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Advection Fog
A fog that forms when warm air flows over a cold surface and cools from below until saturation is reached.
Dense Fog Advisory
Issued when fog reduces visibility to 1/8 mile or less over a widespread area. For marine products: An advisory for widespread or localized fog reducing visibilities to regionally or locally defined limitations not to exceed 1 nautical mile.
Evaporation-mixing Fog
Fog that forms when the evaporation of water raises the dew point of the adjacent air.
(abbrev. F) Fog is water droplets suspended in the air at the Earth's surface. Fog is often hazardous when the visibility is reduced to ¼ mile or less.
A rainbow that has a white band that appears in fog, and is fringed with red on the outside and blue on the inside.
Freezing Fog
A fog the droplets of which freeze upon contact with exposed objects and form a coating of rime and/or glaze.
Ground Fog
(abbrev. GF) Fog produced over the land by the cooling of the lower atmosphere as it comes in contact with the ground. Also known as radiation fog, and in parts of California as tule fog.
Ice Fog
(Also called ice-crystal fog, frozen fog, frost fog, frost flakes, air hoar, rime fog, pogonip.) A type of fog, composed of suspended particles of ice; partly ice crystals 20 to 100 micron in diameter, but chiefly (especially when dense) ice particles about 12–20 micron in diameter, formed by direct freezing of supercooled water droplets with little growth directly from the vapor. It occurs at very low temperatures, and usually in clear, calm weather in high latitudes. The sun is usually visible and may cause halo phenomena. Ice fog is rare at temperatures warmer than -30°C, and increases in frequency with decreasing temperature until it is almost always present at air temperatures of -45°C in the vicinity of a source of water vapor. Such sources are the open water of fast-flowing streams or of the sea, herds of animals, volcanoes, and especially products of combustion for heating or propulsion. At temperatures warmer than -30°C, these sources can cause steam fog of liquid water droplets, which may turn into ice fog when cooled (see frost smoke). See ice-crystal haze, arctic mist.
Radiation Fog
A fog that forms when outgoing longwave radiation cools the near-surface air below its dew point temperature.
Rain Induced Fog
When warm rain falls through cooler air, water evaporates from the warm rain. It subsequently condenses in the cool air forming fog. Such fog can be quite dense. It generally will persist as long as the rain continues. Since temperature rises little during the day, there is little diurnal variation in rain induced fog. Improvement in visibility cannot be expected until the rain stops or moves out of the affected area.
Sea Fog
Common advection fog caused by transport of moist air over a cold body of water.
Shallow Fog
Fog in which the visibility at 6 feet above ground level is 5/8ths statute mile or more and the apparent visibility in the fog layer is less than 5/8ths statute mile.
Steam Fog
Fog formed when water vapor is added to air which is much colder than the source of the vapor. It may be formed when very cold air drifts across relatively warm water. At temperatures below about-20°F, ice particles or droxtals may be formed in the air producing a type of ice fog known as frost smoke.
Tule Fog
Radiation fog in the Central Valley of California. It forms during night and morning hours in late fall and winter months following the first significant rainfall. A leading cause of weather related casualties in California.
Upslope Fog
A fog that forms when moist, stable air is carried up a mountain slope.

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     Page last Modified: 25 June, 2009 1:01 PM