CONDITIONS FAVORABLE FOR UPSLOPE FOG AND STRATUS

Upslope fog and stratus occur in those regions in which the land slopes gradually upward, and those areas accessible to humid, stable air masses. In North America, the areas best meeting these conditions are the Great Plains of the United States and Canada and the Piedmont region east of the Appalachians. 

The synoptic conditions necessary for formation of this type of fog or stratus are the presence of humid air and a wind with an upslope component. The stratus is not advected over the station as a solid sheet. It forms gradually overhead. The length of time between the first signs of stratus and a ceiling usually ranges from 1/2 hour to 2 hours; although at times, the stratus may not form a ceiling at all. A useful procedure is to check the hourly observations of surrounding stations, especially those southeastward. If one of these stations starts reporting stratus, the chances of stratus formation at your station are high.

CONDITIONS FAVORABLE FOR FRONTAL FOG

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

Prefrontal (warm front) fogs occur in stable continental polar (cP) air masses when precipitating warm air overrides the colder air. The rain raises the dewpoint in the cP air mass sufficiently for fog formation. Generally, the wind speeds are light, and the area most conducive to the formation of this type of fog is one between a nearby secondary low and the primary low-pressure center. The northeastern area of the United States is probably the most prevalent region for this type of fog. Prefrontal fog is also of importance along the Gulf and Atlantic coastal plains, the Midwest, and in the valleys of the Appalachians. A rule of thumb for forecasting ceiling during prefrontal fog is as follows: If the gradient winds are greater than 25 knots, the ceiling will usually remain 300 feet or higher during the night.

As with the prefrontal fog, postfrontal (cold front) fogs are caused by falling precipitation. Fogs of this type are common when cold fronts with east-west orientations have become quasi-stationary and the continental polar air behind the front is stable. This type of fog is common in the Midwest. Fog, or stratiform clouds, may be prevalent for considerable distances behind cold fronts if the cold fronts produce precipitation.

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 the summer if the surface is cooled sufficiently by evaporation of rain that fell during a frontal passage, provided that the moisture addition to the air and the cooling are great enough to allow for fog formation.

Sea fogs are advection fogs that form in warm moist air cooled to saturation as it moves over colder water. The colder water may occur as a well-defined current, or as gradual latitudinal cooling. The dewpoint and the temperature undergo a gradual change as the air mass moves over colder and colder water. The surface air temperature falls steadily, and tends to approach the water temperature. The dewpoint also tends to approach the water temperature, but at a slower rate. If the dewpoint of the air mass is initially higher than the coldest water to be crossed, and if the cooling process continues sufficiently long, the temperature of the air ultimately falls to the dewpoint, and fog results. However, if the initial dewpoint is less than the coldest water temperature, the formation of fog is unlikely. Generally, in northward moving air masses or in air masses that have previously traversed a warm ocean current, the dewpoint of the air is initially higher than the cold water temperature to the north, and fog will form, provided sufficient fetch occurs.

The rate of temperature decrease is largely dependent on the speed at which the air mass moves across the sea surface, which, in turn, is dependent both on the spacing of the isotherms and the velocity of the air normal to them.

The dissipation of sea fog requires a change in air mass (a cold front). A movement of sea fog to a warmer land area leads to rapid dissipation. Upon heating, the fog first lifts, forming a stratus deck; then, with further heating, this cloud deck breaks up into a stratocumulus layer, and eventually into convective type clouds or evaporates entirely. An increase in wind velocity can lift sea fog, forming a stratus deck, especially if the air/sea temperature differential is small. Over very cold water, dense sea fog may persist even with high winds.

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