The temperature where air gives up
What Is the Dew Point?
Cool any air far enough and its invisible water has to become visible.
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Warm air holds more water than cold air.
The limit rises steeply with temperature. Air at twenty degrees can hold roughly twice the vapour that air at ten degrees can.
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Cooling shrinks the limit, not the water.
The vapour in the air does not go anywhere as things cool. The container it is allowed to sit in simply gets smaller.
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At one temperature the two meet.
That crossing point is the dew point. Below it the air is over-full and water must condense onto the nearest cold surface.
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Which is why cold things get wet.
A glass from the fridge chills the air touching it below the dew point, and that air dumps its water onto the glass. Nothing leaks out of the drink.
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Step 5 of 5
It tells you far more than humidity does.
The dew point is an actual amount of water. A dew point above about eighteen degrees is what people mean by muggy, whatever the humidity percentage says.
A cloud forms where rising air cools to its dew point, which is why cloud bases over an area are all at the same height.
The short version
The dew point is the temperature at which air becomes full of water, so cooling anything below it makes water appear.
Try it yourself
Put a metal spoon in the freezer for ten minutes. When you take it out, watch how fast it fogs — that is your room's dew point.
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