Air letting go of water
Why Is the Grass Wet in the Morning?
Cold surfaces force the air touching them to release water it can no longer hold.
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Step 1 of 5
Air always carries invisible water.
Even on a clear dry day there is water vapour mixed through the air, spread out as separate molecules far too small to see.
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Step 2 of 5
Warm air can hold much more than cold air.
How much vapour air can carry depends strongly on temperature, and the capacity falls quickly as the air cools down.
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Step 3 of 5
At night, surfaces cool faster than air.
Grass and car roofs radiate their heat away to the clear sky and end up colder than the air around them, sometimes by several degrees.
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Step 4 of 5
Air touching them is forced to let go.
Chilled below the point where it can hold its vapour, the air must release the excess, which settles on the cold surface as dew.
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Step 5 of 5
Below freezing you get frost instead.
If the surface is below zero the vapour turns straight to ice crystals without ever being liquid, which is why frost looks feathery.
Cloudy nights rarely produce dew, because clouds reflect heat back down and surfaces never get cold enough.
The short version
Dew and frost appear because surfaces cool below the temperature at which the air beside them can hold its water, forcing it to be released.
Try it yourself
Take a cold drink outside on a warm day. The outside of the glass wets itself from the air, exactly as grass does at night.
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