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Name: sport fisherman
Status: other
Age: old
Location: N/A
Country: N/A
Date: Sunday, December 01, 2002

What causes sea fog (advection fog) to form? Is there a "rule of thumb" regarding the relationships between current air temperature and the current dew point, which would be a predictor that fog is likely to form? e.g. when they are 6 degrees or less apart?

I am a sport fisherman who regularly goes from 28 to 54 nautical miles to the Gulf of Mexico offshore of Sarasota. Thank you!

Fog forms when the air becomes "saturated", which over water, especially the ocean, can occur even if the air is not at 100% relative humidity. Salt from the ocean serves very effectively as the hygroscopic nucleus of fog droplets; this produces a solution of water and salt that has a reduced droplet surface tension allowing fog to form when the relative humidity is well below 100%, perhaps even as low as 97%.

To get high relative humidities like that, the air temperature must be sufficiently low. The required cool air over the water may be flowing from the land (cold high pressure system invading the southern United States) or may be produced by radiational cooling on a clear, still night.

Using these criteria as a guide, the air temperature would need to be low enough to produce a fairly high relative humidity and the relative humidity would need to be around 97% or higher. The difference between air temperature and dewpoint that would produce the required high relative humidity would not always be the same, it varies with air temperature, so using 6 degrees would not work (this will become obvious below).

So, you would need to know the air temperature and the dew point (available from the National Weather Service maps on the Internet). Then using a table of dew point depression (air temperature minus dew point versus air temperature), or more conveniently a relative humidity calculator, which you can find at on the Internet, you can determine the temperature and dew point that will give you a high relative humidity. To use the calculator, put in an air temperature (dry bulb) and then a wet bulb temperature that is just slightly lower. Then adjust the wet bulb temperature to get the dew point temperature that you need. You will find that the dew point temperature generally needs to be less than a degree Fahrenheit different from the air temperature to get 97% or greater relative humidity (and thus fog).

For instance, I used the calculator to find the following for three temperatures; 98, 68, and 38 degrees:
Dry Bulb  98.0   68.0   38.0
Wet Bulb  97.18  67.41  37.66
RH        97.0   97.0   97.0
Dew Point 97.0   67.1   37.2

Using this calculator you can determine the relative humidity, and thus the likelihood of fog.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory

Dear "sport fisherman.."

There are several factors affecting the formation of sea fog... The temperature-dew point spread is one of them, but wind speed and direction, and cloud cover are also considerations... Usually it is a combination of those parameters that is a determinant whether the fog will form or not.

Here are a couple of links that describe advection fog in more detail. You might also do a search on sea fog in There are many references to that subject accessible through the Internet.

Wendell Bechtold, meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO

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