Thread: Discussion which way you turn?
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Old Nov 29, 2012, 03:32 PM
rrcdoug is offline
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Canada, BC, Kelowna
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hot air?

Yes , "hot" is relative.

The most "visible" thermals I have encountered happened in late fall over a large fresh water lake.

We have some very deep large lakes in the Okanagan and they stay "warm " in the fall. If the north wind of winter is cold and early enough, the lakes produce mist. The air is very cold and locally unstable above the lake. The mist gathers together in several places on the surface of the lake where it is swept upward in a slender column. At about 800- 1000 ft above the lake, the mist gets slightly thinner as the column widens and a tiny cumulus cloud forms!

There was no obvious rotation, although I might not have noticed a slight amount.

From soaring hang gliders, my take on thermals is that they usually are surrounded by turbulence. Airspeed will vary, the nose may be pitched up on entering and down on leaving, there will be unintentional roll and yaw. There is a lot of energy in these invisible things. I have been stalled and had the nose pitched down simultaneously! I lost close to 1000 ft that time! This may have been a wind shear tearing apart the thermal or I may have climbed in the core to the top of the thermal and then been spit out in a descending tailwind.

In model soaring I often see the glider accelerate relative to the ground before entering a thermal. This probably happens when my glider is near the bottom of a detached thermal and I am flying my glider in the air being sucked into a classic toroidal thermal. Easy to core the thermal then!

If approaching the thermal from the top, we may see our glider slow down. The glider will be pushed away from the thermal. I think this is more rare for model flying since we usually find our thermals from the bottom, due to wind shifts on the ground.

I have found the toroid model of the thermal very useful but there are many cases where it doesn't explain what's happening. Thermals obviously form long skinny columns too. I've seen them!
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