Why a Cold Glass Sweats, and When It Sweats Most

Condensation on a glass is so familiar that few people think about the conditions that cause it, which is a shame, because those conditions explain why a table survives one party and is ruined by the next.

The Mechanism

Air holds water vapour, and warm air holds considerably more than cold air.

When air touches a surface colder than its dew point, it can no longer hold all its vapour, and the excess condenses as liquid on that surface. That is the water on the outside of the glass, and it came from the room rather than from the drink.

Dew Point

The temperature at which air becomes saturated. It depends on how much water vapour is present, which is what relative humidity describes.

A glass colder than the dew point sweats. A glass warmer than it does not, regardless of how cold the drink feels to hold.

When It Is Worst

• Humid summer weather, which is the obvious case
• A warm room full of people, since bodies add both heat and moisture
• After cooking, particularly boiling and steaming
• Bathrooms and kitchens generally
• Anywhere air conditioning is off and the outside air is damp

The party case is the significant one for furniture: a full room raises both temperature and humidity, so glasses stream with water exactly when nobody is watching what is underneath them. Materials designed around that scenario, of the kind at thirsty-stone.net, are addressing the condition rather than the drink.

Why Winter Feels Different

Heated indoor air in winter is very dry, because cold outside air holds little vapour and heating it lowers the relative humidity further.

In a dry heated room a cold glass may barely mark at all. The same glass in August will pool water within twenty minutes.

People conclude their coasters are unnecessary in January and discover otherwise in July.

The Same Effect Elsewhere

Condensation on windows, on cold water pipes, on a car windscreen and inside a poorly ventilated bathroom are all the same phenomenon.

Window condensation is worth knowing about for a different reason: persistent condensation and the mould that follows is a ventilation and humidity problem rather than a glazing one, and it is a health issue in a way a water ring is not.

Reducing It

Lower humidity helps, through ventilation or a dehumidifier. So does a less cold drink, which nobody wants.

In practice the useful response is not to prevent condensation but to deal with where it goes, which is what a coaster does.

Bottles Are Worse Than Glasses

A bottle straight from a fridge has more cold mass and a larger cold surface, so it sweats more and for longer.

It also has a narrow base, which concentrates the resulting water into a small area. A bottle left standing is one of the more reliable ways to mark a table.

Ice Makes It Continuous

A drink with ice stays cold as the ice melts, which means the sweating continues for as long as the glass sits there rather than tapering off.