FOG - what is fog?

Fog is a cloud at the Earth’s surface formed by the condensation of water vapor from the atmosphere. The fog forms as atmospheric moisture increases and condensation occurs on numerous nuclei, reducing visibility to less than 0.62 miles (1 km). Fog that forms in dirty city air tends to be thicker than fog that forms at the same atmospheric moisture count over oceans. This is because city air has abundant nuclei that grow more but smaller water droplets than the air over oceans, which typically has fewer nuclei and produces fewer but larger water droplets. Fog that forms in polluted air may be acidic and harmful to human health, particularly if the water droplets combine with sulfur and nitrogen oxides. One of two main mechanisms is usually responsible for the formation of fog. Fog may form by condensation when the air is cooled past its saturation point (dew point), or it may form by continuous evaporation and mixing of vapor into the air.


Radiational cooling of the air near the ground can lower the temperature of the surface layer below the dew point, forming radiation or ground fog. This type of fog forms best on clear nights when a layer of moist air near the surface is overlain by a layer of dry air, forming an atmospheric inversion. Radiation fog forms commonly in the late fall and winter, when nights are longest and cooling of the surface layer lasts the longest. Low winds also help the formation of radiation fog, as the wind promotes interaction of the surface air with the rapidly cooling ground, promoting faster cooling, but strong winds would mix the moist surface layer with dry air aloft, preventing fog from forming. Since the fog is heavy, it typically collects in valleys and low-lying areas. During the day the fog dissipates (it does not burn off) when the ground and low level air warms, causing the water droplets to evaporate.

Advection fog is formed when warm moist air moves over a cold surface, causing the air to cool below the dew point, initiating condensation. This type of fog is common along coastlines and is especially common in central California in the summer. Here, warm ocean water moves over cold upwelling ocean water near the coast, produced by westerly winds, causing the surface air to cool below the dew point and forming fog. Production of advection fog is enhanced by winds that produce rolling clouds of fog moving inland during summer months. Advection fog is also common along headlands of other coasts where converging warm air is cooled by the surface and forced to rise by the convergence. Upslope fog forms as moist air flows up a mountain or other slope, cooling below the dew point. This type of fog is common around some mountain ranges including the Rockies and also forms around isolated mountains such as volcanoes where the fog may look like a small eruption in progress. A final type of fog is known as evaporation fog, evaporationmixing fog, or steam fog. This is produced when cold air moves over warm water. This type of fog may commonly be fog seen forming above warm lakes, rivers, and other water bodies on cold autumn mornings. Steam fog may form over a warm wet surface on a warm summer day, such as after a brief shower drops water on a hot surface. The water quickly evaporates and mixes with the air above, disappearing quickly.