It is condensation. The temperature difference between your breath and the surface of the glass allow the condensation to be removed from your breath. Obviously the air that you breathe onto the glass doesn't stick to the glass but the moisture it contains does. This is actually very similar to how an air conditioner works. Notice on certain days the "steam" will last longer or be even more wet than others...this is because of temperature changes and the amount of humidity already in the air.
Because the cold glass condenses the moisture in the breath.
The glass has a low insulation value and this means that there is often a temperature change near the surface. Warm air can hold more moisture than cold air, so when your arm breath hits a window, it is cooled and the air cannot hold the moisture so it becomes liquid water on the glass surface (this effect is the dew point and why grass is often wet in the morning as well).
Glass is a non porous substance and the water in breath stays on the outside of the glass. If you breathe on a piece of cloth the same amount of water will be there but most of it will be absorbed into the material. It may feel a little damp, but not wet like the non porous glass.
One of the components of glass is "damp detector"... a colorless, odorless chemical. When you breathe on to a piece of glass, this damp detector is activited and that shows up as moisture ("dampness").....
Damp detector does not work at temperatures in excess of 104 deg F, (40 deg C).....
by Caerleon3 years ago
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