Causes of Fog


What causes fog?  What tells you to include it in your forecast?  Do you use an instrument that indicates when it will appear? I heard somewhere that if the relative humidity and temperature are within 5 degrees/points of each other, you get fog.  Any truth to that folklore? Please explain in laymen's terms.  BTW, you do a great job with your forecasts.


Thank you so much for the forecast compliment.  Now to the fog questions(s).
Please excuse the long-winded answer . . . I am not sure how much to assume you already know, so apologies if much of this is re-hash for you.
Fog is produced when the air becomes saturated (holding all of the water vapor that it can).  Without getting to complicated, just remember that warm air can hold more water vapor than cold air.  Example in rough numbers:  air at about 95°F can hold about twice as much water vapor as air at 75°F, and about four times as much water vapor as air at 55°F.  So cold air at 100% relative humidity is holding much, much less water vapor than warm air at 100% relative humidity -- even though both temperature conditions are at 100% relative humidity.
At the point where the air becomes saturated (holding all the water vapor that it can based on its temperature), then the water vapor (which is invisible) will begin to condense.  By condense, we mean that some of the water vapor is chanced to either liquid water micro-droplets or extremely fine ice crystals (under extremely cold conditions).  Water droplets and ice crystals are (obviously) visible - that's why we can see clouds.
In fact, in general terms, fog formation is exactly the same process involved in the formation of clouds . . . fog is simply a cloud that forms over the ground.
Fog is possible, but not entirely guaranteed, if the relative humidity is 100% (when RH% = 100%, that means that the air is saturated).  When the air is saturated, the air temperature is the same as the dewpoint temperature.  (Both indicate that the air is "saturated.") 
The dewpoint temperature is determined basically by finding out how much you must cool the air to bring it to saturation.  As an example, in very rough numbers, summertime air at 90°F with a relative humidity of 50% has a dewpoint temperature of about 70°F.  Meaning that if you took a sample of that 90° air and cooled it to 70°, water would begin to condense out of it.
Now understand that saturated air is not dripping wet, especially when it is cold.  By the way, another misconception is that when it rains, the air near the ground is at RH%=100% -- not true.  The rain drops formed in a cloud (which had RH%=100%), but the humidity near the ground can be MUCH less than that.
So, fog is the result of a "moist" air mass near the surface, which has reached saturation.
In forecasting fog, two key characteristics that we look is to see if (1) the humidity is expected to approach or reach 100% (which is the same as saying the air temperature is expected to be very near the dewpoint temperature) and (2) the winds are expected to be very light to calm.   An inversion above the fog layer (a layer of warmer air above the fog layer) also helps promote fog, so we can look for the potential for low-level inversions in the forecast products (forecast products are mainly the output from meteorological computer models.)
Actually, when the air is "moist" (air temp is close to dewpoint temp),  clear skies ENHANCE the chances for fog, because clear skies allow the temperatures near the ground to drop more rapidly (cooling the air layer), especially when the winds are light.  Light winds reduce "mixing" of warmer and cooler air.  Ever notice how cloudy nights tend to be a bit warmer than clear nights, especially in the winter?
Now . . . your barroom buddies may have actually said that when the air temp and dewpoint temp are nearly the same, then there is a good chance for fog.  Or they may have said that if the RH% is within a few points of 100%, then fog is likely.
I often get asked, why doesn't RH% have to be exactly 100%?  Well, indeed it does, but RH% chances slightly over very short distances, just like the temperature in your front yard is probably not exactly the same as the temp in your backyard at any given moment.  So, if the RH% at Metro Airport (our standard observation site) is near 100% (say at or above 95%), then there are going to be places around the area that reach 100% -- the result, "patchy fog."
But if the entire area is at or very near RH%=100%, and it stays that way for several hours, widespread DENSE fog may result.
Hope this helps some.  More questions?
Thanks for the note!
Jay Grymes
Chief Meteorologist
WAFB Storm Team