Why Do Cold Glasses Sweat? Where the Water Outside Your Drink Comes From
Pour a cold drink, set the glass down and watch droplets collect on the outside. It looks as if the glass is leaking or sweating. In an intact glass, the familiar wet coating is condensation: water vapor from the surrounding air becoming liquid on a cold surface.
The water was already in the air
Air contains water vapor, which is water in its gaseous state. You usually cannot see it. The US Geological Survey describes condensation as the change from gaseous water to liquid water, the opposite of evaporation.
The glass does not need pores that let the drink escape. NOAA's classroom demonstration uses a chilled glass to show that the water on its exterior comes from the atmosphere. It can happen on a cold can as well as a glass.
The cold surface makes the difference
Your drink chills the glass. Air meeting the outside surface cools too. If it cools below its dew point, water vapor can condense there, leaving visible droplets.
Dew point is the temperature at which condensation begins for the air's moisture conditions. This makes the explanation more useful than saying "a glass sweats when it is cold". A surface must be cold enough relative to the surrounding air and its water vapor, not simply cold by your personal judgement.
Why it is more noticeable on humid days
NOAA explains that the amount of water collecting on the glass depends on humidity. More moisture in the surrounding air means more water is available to condense.
That is why the same cold drink can leave an obvious wet patch in one room and much less in another. Surface temperature and atmospheric moisture both matter. A glass that feels cool may not be below the local dew point, so a lack of droplets is not evidence that condensation has stopped being possible.
Why the droplets keep appearing
While the surface remains cold enough and moist air continues reaching it, condensation can continue. The droplets are supplied by the air around the glass, rather than a fixed layer of water stored inside the glass wall.
The visible liquid may gather and run down onto the table. The USGS points to this everyday effect as the reason people use coasters under chilled drinks. If you wipe the exterior dry and it becomes wet again, condensation can explain that return without a leak.
The same process fogs glasses and windows
The USGS gives several familiar examples. Eyeglasses can fog when someone leaves a cold, air-conditioned room for humid outdoor air. Warm, moist indoor air can produce water on a cooler window. Moisture from people's breath can contribute to condensation on car windows.
What these examples share is water vapor meeting conditions that allow it to become liquid. The droplets may be tiny enough to look like fog, or large enough to run down the surface.
Is this how clouds form too?
Condensation is part of cloud formation, but a cloud is not simply a giant cold glass. The USGS explains that rising air can cool and water can condense around small particles such as dust or salt, forming cloud droplets.
The shared idea is the change of state from vapor to liquid. The setting is different: droplets form in the air rather than coating the outside of your drink. Condensation connects an ordinary kitchen observation to the water cycle.
Try a simple observation
Dry the outside of a clean glass, add cold water and ice, and set it on a coaster. NOAA uses a similar experiment to demonstrate condensation. Watch where new droplets appear. Keep electrical equipment away and avoid sudden temperature changes that might damage glass.
This demonstrates condensation, not a guarantee that every wet container is sound. A crack, spill or wet rim can also put water outside a drink. Check those separately if the container seems to be leaking.
Quick answers
Does the water come through the glass? Ordinary condensation comes from water vapor in the outside air. It does not require liquid to pass through intact glass.
Will an empty cold container do it? The relevant conditions are a cold surface and sufficiently moist air, not the presence of a drink itself.
Is visible mist water vapor? Water vapor is gaseous water and is normally invisible. The familiar foggy coating on a cold surface consists of liquid droplets.
Facts checked October 7, 2026.
Sources
- US Geological Survey, "Condensation and the Water Cycle": cold glasses, windows, fog and cloud formation.
- NOAA JetStream, "Sweatin' to the Coldies": cold-glass experiment, dew point and humidity.
- National Geographic Education, "Condensation": dew point and droplets on cold containers.