Where do the droplets on a cold cup come from?

On a hot day, a can or iced cup from the fridge gets wet again soon after you wipe it. Some people wonder whether the cup is cracked, but a tightly capped bottle or an unopened can gets just as wet. A bottle of water fresh from the fridge soaking the books in your bag is the same thing.

You can even tell by taste. The water on the outside of a cola can is not sweet. It is plain water from the room air, not the drink inside. People often say a cup is sweating, but the cup is not giving off water; the air is setting water down.

How much water vapor air can hold depends on temperature. Warm air holds more, cold air holds less. So if you keep cooling air, it reaches a temperature where it can no longer hold all its vapor. That temperature is called the dew point.

For example, summer air at 30°C and 70% humidity has a dew point of about 24°C. The outside of a glass of ice water is far colder than that, so as the air touching it cools, the leftover vapor settles as droplets. Winter windows fogging up and the bathroom mirror clouding after a shower work the same way.

Weather forecasts use the dew point too. The higher it is, the stickier the air feels, and when the temperature drops to the dew point overnight, dew forms on the grass.

The same iced cup behaves differently with the weather. In a humid summer kitchen it is soon dripping, while in a dry, heated winter room it may barely get wet. Less vapor in the air means a lower dew point. The water running out of an air conditioner's drain hose is also this kind of condensation, collected on the cold coils of the indoor unit and carried outside.

If you dislike water rings on the table, a coaster is the simplest fix. Double-walled tumblers rarely get wet because their outer surface does not get cold enough to drop below the dew point. As the cup warms up, the droplets fade on their own.