On a cold, dry winter day your glasses fog up when you go indoors after being outside for a while. Why is that?

And if you go back outside with your glasses still fogged up, they quickly clear up. Why is that?

Question

On a cold, dry winter day your glasses fog up when you go indoors after being outside for a while. Why is that?

And if you go back outside with your glasses still fogged up, they quickly clear up. Why is that?

anonymous · Answer

Isn't is condensation effect? The glass is comparatively cooler than the room temperature. Water vapour become tiny little fluid drops on the outer most layer of the glass where the temperature difference is highest. You maybe able to be more quantitative and use the following two equations.

$$Q = \Delta m L $$ where Q is the energy needed to convert a certain amount of mass and L is the latent heat constant for water in this case.

I do not know how much the mass of the condensed water molecules would be but it might be tiny maybe on the order of 10^-6 kg. 

Now by using the first equation you can find out the energy needed to convert one phase to another. Then you can assume that this energy is transferred only via convection. Use the Newton's law of cooling.
$$P = k A (Tg-Ta)$$where P is the rate of energy dissipation, the temperature for the glass is Tg and Ta is the one for the ambient room temperature. Assume A is about 5 cm^2 and k is just a certain constant for which value I do not know right now. You may be able to find a good approximate value on wikipedia.

$$Q/P=\Delta t$$ and you would know the time it takes for you to see nothing but a white cloud through your glass.

Of course by energy here I mean heat transfer. Maybe I should be more explicit.

anonymous · Answer

Lol, this was on my exam two days before now.

anonymous · Answer

thnx u dude i appreciate the answer!

anonymous · Answer

No problem, I think it is not entirely correct but you can get some approximate values.

anonymous · Answer

because when you go out your glasses get cold and when you come back inside room, your room is hotter and has more humidity than outside so that water present in the air deposits on your glasses.(same act as drops comes on outer surface of glass full of cold water)
when you go out that fog again evaporates and temperature of your glasses and environment becomes equal.