Sea Fog Formation
Name: sport fisherman
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 22.214.171.124/calc/humidity.html 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
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 www.google.com. There are many
to that subject accessible through the Internet.
Wendell Bechtold, meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO
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Update: June 2012