Fog is really nothing more than a cloud on the ground or sea surface. The most common types of fog form when humid air is cooled to the dew point (saturation), which causes water vapor to condense into water droplets. Sometimes fog occurs when extra water vapor evaporates into the air, which raises the dewpoint enough to cause saturation. That said, fog develops in a range of ways and each type of fog is named according to how it occurs.
Sea fog is a type of advection fog that forms when air over warm water is transported by the wind over colder water. This air, already humid, is cooled to its saturation point in the layer closest to the sea surface and fog develops. The places best known for this type of fog are those with cold currents close to the shore. The Pacific coast, particularly in California and the African west coast around Namibia are two famous places for this type of advection fog.
The fogs of San Francisco are famous and life only exists in the Namib Desert (on the coast of Namibia) because of the dense coastal fogs that advect ashore. What do these places have in common? They both have cold currents that flow along the coast and warm, moist air is advected ashore over the currents.Another place famed for its dense sea fog is the Labrador Sea. Here, air warmed and moistened by the warm Gulf Stream is advected over the cold Labrador Sea and the fog that forms as a consequence can be extremely dense and persistent. It is at its worst in winter in the area where the warm Gulf Stream encounters the cold Labrador Current, especially when a warm front moves over the area.
Many ships have been lost to collisions or foundering on rocks in dense fog. Similar conditions occur around the Aleutian Islands of Alaska. In both places, extremely dense fog can be advected along on very strong winds. The effect is like standing in the middle of a fast-moving cotton ball.
You do not have to have strong winds or big bodies of water for advection fog to occur. Steam fog is commonly seen over lakes and ponds in fall and early winter as cold air is advected over warmer water where it gains enough water vapor to form wisps of fog (steam). This does not have to be a big area either. Take a cup of coffee outside or boil a pot of water on the stove and you can see the same effect in action.
Upslope fog operates a little differently. It occurs when the wind blows humid air up rising elevation. Air cools as it rises and its capacity for holding water vapor in suspension is lower and lower. Fog forms when the air saturates. This occurs at the lifting condensation level, so where fog begins to form can be predicted if you know the temperature and dewpoint of the air at the foot of the rising terrain. Remember, what is fog in higher terrain looks like clouds that cloak the slopes from below.
Radiation fog can not survive strong winds, stronger winds mix the air too well for saturation to occur, but it does need light winds to develop. Radiation fog develops in low-lying areas or valleys under radiation inversions. Radiation inversions develop overnight as the air layer close to the surface cools faster than the layer above it. This makes for a very stable condition and the air below the inversion gradually cools to its saturation point.
Once that happens, fog develops. How dense the fog gets depends on several variables, among them the height of the inversion, the strength of the wind, and the amount of moisture available. Radiation fogs can be so shallow they fill only dips in the road or other low spots. They can also be thick enough to fill an entire river valley. Radiation fogs dissipate from the outer edges inward because that is how the inversion that allows it to develop breaks down too.
One last mechanism: precipitation. Fog forms when the air becomes saturated. Precipitation fog, also called frontal fog, develops from the top down rather than from the bottom up. Precipitation falls and some of it evaporates into the air below the clouds. Gradually, the air just below the cloud layer becomes saturated and new cloud forms. As precipitation continues, cloud keeps forming lower and lower until it finally hits the surface.
Precipitation fogs are generally divided into three types. Prefrontal fog, post-cold frontal fog, and frontal passage fog. The first two result from precipitation falling into cold, stable air and raising the dewpoint to saturation. Frontal passage fog can result from the mixing of cold and warm air masses in the frontal zone itself or by a sudden cooling of air over wet ground.
By now you probably see a trend here. Fog forms when a air layer above the surface reaches its saturation point. How that occurs varies, but the result is always the same. If you take the time to understand the mechanisms at work where you are, you can forecast fog much more effectively. Your customers will thank you for it!
Written by Melody Higdon, 14 WS/DOPA