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 Fog 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. Postfrontal Fog 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. Frontal Passage Fog 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. CONDITIONS FAVORABLE FOR SEA FOG 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. CONDITIONS FAVORABLE FOR ICE FOG When the air temperature is below about -25°F, water vapor in the air that condenses into droplets is quickly converted into ice crystals. A suspension of ice crystals based at the surface is called “ice fog.” Ice fog occurs mostly in the Arctic regions, and is mainly an artificial fog produced by human activities, It occurs locally   over   settlements   and   airfields   where hydrocarbon   fuels   are   burned—the   burning   of hydrocarbon  fuels  produces  water  vapor. 5-24