Short · Weather and water
Why does a cold glass sweat?
The water on a cold glass comes out of the air. When the air touching the glass chills below its dew point, vapor condenses into droplets; on a dry day, the glass stays dry.
Try it yourself
Opens Dew Point Lab. Change the same thing yourself.
Transcript
Why does a cold glass sweat? The water isn't leaking through. It's coming out of the air. Air carries invisible water vapor, and the colder it gets, the less vapor can stay a gas. The air touching your glass chills below its dew point, so the extra vapor condenses into droplets. Now drag the humidity down. On a dry day, the dew point is colder than the glass, so it stays dry. Try it yourself at Explainer Tools dot com.