interested to hear about the modelling work of Reno.

Regards,

Kurt Sermeus

Many strong thermals to you :-)

[Grant & Candace]

I fly a Lark in the skys over Las Vegas and we get huge thermals. I
have found that the thermals turn in both directions. The thermal
direction of rotation seems to be influenced by mountains and valley
converjents.
If you find yourself working the rudders alot try thermalling in the
opposite direction. Not only will the rudders settle down but the
handling will improve. Many times you can slow your airspeed down to
just above stall and not have to use as much control imput.
Check it out on a real punchy day. Grant