Fog

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If the atmospheric visibility near the earth's surface is reduced to 0.62 mi (1 km) or less due to floating water droplets in the air, it is called fog. Fog can form in two ways: either by cooling the air to its dew point (e.g., radiation fog, advection fog, upslope fog), or by evaporation and mixing, when moisture is added to the air by evaporation, and then it is mixed with drier air (e.g., evaporation fog, 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 its effects on atmospheric visibility for ground and air transportation, acid fog and smog can pose additional risk to human health, causing eye irritations or respiratory problems.

Radiation fog (or ground fog) occurs at night, when radiational cooling of the earth's surface cools the shallow moist air layer near the ground to its dew point or below, so the moisture in the air condenses into fog droplets. It occurs under calm weather conditions, when light wind, or no wind at all is present, since a strong wind would 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 fog. The presence of clouds at night can also prevent fog formation of this type, because they trap the earth's heat, not allowing the cooling of the air for condensation. Radiation fog often forms in late fall and winter nights, especially in lower areas, because cold and heavy air moves downhill, and gathers in valleys. Accordingly, radiation fog is also called valley fog. In the morning it usually dissipates or "burns off" when the Sun's heat warms the ground and air.

Advection fog forms when warm, moist air horizontally moves (which is called advection) over a cold surface, which cools the air to its dew point. Advection fog can form any time, and can be very persistent. It is common along coastlines where moist air moves from over the water to over the land, or when an air mass moves over a cold surface (e.g., snow), and the moisture in the air condenses into fog as the surface cools it. Advectionradiation fog forms when warm, moist air moves over a cold surface, which is cold as a result of radiation cooling. When warm, humid air moves over cold water, it is called sea fog.

Upslope fog forms in higher areas, where a moist air mass is forced to move up along a mountain. While the air mass is moving up the slope, it is cooled beyond its dew point and produces fog. It requires a fast wind, and warm and humid conditions at the surface. Unlike radiation fog, this type of fog dissipates when no more wind is available, and it can also form under cloudy skies. Upslope fog is usually dense, and extends to high altitudes.

Evaporation fog forms by the mixing of two unsaturated air masses. Steam fog is a type of evaporation fog, which 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 condenses some of its water vapor. Over oceans, it 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 common, especially in the fall season, when winds are getting colder but the water is only slowly turning colder.

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 it is mostly associated with warm fronts, it can occur with slow cold fronts or stationary fronts as well, hence the name frontal fog is also used.

Hydrologic Cycle