Chapter 15 — METEOROLOGICAL ELEMENTS 



WARM OCEAN 



FOG OR LOW 

 STRATUS 



COLD COASTAL WATER 



COLD CONTINENT 





Figure 15-10. — Fog caused by an onshore wind and cold coastal water. 



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most often found in breaks of the surface ice 

 in this area. It may also occur over the ocean 

 surface following a cold frontal passage when 

 the water is approximately 40° F warmer than 

 air passing over it. 



UPSLOPE FOGS. — There is a type of fog, 

 called upslope fog, which is caused by adiabatic 

 cooling of rising air (adiabatic cooling is the 

 cooling of the air by expansion as it rises). 

 Upslope fog is formed when moist, warm air 

 is forced up a slope by the wind. The cooling 

 of the air is almost entirely adiabatic, since 

 there is very little conduction to the surface 

 of the slope. The air must be stable before 

 it starts its motion so that the lifting does not 

 cause convection, or vertical currents, which 

 would dissipate the fog. 



Some wind speed is needed, of course, to 

 cause the upslope motion. The fog is usually 

 found where the air moves up a gradual slope. 

 This type of fog is deep and requires consider- 

 able time to dissipate. The most common fog 

 of this type is called CHEYENNE FOG and is 

 caused by the westward flow of air from the 

 Missouri Valley, which produces fog on the 

 eastern slope of the Rockies. 



FRONTAL FOGS. — Frontal fogs are another 

 hazard which must be added to the list of weather 



troubles associated with fronts. The actual fog 

 occurs under the frontal surface in the cold 

 air mass. It is due to the evaporation of falling 

 rain. This additional water vapor gradually sat- 

 urates the air. Precipitation falls from the lifted 

 warm air through the cold air. Evaporation from 

 the rain continues as long as the temperature 

 of the raindrops is higher than the temperature 

 of the air, even though the cold air is already 

 saturated. Naturally, the upper regions become 

 saturated first because the temperature and dew- 

 point are lower at the higher altitude. As the 

 evaporation from the rain continues, a layer 

 of clouds begins to build down from the frontal 

 surface. Eventually, this cloud layer extends 

 to the ground and becomes fog. During the day, 

 there may be enough turbulence caused by solar 

 heating to keep this cloud off the ground. However, 

 after dark, because of dying convection currents 

 and the nocturnal cooling of the air, the ceiling 

 drops very suddenly. It is this sudden closing 

 in after dark that makes this type of fog so 

 dangerous. 



Cold fronts usually move so rapidly and have 

 such narrow bands of precipitation and high wind 

 speeds that COLD-FRONT FOG is comparatively 

 rare and short lived. 



WARM-FRONT FOG, on the other hand, Is 

 common and dangerous. Since frontal systems 

 are quite extensive, warm-front fog may cover 



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