Why does a cold glass get wet on the outside?
The water comes from the air, not from inside the glass. Air always carries invisible water vapor. When it touches the cold glass it cools down, and the vapor turns back into tiny droplets on the surface.
The air carries invisible water
Air is never completely dry. It holds water as an invisible gas, water vapor. Warm air can hold much more than cold air, which is why summer air often feels sticky.
Share this stepThe glass chills the air around it
The ice keeps the glass far colder than the room. A thin layer of air touching the outside of the glass cools down quickly, and cold air can’t keep as much water vapor.
Share this stepVapor turns back into water
Once the air is cooled past a point called the dew point, the extra vapor condenses into tiny droplets on the glass. They grow, merge and run down, leaving a ring on the table.
Share this stepThe water comes from the air, not the glass.
Moist aircold glassdroplets
It’s the same thing that fogs the bathroom mirror after a shower, puts dew on the grass on cool mornings and makes your breath fog a cold window. The more humid the air, the wetter the glass gets.
Common follow-up questions
Is the glass leaking?
No. Try it with a colored drink: the droplets on the outside are still clear, because they come from the air around the glass.
Why is it worse in summer?
Warm, humid air carries much more water vapor, so there’s more to condense. In dry winter air indoors, a cold glass may barely get wet at all.
How do I stop the ring on the table?
Use a coaster, or a double-walled or insulated glass. The outer wall stays close to room temperature, so no water condenses on it.
Why do my glasses fog up when I come inside in winter?
It’s the same effect in reverse: your cold lenses meet warm, moist indoor air, and the vapor condenses on them until they warm up.
