When the air temperature is below about 04C, any water vapor in the air condensing into droplets is quickly converted into ice crystals. A suspension of ice crystals in the air at the surface of Earth is called ice fog. Ice fogs occur mostly in the Arctic regions and are mainly artificial fogs produced by human activities, occurring locally over settlements and airfields where hydrocarbon fuels are burned. (Burning 1 pound of hydro-carbon fuel produces 1.4 pounds of water.)

When the air temperature is approximately 01C or lower, ice fogs form very rapidly in the exhaust gases of aircraft, automobiles, or other types of combustion engines. When there is little or no wind, an aircraft can generate enough ice fog during landing or takeoff to cover the runway and a portion of the airfield. Depending on atmospheric conditions, ice fogs may last from a few minutes to several days.

A fine mist of ice crystals persists as haze over wide expanses of the Arctic basin during winter. The haze may extend upward through much of the tropospherea sort of cirrus cloud reaching down to the ground.

Frontal fogs are of three types: prefrontal (warm front), postfrontal (cold front), and frontal passage.

PREFRONTAL FOGS. Prefrontal (warm-front) fogs occur in stable continental polar (cP) air when precipitating warm air overrides it, the rain raising the dew point sufficiently for fog formation. Generally, the wind speeds are slight, and the area most conducive to the formation of this type of fog is one between a nearby secondary low and a low-pressure center. In the entire world, the northeastern part of the United States is probably the most prevalent region of this type of fog. Prefrontal fogs are also of importance along the Gulf Coast, the Atlantic coastal plain, in the Midwest, and in the valleys of the Appalachians.

POSTFRONTAL FOGS. As is the case with prefrontal fogs, postfrontal (cold-front) fogs are caused by falling precipitation. Fogs of this nature are widespread only when a cold front of an east-west orientation has become quasi-stationary and cP air is stable. This type of fog occurs frequently in the Midwest. Fog or strati form clouds are prevalent for a considerable distance behind cold fronts associated with stable cP air masses in that region if the cold fronts have produced general precipitation.

FRONTAL PASSAGE FOGS. During the passage of a front, fog may form temporarily if the winds accompanying the front are very light and the two air masses are near saturation. Also, temporary fog may form if the air is suddenly cooled over moist ground with the passage of a precipitating cold front. In low latitudes, fog may form in summer if the surface is cooled sufficiently by evaporation of rainwater that fell during frontal passage provided that the moisture addition to the air and the cooling are great enough to cause fog formation.

Learning Objective: Define density altitude, pressure altitude, vapor pressure, and specific humidity and recognize their importance in air operations.

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