Dew Point and Minimum Temperature

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 nighttime temperature.

Here's the forecasting technique:

  1. 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.

  2. 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.

  3. Make a minimum temperature forecast, by assuming that the minimum temperature will be within a couple of degrees of your forecasted dew point.

  4. 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.

An example

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?

  1. The temperature will not fall below the current dew point. I forecast a minimum temperature of 21 C.
  2. The dew point will drop to around 18 C as air from central Georgia reaches Savannah. The temperature should fall to within a degree of the dew point again, so I forecast a minimum temperature of 19 C.
  3. Because it's clear and calm, there's probably more chance of dew forming than before. I'll forecast that the temperature will drop down to 17 C, with a little bit of dew on the ground.
  4. The dew point will drop to around 18 C. Then, as the night continues, dew and fog will form. The temperature and the dew point should both drop to around 15 C.

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Questions or Comments

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