dew on a spider web

How does dew form?

Dew forms when a surface cools through loss of infrared radiation down to a temperature which is colder than the dewpoint of the air next to that surface.

Dew most often forms on on evenings or nights when there are few clouds, since the greenhouse effect from clouds can keep surfaces from cooling by infrared radiation loss to outer space. Calm winds at night also contribute to dew formation because a windy night keeps the lowest layers of the atmosphere warmer, and also helps evaporate any dew that might begin to form on surfaces.

Dew is made of liquid water that has condensed from some of the water vapor contained in the air.

If the layer of air next to the ground also cools to the dewpoint temperature, then fog forms as well.
Interesting facts:
DEW FORMS FIRST ON... One of the most efficient surfaces for the formation of dew is the top of a car. On a cool evening just after the sun sets, you will often see dew form on the top of cars before it forms on grass or other surfaces.
(page last updated 12/13/2019)
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