Thermalling with Corn Stalks - CW or CCW?

All,

It is corn havest time here in the upper midwest. After pulling the
corn kernels off the cob, the harvestors grind up the left over corn
stalks and spray them out onto the field for mulch. The lighter corn
stalk leaves sometimes get sucked into thermals and I have seen them
at 7-8K AGL. The first time this happens you get freaked out when
you see something flash by out of the corner of your eye. I, for one,
never expected seeing a corn stalk that high up. Insects? Sure.
Birds of prey and carrion eaters? Yep. Vegetable matter? No way.

Anyway, it is quite a sight. I was chasing one on Sunday trying to
keep it off my wing tip. Had to bank pretty hard and it did out climb
me. No surprise there.

What was surprising is that I noticed it was thermalling the same
direction I was. I guess I never thought about it that a thermal
might be a rotating column of air. I suppose a dust devil is a good
example of a rising and rotating column of air.

So now the questions...

- Do all thermals rotate?
- If they rotate, do they all rotate in the same direction (north of
the equator)? CW or CCW?
- If they all twist the same way, should I turn into thermals with or
against the rotation? (I know, I know, you should turn into the
core ... but this is a hangar flying thought exercise).

Expiring minds want to know.

-- John DeRosa

Yes, No, Against

How about a thermal that smells like Cotton Candy. Down in Sugarcane
County (Palm Beach, Fl.), they burn the leaves to make it easier for
the harvesters to do the cutting and stack the cane. They'll set a
hugh section on fire and the leaves have sugarwater in them just like
the cane and the smoke will drive you up a over 1400'/m and you will
see burning leaves on fire climbing with you.
The air is full of hawks and falcons looking for an easy meal with the
escaping snakes and mice and all other poor creatures. Of Course, the
ash and soot will load up your leading edges and I have caught a few
cane leaves on the leading edge.
R

I saw pretty large corn stalks at cloudbase when I flew from Magalies
Gliding Club near Johannesburg many years ago. I remember meeting
them on each circle - their climb rate was about the same as my
Jantar. I don't recall any obvious rotation and if you calculate the
numbers from the conservation of angular momentum, a rapidly rotating
dust devil slows down as it rises and expands. The best advice is to
look at the cloud overhead and if that is rotating, leave quickly!

I have noted that soaring birds will choose a direction in well-
organized stable thermals, but don't know if this is chosen to
maximize their soaring energy using thermal rotation or just randomly
by the first one up. I have experimented by flying the opposite way
to flocks of soaring birds. They do look annoyed, but eventually give
in and join the bigger bird!

To answer the question of direction of rotation, thermals are too
small to be influenced much by Coriolis force and they rotate randomly
both ways. In fact, it is not uncommon in Arizona to see very large
thermals spin off daughter dust devils on their periphery, these often
spinning in random directions.

Mike