Question

If you have a glass of ice water outside on a hot day, sometimes people say the glass is sweating. What is really happening to the glass of water?

Answer 1

Water vapour condenses from the air onto the cooler surface of the glass forming droplets.

Explanation:

It would be better to ask what happens to the water vapour in the air around the glass surface.

Because the surface of the glass is much colder than the temperature of the air, when warm, moist air (higher vapour pressure) comes into contact with the glass, it cools significantly. This will lower the vapour pressure of the air, meaning that moisture in the now super-saturated air will condense into liquid on the surface of the glass.

As the droplets gain mass, they eventually can run down the side of the glass, making it appear as though the glass was sweating.