Why Do Drops of Moisture Form on a Cold Glass?
The beads of moisture that form on a glass of ice-cold water on a warm day have nothing to do with the liquid inside the glass. They are beads of water that have condensed from the air around the glass. There is always some water in the air, even though you cannot see it.
The water is in the air in the form of an invisible gas called water vapor, which has evaporated from surface bodies of water. When the air touches the chilled glass, some of the water vapor in the air condenses. It changes back to liquid water, and collects on the glass like dew.
That’s called condensation! If you wear glasses, you may have noticed condensation in another form. If you’ve been wearing your glasses inside where it’s cool and suddenly walk outside where it’s warm, you may have noticed your glasses fog up. This is the result of the water vapor in the hot outside air suddenly condensing on your cooler glasses.
Particularly during fall and spring, when temperatures vary more than at other times of the year, you may notice condensation on the walls or windows of your house. You can also see it on the windows of your car. These are all examples of the same scientific process of condensation.