Ask A Scientist© Weather Archive

name         Steven
status       other
age          50s
Question -   What is the relationship between air tempature and dew
point in cold weather <32 degrees?  The meteorologist on the news stated
that the tempature can never be less then the dew point.  How is this
possible at tempatures below zero?  Thank you

The dew point is the temperature at which water will condense out of the
air.  If the dew point were above the temperature, that would mean that some
of the water vapor in the air should have already condensed out.  In such a
situation, the excess water WOULD condense out, lowering the amount of water
vapor and consequently the dew point.

As far as dew points are concerned, there is nothing special about the
temperature being below freezong or below zero.  All that those temperatures
would mean is that water will condense out of the air as solid ice (frost)
rather than as liquid water (dew).

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
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Steven -

The dew point is the temperature to which the air must be cooled before it
becomes saturated. As air cools its ability to hold water diminishes. It
does not matter if the temperature is 80 degrees or 20 degrees, if you cool
it, it can hold less water. When it reaches saturation, water will
precipitate out.... rain, snow, dew, frost....

In reality the temperature need only approach the dew point (within about 3
degree F) before we begin - because of the mixing of the air - to see
precipitation.

Larry Krengel
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