AEROGRAPHER'S MATE 3 & 2 



Figure 15-9. — Advection fog. 



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to hear of sea fogs which have lasted for weeks. 

 A good example of sea fog is that off the coast 

 of Newfoundland. 



LAND ADVECTION FOG is found near large 

 bodies of water; that is, along seacoasts and 

 large lakes. Onshore breezes bring maritime 

 air over a land surface which has cooled by 

 radiation at night. (See fig. 15-10.) Also, fogs 

 may form over the ocean and be blown over the 

 land, during either the day or the night. Another 

 situation favorable to fog formation is one in 

 which air flows from warm, bare ground to 

 snow-covered ground nearby. 



Land advection fog cannot exist with as high 

 wind speed as the sea type because of the greater 

 turbulence. If only a slight amount of cooling is 

 necessary to cause condensation, even a cloud 

 cover may permit the land surface to cool enough 

 to cause fog. This type of fog dissipates over 

 a land surface in much the same fashion as 

 radiation fog; however, since it is usually 

 deeper, it requires a longer time to disperse. 



STEAM FOG, another type of advection fog, 

 occurs within air masses; but, unlike other 

 air-mass fogs, which are formed by the cooling 

 of the air temperature to the dewpoint, steam 

 fog is caused by saturation of the air through 

 evaporation of water. It occurs when cold air 

 moves over warm water. Evaporation from the 

 surface of the warm water easily saturates the 

 cold air, causing fog. It rises from the surface 

 like smoke. It should be noted that the actual 

 process, heating cold air over a warm surface, 

 tends to produce instability. Denseness and per- 

 sistence are favored by the presence of in- 

 version above the surface, which prevents the 

 fog from rising very high. 



This type of fog forms on clear nights over 

 inland * "tea and rivers in late fall before they 

 are fro-en. They are prevalent along the Mis- 

 sissippi River and Ohio River at that time of 

 year. 



ARCTIC SEA SMOKE (name given to steam 

 fogs in the Arctic region) forms when cold air 

 moves over a warmer water surface, which is 



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