Land Advection Fog.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. 5-1-4.) 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 gound to snow-covered ground nearby. 

Land advection fog cannot exist with as high wind speed as the sea type because of the greater turbulence. It dissipates in much the same fashion as radiation fog. However, since it is usually deeper, it requires a longer time to disperse.

Steam Fog. Steam fog occurs within air masses; but, unlike other air-mass fogs, which are formed by the cooling of the air temperature to the dew point, 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 which rises from the surface like smoke. It should be noted that the actual pro-cess of heating cold air over a warm surface tends to produce instability. The presence of an inversion above the surface prevents steam fog from rising very high; it is usually fairly dense and persistent. This type of fog forms on clear nights over inland lakes and rivers in late fall before they freeze. It is prevalent along the Mississippi River and Ohio River at that time of year.

Arctic sea smoke is the name given to steam fogs in the arctic region. It forms when cold air moves over a warmer water surface, which is most often found in breaks of the surface ice. It may also occur over the ocean surface following a cold frontal passage when the water is approximately 40F warmer than the air passing over it.

Figure 5-1-4.Land advection fog caused by an onshore flow over cold coastal water.

Upslope Fogs.Upslope fog is caused by adiabatic cooling of rising air. It 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 little conduction taking place between the air and 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. Upslope fog is usually found where the air moves up a gradual slope. This type of fog is deep and requires considerable 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 Fog.Frontal fog is another hazard which must be added to the list of weather prob-lems associated with fronts. The actual fog is due to the evaporation of falling rain and occurs under the frontal surface in the cold air mass. This addi-tional water vapor gradually saturates the air. Pre-cipitation 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 tur-bulence caused by solar heating to keep this cloud off the ground. However, after dark, because of dying convection currents and the nocturnal cool-ing of the air, the ceiling drops suddenly. It is this sudden closing in after dark that makes frontal 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 fairly common. Since warm frontal systems are quite extensive, warm-front fog may cover a wide area. This type fog is also deep because it extends from the ground to the frontal surface. The clouds above the frontal surface also slow down the dissipating effect of solar heating. These factors make the warm-front fog among the most dangerous. (See fig. 5-1-5.)

Dew does not actually fall; rather the moisture condenses from the air which is in direct contact with the cool surface. During clear, still nights, vegetation often cools by radiation to a temperature at or below the dew point of the adjacent air. Moisture then collects on the leaves just as it does on a pitcher of ice water in a warm room. Heavy dew is often observed on grass and plants when there is none on the pavements or on large, solid objects. These objects absorb so much heat during the day or give up heat so slowly, they may not cool below the dew point of the surrounding air during the night.

Figure 5-1-5.Warm-front fog.

Another type of dew is white dew. White dew is a deposit of white, frozen dew drops. It first forms as liquid dew, then freezes.

Frost

Frost, or hoarfrost, is formed by the process of sublimation. It is a deposit of ice having a crystalline appearance and generally assumes the form of scales, needles, feathers, or fans. Hoarfrost is the solid equivalent of dew and should not be confused with white dew, which is dew frozen after it forms.

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