Fog

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Fog

Fog is caused by the condensation of water at or near Earth's surface. The atmosphere is obscured—essentially by cloud formation—near the surface and fog conditions are generally characterized as existing when atmospheric visibility is reduced to about one-half mile (approximately 0.8 km).

Causes and types of fog

Fog forms either by air cooling its dew point—resulting in radiation fog, advection fog, or upslope fog—or by evaporation and mixing, when moisture is added to the air by evaporation and then mixes with drier air to form evaporation fog or frontal fog.

Other types of fog include ice fog (a fog of suspended ice crystals, frequently forming in Arctic locations), acid fog (fog forming in polluted air, and turning acidic due to oxides of sulfur or nitrogen ), or smog (fog consisting of water and smoke particles). While any type of fog can be hazardous because of the potential dangers of reduced atmospheric visibility—especially for ground and air transportation—acid fog and smog can pose additional risk to human health, causing eye irritations or respiratory problems.

Radiation fogs

Radiation fog (or ground fog) generally occurs at night, when radiational cooling of Earth's surface cools the shallow moist air layer near the ground to its dew point or below. This causes moisture in the nearby layers of air to condense into fog droplets. Radiation fog usually occurs under calm weather conditions, when no more than light winds mix the air layers near the surface. Strong winds normally mix the lower-level cold air with the higher-level dry air, thus preventing the air at the bottom from becoming saturated enough to create observable fog. The presence of clouds at night can also prevent fog formation of this type, because they reduce radiational cooling. Radiation fog often forms in late fall and winter nights, especially in lower areas, because cold and heavy air moves downhill to gather in valleys or other relatively low-lying areas. Accordingly, radiation fog is also called valley fog. In the morning, radiation fog usually dissipates or "burns off" when the Sun's heat warms the ground and air above the dew point temperature (i.e., the temperature at which moisture in the air condenses).

Advection fogs

Advection fog forms when warm, moist air moves horizontally over a relatively cooler surface. During such contact, the layer of air near the surface may cool to below its dew point to form advection fog. Because advection fog can form at any time, it can be very persistent. It is common along coastlines where moist air moves from over a water surface to a cooler coastal land mass. Advectional fog can also occur if an already cool air mass moves over a still colder surface (e.g. snow), so that even the reduced levels of moisture in the cold air can condense into fog as the surface continues to cool the air mass. Advection-radiation fog forms when warm, moist air moves over a cold surface that is cold as a result of radiation cooling. When warm, humid air moves over cold water, a sea fog may form.

Upslope fog forms in higher areas, where a moist air mass is forced to move up along a mountain incline. As the air mass moves up the slope, it is cooled below the dew point to produce fog. Upslope fog formation generally requires a stronger wind along with warm and humid conditions at the surface. Unlike radiation fog, this type of fog dissipates as wind dissipates, and it can form more easily under cloudy conditions. Upslope fog is usually dense, and often extends to high altitudes.

Evaporation fogs

Evaporation fog forms as a result of the mixing of two unsaturated air masses. Steam fog is a type of evaporation fog that appears when cold, dry air moves over warm water or warm, moist land. When some of the water evaporates into low air layers, and the warm water warms the air, the air rises, mixes with colder air, cools, and water vapor condensation occurs to form a fog. Over oceans, this form of evaporation fog is referred to as sea smoke. Examples of cold air over warm water occur over swimming pools or hot tubs, where steam fog easily forms. It is also common in the autumn, when winds and air fronts turn cool as water bodies remain warm.

Precipitation fog

Precipitation fog is a type of evaporation fog that happens when relatively warm rain or snow falls through cool, almost saturated air, and evaporation from the precipitation saturates the cool air. It can turn dense, persist for a long time, and may extend over large areas. Although usually associated with warm fronts, precipitation fog also occurs with slow moving fronts or stationary fronts to form frontal fogs.