re: Sitting in an open thermos, which reaches room temperature (20°C) faster?

Assuming the fluid is filled to the top of the thermos, and the lid is on, so no air, and no evaporation / condensation, and both thermoses are shaped the same.

I believe both will require the same amount of energy transfer to reach the desired temperature.

Also assume no or at least the same air motion around both thermoses to allow equal opportunity for heat transfer, and it comes down to the natural convection within the thermos maximizing the temperature gradient between the air – thermos – water.

So which will have the greater convection? I would guess the hot 1, due to slightly less dense?

The hot one would have the fluid around the walls falling and an internal / Central updraft, while the cold would see the fluid Rising at the walls and falling centrally. Assuming identical geometry, I don't think that would create an intrinsic difference, but it's interesting to visualize

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I know that it can, that goes with the assumption that heat transfer only occurs on the top surface of the water. I was only talking about the thermos, hence the commentary about thermos effectiveness.

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Gotcha. Fair enough.

The point about humidity is still valid - you will see heat added to the “cold” system (meaning water inside the thermos) via condensation if the humidity is sufficiently high. And you will see heat removed from the “hot” system via evaporation if the humidity is sufficiently low.

However, I think evaporation would be more impactful than condensation over the long run (at a given rate of evaporation/condensation) because evaporation removes both heat and mass from the system, while condensation adds heat but also adds mass. So you would see diminishing returns from the effects of condensation.

I’m also not convinced that evaporation/condensation rates are the primary factors determining which system heats/cools faster in the first place. Maybe at the humidity extremes, but I would think under most conditions the convection from the hot system would have a larger impact.