Chapter 15 — METEOROLOGICAL ELEMENTS 



201.60 



Figure 15-8. — Radiation fog. 



(2 to 5 knots) and clear skies are frequent. 

 This permits maximum radiational cooling. 



ADVECTION FOG. — Advection fog is the 

 name given to fog produced by air in motion 

 or to fog formed in one place and transported 

 to another. This type of fog is formed when 

 warmer air is transported over colder land or 

 water surfaces. Cooling from below takes place 

 and gradually builds up a fog layer. The cooling 

 rate depends on the wind speed and the difference 

 between the air temperature and the tempera- 

 ture of the surface over which the air travels. 



Advection fog can form only in regions where 

 marked temperature contrasts exist within a 

 short distance of each other, and only when the 

 wind blows from the warm region toward the 

 cold region. (See fig. 15-9.) It is easy to locate 

 areas of temperature contrast on the weather map. 

 They are usually found along coastlines or 

 between snow-covered and bare ground. 



SEA FOG is always of the advection type 

 and occurs when the wind brings moist warm 

 air over a colder ocean current. The greater 

 the difference between the air temperature and 

 the ocean temperature, the deeper and denser 

 the fog. Sea fog may occur during either the 

 day or the night. Some wind is necessary, not 

 only to provide some vertical mixing, but actually 

 also to move the air to the place where It is 

 cooled. Most advection fogs are found at speeds 

 between 4 and 13 knots. Sea fogs have been 

 maintained with wind speed as high as 26 knots. 

 They exist at such speeds because of the lesser 

 frictional effect over a water surface. Winds 

 of equal speed produce less turbulence over 

 water than over land. 



Sea fogs, which tend to persist for long 

 periods of time, are quite deep and dense, Since 

 the temperature of the ocean surface changes 

 very little during the day, it is not surprising 



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