Altitude, Frost, and Dew
How does altitude affect the frost point and dew
point? Would a town at 1300' ASL be more likely to get a frost
than a neighboring town (7 miles away)at 1100' ASL?
Frost and dew formation depends on the temperature
and the absolute humidity of the air, and the temperature
of whatever dew or frost may form on.
Sometimes grass, branches, vehicles, etc. will loose
energy and decrease in temperature more than the air itself,
allowing dew or frost to form on these exposed surfaces even
though the air temperature itself does not decrease to the
frost point or dew point.
Since temperature generally decreases with height (unless
there is a nighttime temperature inversion on a clear night),
it is more likely that the air temperature will be lower
at 1300 feet than at 1100 feet. Dew or frost would then
be more likely to form at 1300 feet than at 1100 feet,
assuming that the absolute humidity at the two towns is the
However, the absolute humidity can also change with
altitude, as well as from one place to another depending
on soil moisture, whether a water body is nearby, etc.
So saturation of the air (dew point, frost point) may
occur at the 1100 foot high town and not at the 1300 foot
town in some cases. There are just too many variables
to make a hard and fast rule on this.
David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory
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Update: June 2012