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This evening after a walk in the cold weather, about -5 degree Celsius, my glasses were misting up when I got back into the warm house, about +20 degree Celsius.

However, shortly after I went outside in the cold again, and the mist on my glasses disappeared. This puzzles me, since I guess the outside air to have nearly 100% relative humidity, since it is cooling down in the evening.

How come that the mist on my glasses can disappear in the freezing weather?

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The $absolute$ humidity is much lower outside. The lenses of your glasses have some thermal inertia, and while you were outside they got pretty cold. When you step inside, where the absolute humidity is higher, a thin layer of air near the lenses cools to below the dew point, and condensation occurs.

Then when you go back outside, the droplets evaporate. They would eventually even if the glasses were at ambient temperature, but they’re warmer than that—they warmed up a bit during the brief period you were inside, and they get heat from your head. So now the thin layer of air around the lenses is warmer than ambient, leading to low relative humidity and rapid evaporation.

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  • $\begingroup$ Thanks for explanation in the answer. It made me consider if the high heat of fusion for water (333 kJ/kg) has an impact. So when the hotter vapor hits the glasses it only cools to 0 degree Celsius, since it would then start to freeze, but the glasses from outside are not cold enough to freeze the vapor (or can't conduct the cold). When going outside again, the mist on the glasses is thus 0 degree Celsius, thus at higher temperature than the outside, and this (also) gives the effect of decreasing the relative humidity that you mention. $\endgroup$
    – EquipDev
    Feb 11, 2021 at 5:24

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