Why exactly do eyeglasses steam up when we enter buildings in the winter?

Question

A friend tells me it is a sign of high humidity in my home, and the fogging happens only when this is the case. Is this so?

Answer 1

The eyeglasses are very cold. ANY moisture will condense on the cold glasses.

Put them in the freezer in the summer and take them out, and they'll fog up too.

Answer 2

Well, if it's cold out and you breathe on a window, it fogs up from the heat. The same priciple applies to your glasses. Since it is warm inside and you were just out in the cold, the combination of the two on the glass causes a fog-like effect on the glas.

Answer 3

Its the same process that causes FOG.  

Cold air meets Warm air,  they date, get married, and they have a "baby"

They name it "FOG"

Answer 4

Or when we get out of an air-conditioned car in Houston in the summer?

Your friend is correct, it has to do with the humidity in the air where you are going, compared to the humidity in where you are going.

There is also the difference of temperature to consider.

If you go from a cold, dry area to a moist, heated area, the moisture in the warm area is going to "condense" on the glasses, thus causing them to steam up/fog over. It's simple science, really.