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!

I've had my share of near death moments in HG and while it continues to be the most exciting thing I've ever done looking back on it now I can more clearly see the big downside risk to flying in radical air with only weight shift control.

Excuse the off topic drivel but don't often run across old living HG pilots.

Originally Posted by Gary Evans

It doesn't have to be hot air to initially rise or continue to rising but only to be warmer than the surrounding air. Thermals can originate from a water surface triggered by the temperature difference caused by cloud shadows.

definition of thermal-from the dictionary:
ther·mal (thûrml)
adj.
1. Of, relating to, using, producing, or caused by heat.

so we need to get into details: when i said'hot air' i meant 'air at higher temperature' (of course, than the air around it). i know that may be thermals even at temperatures below freezing. it's the temperature differential what makes air rise.
sounds good?

Old hang glider pilots, Gary? I thought they were ALL old... around here anyway. Anyone under 40 flies paraglider. I have given up hang gliding but I do miss it.

Actually, I have found that the presence of gusts near the ground away from any obvious wind shadowing object probably indicates some form of thermal lift. It sure feels cold though sometimes.

A couple of observations made yesterday. We've had a huge run of very light tumble weed seed heads from grasses around work, and they've proved to be an excellent visual aid for lift as they are picked up easily, and there are thousands of them and are big enough to be seen clearly at a fair height (though I wouldn't go so far as to guess, but certainly 100's of meters up). Each is probably about 1 foot/30cm in diam.

1. First thermal was a very large (200m+ diam) slowly clockwise rotating and rising mass of air, in which there was a central core of very fast rotation and high vertical speed air. This was clearly visible from the grass pods. Across ground speed was moderate, and once the lift had passed on, everything fell out of the sky quite quickly.

2. Same conditions, day and near exact same path of 2nd thermal. A very large (200m+) mass of slowly counter clockwise rotating mass of air with no core, but with slower ground speed and less sink once it had passed on. Interestingly, neither thermal attracted bird attention despite the fact that there are large numbers of soaring birds around (raptors as well as others).

3. Same day, different thermal path, soaring raptor. The kestrel thermaled up from just above tree height to out of sight circling both directions freely in what appeared to be the same thermal.

4. I need to take my DLG to work so I can take advantage of these well defined and often big areas of lift

Steve

thank you, Steve, for these fantastic comments.

this is exactly the kind of input that i have been hopping to get.

amazing experiences that add to the awareness of these phenomenons of the air.

Xc is often flown from ground height up to a mile high. Higher up the thermals can be huge. On the scale of miles. Often I skirt the edge of cloud formations. At that altitude I don't think there is much rotation and I usually don't circle because it is a race and I want some forward component towards the end at all times. Likewise sink can go for miles. I often find good lift at the edge of a cloud where there is a lot of activity meaning cloud formation and destruction. I usually don't see rotation in these areas. Pick a spot away from the sun some day and watch the cloudshow. It is very interesting.

Great input......from the hang/para pilots as well.

Interesting question...my observation is most right handed people prefer to thermal CCW and left handed people like to thermal CW. Left handed people are usually better at going either CW or CCW.

When you are the first to initiate a thermal, you will probably have a tactical advantage if you can thermal well CW because you will out climb those who follow you into the thermal.

This is also very true in full scale soaring. I am more coordinated when I "push" with my hand than when I "pull" with it . For this reason, there are right handed pilots that have learned to thermal CW with their left hand.

Now that you mention it....I do that! Right handed and turn to the left almost always! Never thought of it before......

I think it goes deeper than hand coordination... I mentioned a little experiment with a bolt and a mirror.. no don't break the mirror! Take a regular right hand thread bolt and view it in a mirror while comparing it to it's "natural" appearance. Look at the pitch of the threads! Our perception is not symmetric! We aren't all the same either.. even left-handed people aren't all equally left handed. I write with my left hand but I throw with my right and turn left best. Our field made right hand circuits safer so I learned them as a beginner... but the expert always came in left-handed.

Originally Posted by Gary Evans

I've had my share of near death moments in HG and while it continues to be the most exciting thing I've ever done looking back on it now I can more clearly see the big downside risk to flying in radical air with only weight shift control.

Excuse the off topic drivel but don't often run across old living HG pilots.

As an avid Rc glider guy, I've often thought of going hang gliding....I think I'll pass now