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

quote:

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And the humidity

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I'm assuming no heat exchange with the thermos and the surroundings. So it's just out the top.

I would think the boundary layer would be thinner for the warm water because of natural convection mixing the air. Additionally while evaporation probably isn't the most important part, you'd get more evaporation off the warm water. So the mass of the warm one would be a little less over time.

So the hot one seems like the right answer, but it depends on the assumptions you make about the system.