Air losing its grip on water
Why Do Windows Fog Up?
Warm air holds invisible water, and a cold window makes it condense into visible drops.
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Step 1 of 5
Air carries invisible water.
Water vapour is a gas — individual molecules, far too small to see. Ordinary room air holds a surprising amount of it.
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Step 2 of 5
Warm air holds much more than cold.
Capacity rises steeply with temperature. Air at 25°C can hold roughly twice the water of air at 15°C.
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Step 3 of 5
Cold glass cools the air touching it.
The thin layer of air against the window drops to near the glass temperature, and its capacity falls with it.
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Step 4 of 5
Excess water has to leave the air.
Once the air holds more than it can, the surplus condenses onto the coldest surface available — the glass.
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Step 5 of 5
Which is why bathrooms fog worst.
A hot shower loads the air with vapour while the window stays cold. Ventilation removes the moist air and fixes it.
Double glazing works because the inner pane stays near room temperature, so the air beside it never cools enough.
The short version
Windows fog because chilled air cannot hold as much water vapour, so the surplus condenses onto the cold glass.
Try it yourself
Breathe on a cold window, then on a warm one. Only the cold one fogs — the glass temperature is what matters.
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