Oh, I think I misunderstood how the mechanics of the frame display works. I was thinking the lower the number the slower the frame animation. Seems to be the other way around. In that case, yes, 10 looks better. But that's just me. It's the whole atmosphere thing for that area of the game. Convection currents add to the feel of the area, just not too fast. Of course there is the possibility of using a gradient. For upper areas of Norfiar use less bright graphics and slower animation because those areas are naturally cooler, but for lower Norfair use more bright gradients and faster animations because it's hotter down there. Only ideas. I have no idea what the code can do or what limitations are at play..
If framerate (of an animation) is too low, your brain will see it as a series of separate images instead of as continuous motion (and conversely, rapid enough is how apparent animation via still images works). The 10 frame delay for the heat distortion shows this problem especially: it doesn't look like heat distortion, it looks like the background is snapping between images. While the 6 frame delay may look like it's wiggling a little fast, it also looks the most continuous. 8 is right at the threshold; to me it looks a little choppy, but I expect some people see it as fluid and I expect that's why it was the initial version shown off (it seems to be an acceptable trade-off between animation speed and apparent continuity).
Complicating matters is that different people have a different tolerance for how low a framerate can get before they stop seeing a series of images appearing as a continuous animation. So, good luck. Personally my vote is for 6, but it's just nit-picking, they all actually look pretty great.
Very good points. To be fair, the above examples are GIFs, the look in actual gameplay is likely to be a bit different.
Well yes, good points.
8-ish seems like a good measure. maybe 7!?

gonna fiddle around and by the time i'm convinced myself i'll upload a youtube vid for reference.
Also like the idea of having some variation in its look with a transition to an even hotter (sub-)area.
In my level layout this area will be nearby a marine area, so some underwater volcano idea came to mind.


...in which case a faster animation would make sense, because it seems more fluid then.
Gosh, i like experimenting in that creative fashion

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The sudden shift from mostly dark and moody atmospheres, to a bright blue sky and bright desert, may be a bit harsh and not very cohesive.
You could try having the desert at night. Or making the sky more grey.

I haven't yet tried out, what impression the transition to the desert looks and feels like, but you made a valid point.
Well for one, i wanted to have a different feeling area, but, your point really makes sense and i'm gonna try it out anytime with nighttime or maybe some twilight. I like that idea.
Thanks for all your feedbacks, that's worth a lot!