Highs and Lows

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Craig

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Jan 30, 2003
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543
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Jupiter, Florida
Strange thoughts sometimes circulate in my brain. So i will share with all ya'll (plural of ya'll) the latest thinking:

We are all taught that air circulates clockwise around a high pressure area, and counter-clockwise around a low. Basic weather, right?

So in a thermal, which way does the air circulate? I know from glider flying that I am usually more comfortable circling in a thermal to the left - CCW. Comes from learning to fly first in a side-by-side airplane, instead of a tandem-seat glider, I guess. And thermals do travel with the prevailing wind, in an ascending slant to the ground over which they are formed.

But is there a circulation within the thermal? And if so, which way would it go? And would it be more beneficial to travel with the circulation, or against it? Certainly one way would give you a higher apparent ground speed than the other - good or bad?

Just something to tease your brains.
 

Craig

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Joined
Jan 30, 2003
Messages
543
Location
Jupiter, Florida
Thanks, Rhino. Both of your references refer to "dust devils" - horizontal circulation that sometimes takes dirt, leaves, etc., up into the air with the thermal.
But for the life of me I can't remember which way dust devils circulate - CW or CCW. And I don't know the effect on gliders of flying into or against this internal thermal circulation. Just some food for thought.

Funny that they mention air coming down to replace the hot air going up - we always called this "down air", and I had a glider student once who disregarded my calls of "Down Air!!!". Normally you put the nose down to get out of it in a hurry - he poked along, losing better than 800 fpm until I thought we were going to land in a cow pasture. Finally, I put the nose down, scooted across the pasture, and just managed to land on the airfield. Kinda exciting there for a moment. My fault, tho, as apparently I had not taught him that what goes up must come down - and what to do about it.
 

Othman

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Aug 23, 2004
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355
Location
Ottawa, ON, Canada
Off the top of my head, I would guess that thermals travel upwards without circulation... but may have some mushrooming around the perimeter where mixing (shear) with cooler air occurs.
 

wally

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Mar 31, 2004
Messages
928
Location
southwest TN.
Northern hemisphere, I gues CCW like water drains.

I used to think they were just blobs of heated air that periodically broke free from the hotter surface area where they were formed.
They are probably more like a vertical column since as the heated air rises, more air would be coming into the hotter area and warm up also.
I think glider guys can look at the ground and tell likely areas for thermals to form. The guy who has the other half of the hangar I rent has 2 high perfromance gliders, I will ask him.
Wally
 
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Craig

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Joined
Jan 30, 2003
Messages
543
Location
Jupiter, Florida
When I was flying gliders every day - and teaching others how - I could easily spot places ont he ground that would generate thermals. And yes, some would be rising columns, some would be rising bubbles. Generally, a "cloud street" forms down wind of a good thermal generator, constant type. And little fracto-cumulus puffs would mark a bubble producer.
Sometimes, a little ridge, even as small as a 20' high dune line, will generate orthographic lift with an on-ridge (or, in the case of Florida) an on-shore wind. Small, but can give you 100-200 fpm up.
Thanks all for your thoughts -
 

Yenn

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Mar 16, 2007
Messages
42
Location
Benaraby Queensland Australia
Willy Willy's. The Australian equivalent of dust devils do definitely rotate and very fast also, but I can't for the life of me remember which way. If they rotate I would expect all rising air to follow suit.
 

Topaz

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Jul 29, 2005
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Orange County, California
Well, the dust devils in SoCal definitely rotate counter-clockwise, every one that I can remember.

'Plain' thermals probably don't rotate much, but I'm guessing on that.
 
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