Why Does a Cold Glass Get Wet on the Outside?

Water vapor condenses on the chilled surface

Air contains invisible water vapor. When humid air touches a cold glass, the air next to it cools. Cooler air can hold less water vapor, so some vapor becomes liquid droplets on the outside. The same process can fog eyeglasses when someone moves from a cool room into warm, humid air. [1]

Why humid days make more droplets

Warm, humid air contains more water vapor than dry air. It can therefore produce more visible condensation when it meets a cold surface. The liquid is not sweat made by the glass; the glass is where airborne moisture becomes visible.

Condensation is part of the water cycle

Condensation also helps form dew, fog, and clouds. In each case, vapor becomes liquid on a surface or around particles. The temperature at which air becomes saturated is the dew point. See how windows fog up and why fog forms near the ground.

Frequently asked questions

Frequently asked questions

Does the water come through the glass?

No. It usually comes from water vapor in surrounding air condensing on the cold surface.

Why is the glass wetter in humid weather?

Humid air contains more water vapor that can condense when cooled.

Is this the same as dew?

Yes. Both involve water vapor cooling and condensing into droplets.