Thanks for your explanations, Clive, they have been very helpful. Let me ask you this. Suppose I'm motorcycle going down a straight highway. When I encounter a crosswind of sufficient strength I noticeably have to compensate for it. If I didn't, I would drift off the highway.
Okay. Now suppose your soaring aircraft is going straight relative the highway just like me. You're not in any thermal at this point, so I guess it's just a steady glide down. Suddenly you enter a thermal where for meteorological reasons unknown to me the same crosswind I encountered on the motorcycle is also pushing the thermal into a leaning tower.
When your aircraft enters the thermal, what happens? Do you shift "off" the highway like my motorcycle would if I did nothing to compensate?
Thanks for the vote of confidence UncaYimmy! However, don't rely too much on what I say without confirmation from other sources. I've not been a practising pilot for decades, and in any case have very little actual gliding/soaring experience.
I'm also not 100% sure I understand your question properly but I'll take a stab at it anyway. It actually seems to relate to something I've been trying to get my head around also with respect to this particular puzzle and that concerns how a horizontal wind and a vertically moving thermal might "mix". The answer to your question may partly depend on how realistically we want to treat our thermals!
In the real world (and without any significant wind), a thermal is generated by an area on the ground that heats the air immediately above it. That becomes less dense than the air above it as a result, causing it to rise. But something also has to replace it as it rises, and mostly (I would think) this is air flowing in "sideways" from the surrounding areas. Possibly though some air from above is also sinking down through the rising warmer air simultaneously (maybe more so at the start of the process). Anyway, the "replacement air" also gets heated, starts to rise, and our thermal grows taller and taller. It won't go up forever though. The air warmed at ground level gets cooled and mixed with other air as it rises. Eventually it'll end up being close enough to the same temperature as the other ("non-thermal") air at that same altitude and so the thermal won't go any higher. In fact you end up with a giant circulation pattern, with (as was mentioned earlier by others) the strongest part of the thermal being a relatively small region of quite fast rising air, and a larger surrounding area of slowly sinking air. There's also going to be some turbulence created around the sides of the thermal as air masses with different velocities move past each other.
Now lets add some wind to this, and we'll assume it's also a simplified situation like the puzzle set-up - in the same direction and speed at all altitudes. Imagine a hot air balloon in a wind. It basically gets carried along (sideways) with the wind at the same speed as the wind even though it could be rising at the same time. The air in our thermal isn't physically enclosed like the warm air in a balloon, but in my guess is that it will often tend to act in a similar way. Still, the whole column of rising air can't be sitting there blocking the wind like some kind of solid barrier. I think JB used an analogy earlier of a stream of bubbles rising from a source at the bottom of a flowing stream. I imagine the actual main body of a thermal is going to be a bit like that also if you looked at it closely. Bubbles of warmer air rising (and also moving sideways) each much like a rising hot air balloon, but there will also some "cold air" blowing through (and maybe down and around also) as the surrounding wind meets the thermal at that height and the two streams interact. I really don't know exactly what's happening down at the finest levels - this description is really just me having an educated guess - but hopefully it's close enough to suggest the situation around and inside a real thermal on a windy day is likely to be quite complicated.
Having said all that, whether you are considering a parcel of cooler air that was blown into the region "occupied" by the thermal as part of "the wind", or whether you're looking at a smaller bubble of warm air rising (but also moving sideways "with the wind") you're always basically considering air that is still moving sideways at windspeed. (Mostly!)
Ignoring the real world turbulence and other complications of that kind, in the puzzle we can probably get away with the simplified picture that all the air (in the thermal or outside) is moving horizontally at the same speed and direction as the wind even though the "bubbles" of warmer air that give our thermal it's nature are also rising upwards at the same time.
In that context, if you are gliding (and presumably also descending) in a straight line in an area of "sink" (outside a thermal) and then run into a thermal, you should not expect to be moved sideways like your motorbike rider. All the air (in our simplified world) is moving sideways at the same speed. However, once in the thermal we will now also be in the region where the average vertical speed of the air is "upwards" so as we cross the boundary we should expect to feel a "push upwards" (acceleration) a bit like we've just stepped from a level floor onto an upwards moving escalator. And once we're in the region of air that we've been calling the thermal, we're actually experiencing some kind of quite complex mixture of air moving in different directions. On average though, it is "rising" and that's probably all that counts for aircraft of the sizes we are talking about.
One last warning, as already noted these are really only my own thoughts about roughly what might happen as steady sideways wind mixes with a thermal and I haven't really researched this at all. Therefore, treat what I say with caution!
