7.4.1: Fog
Fog is a cloud that forms near the ground. Fog, like any other product of condensation, requires the air temperature to decrease to the dew point temperature where upon the air is at saturation. Cooling of the near surface air is accomplished either through contact cooling (a diabatic process) or adiabatic cooling.
An advection fog forms when warm and moist air travels over a cool surface. Warm air overlying a cool surface creates a temperature gradient directed toward the surface. Sensible heat is transferred out of the air toward the ground thus cooling the air above the surface. If the air cools to the dew point temperature, condensation will likely result. Advection fogs are quite common. In the Midwest United States during the spring, warm, moist air from the Gulf of Mexico (mT air) streams over the cooler, often snow covered surface. As it does it cools and the water vapor condenses into a fog. San Francisco, California is noted for its fogs as mT air masses from the Pacific travel over the cold California Current as they move toward the coast.
Radiation fog forms during the evening under cloudless skies and with little to no wind. Under these conditions, terrestrial longwave radiation is readily emitted to space without absorption by clouds. The loss of longwave radiation causes the surface temperature to decrease inducing a negative sensible heat transfer between the cooling surface and the slightly warmer air in contact with the surface. As the near surface air cools to the dew point the fog forms.
A radiation fog is shown in Figure \(\PageIndex{2}\) above. To the left of the picture is a grass field and to the right is an asphalt parking lot. As the sun heats the surface during the early morning it "burns off" the fog. Because the grass field has a higher albedo and cooler surface, the fog is closer to the surface than over the warmer parking lot.
Steam fog occurs when cool dry air settles over a warm, moist surface. Such is the case shown in Figure \(\PageIndex{3}\) where a steam fog is forming over a lake. When the drier air lies above the moist surface a moisture gradient enables water to evaporate and humidify the air. Because the air's saturation point is low due to the cool temperature, the water vapor condenses and a steam fog forms.
An upslope fog forms when moist air if forced up a slope. These certainly occur as the air encounters hilly terrain and is forced to rise, or if moist air travels up a very long slope. Such might be the situation when air moves out of the Gulf of Mexico traveling west up the Great Plains toward the east slope of the Rocky Mountains. As the air rises it expands and adiabatically cools. Once the air temperature reaches the dew point temperature the air becomes saturated, and condensation occurs to form the fog.
Frontal fogs are associated with weather fronts, especially a warm front. Warm, moist air rises up and over cooler, drier air at the surface along a warm front. As precipitation falls from the warmer air into the drier air some of the water evaporates and humidifies the cooler air. As the humidification process brings the cooler air to its saturation point a fog forms.
Geographical Patterns of Fog
There is a definite geographical pattern to the formation of fogs. Figure \(\PageIndex{4}\) shows the spatial pattern of fogs over the United States. First note that radiation fog is pretty common over much of the country as no special landscape features like mountains or ocean currents are needed to create it. Advection fog is common along coasts where air originating over warm water travels over cooler water or land surfaces to induce condensation and cloud formation near the ground. Upslope fog is common through the western Great Plains and eastern slope of the Rocky Mountains as warm and moist air, often originating in the Gulf of Mexico, works its way toward higher elevation. The gentle uplift provided induces adiabatic cooling and saturation of the air with subsequent condensation and fog formation.