January 12, 2001

     White Frost

Some January mornings we wake to the beautiful white frost covering every twig, spruce needle and dried meadow grass -- as well as on clotheslines, antennas and other outdoor objects.

Ideal conditions for the formation of dew or white frost, which is deposited on solid surfaces, are a clear sky, little or no wind, a fairly moist atmosphere and relatively long nights.  It is well known that both dew and frost are much more likely to occur on nights that are clear and calm than on those when the sky is overcast and a wind is blowing.

Cloudless skies, or skies with only a few high cirrus clouds, permit a rapid loss of heat from the Earth's surface by radiation.  The adjacent air layer, in turn, becomes chilled.  If the temperature of the lower few inches of surface air is reduced below the dew point -- the temperature at which the air is saturated with water vapor -- condensation takes place.  This may be in the form of dew, if the dew point is above 32 degrees fahrenheit, or white frost if the dew point is below 32 degrees.  Both collect on cold objects close to the ground.