SpruceRoots Magazine - July 2001

Fog is a collection of fine droplets or tiny ice crystals accompanied by often-microscopic saturated hygroscopic particles. In fact, it is identical to a cloud with its base resting on the ground. By convention, horizontal visibility is limited to less than 1 km (5/8 mile) in fog. Otherwise it is referred to as haze or mist. Fog forms in almost exactly the same way as clouds, as air cooled to supersaturation collects around a sufficient quantity of condensation nuclei. Water vapour is added by evaporation from the ground or sea, or by precipitation.



Radiation fog forms overnight or early in the morning, when the air holds enough moisture to allow condensation as a result of radiation cooling. The cooling necessary to reach the dew point must persist or be strengthened by a clear sky or a cloudy sky in combination with light winds. Topography has an important influence on the formation of radiation fog. It frequently flows into valleys or low-lying areas, and is rarer on plateaus, hills or mountainsides. It almost never occurs over large bodies of water.

When a moist warm air mass is pushed over a relatively cold surface, the air is cooled by contact with the surface and its temperature falls to that of the surface. If the air temperature reaches the dew point as it falls, the air will become saturated and fog will form. In some cases, the air temperature and the dew point will fall still further under the influence of the cold surface, producing advection fog. Advection fog may form over water when moist air moves from a warm surface to an area where the water is colder. In this case, the fog will spread out and persist until the wind changes direction.

This type of fog is formed by air cooling owing to expansion as it moves up a slope. Upslope fog often forms with moderate winds. Strong winds will create stratus or stratocumulus cloud (owing to mechanical turbulence), rather than fog.
Near a mountain, an observer in a valley might report cloud, while another observer near the mountain top would consider it fog.

Fronts are often preceded by fog. Prefrontal fog is associated with warm fronts and is caused by the saturation of cold air as rain falling from the warm air evaporates. The fog that often follows warm fronts is due to advection cooling as the warmer air moves over a colder surface.

SpruceRoots Magazine - July 2001