| MadSci Network: Physics |
Dew forms when the air in contact with a surface cools to the point where the air can no longer "hold" the moisture already present in the air. If the air temperature is below freezing, frost forms instead. If the relative humidity reaches 100 percent, the air is saturated and water vapor can condense on any "nucleus" present. Examples include blades of grass and car windows.
If the air does not reach 100 percent relative humidity, it is still possible for dew to form on various surfaces if those surfaces are cooler than the air temperature. A car window is just such an example as it is an efficient radiator of heat. The air in immediate contact with the cooler window becomes saturated, leaving some of its moisture on the surface of the window. This normally only works with the air is calm (or nearly calm) but there are exceptions.
For more information about air's ability to hold moisture, see the
following link:
http://www.madsci.or
g/posts/940285087.Es.r.html
and for
more information about the formation of condensation (which dew is a type
of), see:
http:
//www.madsci.org/posts/archives/may97/864085719.Es.r.html
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