Types of Fog

All fog looks the same, but the mechanisms behind fog formation are not always the same. Common forms of fog besides those associated with frontal systems are advection fog, radiation fog, upslope fog, and evaporation fog.

Advection fog, which makes up the majority of the fog seen on the Cape, occurs when warm air is transported horizontally over a cold surface. This causes the warm air in contact with the cold surface to cool, and if the dew point is reached, fog will form. Fog seen off the coast of the Cape is an example of Advection fog with warm air moving northward over the cooler ocean water.

Radiation fog forms at night in calm air under clear skies. Under clear skies, the ground is able to cool substantially because there are no clouds to reemit energy back toward the surface. This cooling causes the layers of air near the surface to cool, and if the dew point is reached, fog will form. This type of fog "burns off" in the early morning when solar radiation warms the cool air.

Upslope fog occurs when light winds blow moist air up a hillside or mountainside. When the air becomes saturated due to the drop in temperature, fog forms. Upslope fog often forms in the Rocky Mountains in the winter months when cool air behind a cold front drifts westward.

Evaporation or mixing fog occurs when water vapor is added to the air by evaporation, and this moist air mixes with drier and cooler air. Two common types are steam fog and frontal fog. In steam fog, cold air mixes with warm moist air over a water surface. When the moist air cools and reaches its saturation point, fog forms in wisps across the surface.

In frontal fog, raindrops evaporate into a layer of cool air near the surface. Once the saturation point is reached, fog forms.