Question

You can comfortably hold your fingers close beside a candle flame, but not very close above the flame. Why?

In a still room, smoke from a candle will sometimes rise only so far, not reaching the ceiling. Explain why.

Answer 1

Is hot air less dense than cold?

Is the explaination have to do with the density of the smoke, which changes as it goes upward cooling? Is there a vagure correlation to a submarine here?

Answer 2

The combustion products mixed with the air heated by passing through the flame area have a density lower than the ambient air in the room and rise. Therefore the gas is much hotter above the flame where these hot gases are heading.

As You approach the ceiling, the hot gases can not go through the ceiling (hopefully) and start to spread out, mixing with the room air and cooling off. In fluid mechanics this might be modeled by putting a virtual candle above the ceiling blowing smoke down like a mirror. As the two flows met, they would not proceed but would spread out sideways.