Why does a cold glass sweat? The dew point, explained

Transcript

Why does a cold glass sweat? The water isn't leaking through the glass. It's coming out of the air. Air always carries some invisible water vapor. In warm air, a lot of vapor can stay a gas. Cool that same air, and at some point not all of it can. That temperature is the dew point. A cold drink chills a thin layer of air right against the glass. If that layer drops below its dew point, the extra vapor condenses into droplets. On a summer porch, at thirty degrees Celsius and seventy percent humidity, the dew point is about twenty-four. Almost any cold drink will sweat. Now drag the humidity down. In a heated room in winter, the dew point can sit below freezing, colder than even an ice-cold glass, so it stays dry. Now warm the drink. Once the glass is warmer than the dew point, no more droplets form. The same physics fogs your glasses and your bathroom mirror, and it puts dew on the grass at dawn. Try it yourself at Explainer Tools dot com.