Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It only takes a minute to sign up.
Sign up to join this community
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?
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.
