Condensation on a glass: Where the drops come from

Water on the outside of your cold glass can come from the room air – it does not have to leak through the glass. A simple two-glass comparison lets you see the effect and reminds you to protect the table underneath.

Try the two-glass comparison

1. Put two undamaged glasses on separate coasters.
2. Fill one with cold water and ice cubes, and the other with room-temperature water. Avoid splashes and wipe both exteriors dry.
3. Leave them side by side for a few minutes without handling the outside surfaces.
4. Look for tiny drops forming on the cold glass while the room-temperature glass stays dry.

Why water appears on the outside

Air contains invisible water vapour. The cold glass cools the air immediately next to its surface. When that air reaches its dew point, water vapour can condense into liquid: the droplets you see on the glass.

This corrects a useful everyday misconception. A glass can be wet on the outside even when it is completely watertight. The National Weather Service explanation of dew point describes this mechanism.

Protect the table and know the limits

Put cold drinks on a coaster that absorbs or catches moisture from the start. Wipe up collected water, particularly on delicate wooden surfaces.

You may see no drops if the air is dry or the glass is only slightly chilled. The comparison is therefore not a reliable leak test. If you find a puddle, also check for spilled water or a crack rather than assuming condensation is the only possible cause.