The most basic tool for forecasting the minimum temperature is the dew
point. As discussed in the previous section, once the temperature reaches
the dew point temperature, it will fall more slowly than before. Generally,
the minimum temperature will be just a few degrees below the initial dew
point temperature. Of course, if the temperature never drops down to the
dew point temperature, the dew point won't have anything to do with the
Here's the forecasting technique:
- Compare the expected minimum temperature with
the expected overnight dew point temperature. If
they are within a couple of degrees of each other,
assume that the dew point is "controlling" the
minimum temperature. If you don't already have
an expected value for tonight's minimum temperature,
look at this morning's observations to see if the
temperature and dew point were close to each other.
On the other hand, if the forecasted or actual
minimum temperature is several degrees warmer than
the dew point temperature, the dew point probably
doesn't have much to do with determining the
minimum temperature and this technique won't work.
- Make a dew point forecast. Consider the
current dew point, as well as the dew points in
areas upstream of the forecast station. Allow for
the dew point to drop a few degrees if you expect fog or dew
to form. The behavior of the dew point on previous nights can
be useful as a guide of what to expect.
- Make a minimum temperature forecast, by assuming that the minimum
temperature will be within a couple of degrees of your forecasted
- If you have access to a numerical model's forecasts, and you expect
the model's forecast of dew point temperature to be wrong, you should
also expect the model's forecast of minimum temperature to be wrong by
a similar amount.
In Savannah, Georgia, on Tuesday,
Sept. 17, 1996, the wind was expected to shift to the
northwest overnight. While the dew point at Savannah
was 21C, upstream dew points were lower: 19 at
Atlanta (ATL), and 17 at Augusta (AGS). On the
previous two nights, the temperature had fallen to
within a degree or two of the dew point and stayed
there, clear skies were expected, and it was likely that
the dew point would control the minimum temperature
again that night. (Click here for MAP of Georgia.)
What is your forecast?
Questions or Comments
Technical: E-mail John Fulton < email@example.com >
Scientific: E-mail Dr. John Nielsen-Gammon. < firstname.lastname@example.org >
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Texas A&M Atmospheric Sciences Department and
Dr. John Nielsen-Gammon.
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