Temperature Effecting Dew Point
Date: Friday, November 01, 2002
How does temperature effect the dew point?
Temperature does not affect dew point. Dew Point is the temperature to
which the air must be cooled before it becomes saturated. When it is
saturated it is holding all the water vapor it is able to hold.
As you may know, warm air can hold more water than cold air. Or said
another way, as air cools, it loses its ability to hold water. To what
temperature would we have to cool the air in your back yard to have it
holding all the water it can hold? That's the dew point.
If air continues to cool, the water vapor in the air has to condense (change
into a liquid) or sublime (change into a solid). When it condenses it may
form dew, clouds, fog, rain... when it sublimes it forms frost, snow...
Even though we use the same degrees to describe temperature and dew point,
they are really different variables used to describe the atmosphere.
The dew point (the temperature at which moisture will condense out of the
air) correlates with absolute humidity, and is not directly related to
temperature. The only real impact temperature can have on the dew point is
that the dew point cannot really be above the air temperature.
Richard E. Barrans Jr., Ph.D.
Director of Academic Programs
PG Research Foundation, Darien, Illinois
The temperature does not affect the dew point. The dew point temperature
reflects the absolute amount of water vapor in the air. It is the
temperature to which you must cool the air at constant pressure and
constant water vapor content for saturation (relative humidity = 100%)
David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
The dew point and temperature are pretty much independent of each other,
except for this restriction...the dew point cannot be higher than the
temperature. It can be the same, but not higher. The dew point is the
temperature to which the air must be COOLED, in order to become saturated,
or, 100 percent relative humidity.
The dew point is an indication of how much water vapor is in the air. The
more water vapor, the closer the dew point is to the temperature. When the
air becomes saturated, the dew point and the temperature are the same.
So it is really the HUMIDITY that affects the dew point, rather than the
Wendell Bechtold, meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO
The dew point is a way of describing how much water vapor is present in
air at a given temperature, so it is not so much that temperature affects
the dew point, but the amount of water vapor in the air determines the dew
point. The dew point is defined as the temperature where the amount of water
vapor present in the air, expressed as a partial pressure is equal to the
vapor pressure of water at that temperature. It can be related directly to
the relative humidity, which measures the same thing but is expressed as the
percentage of water vapor present in the air at the given temperature
compared to the vapor pressure of water at that temperature.
For example, if today we read that the dew point of the air is 44 degrees F.
and the present temperature is say, 75 degrees F. It means that if the
temperature were to fall to 44 degrees F. tonight then water would condense
(forming dew). If the temperature were to only fall to say 50 degrees F. no
dew would form.
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Update: June 2012