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RADIATION FOG.— Radiation fog, which generally occurs as ground fog, is caused by the radiational cooling of Earth’s surface. It is primarily a nightime occurrence, but it often begins to form in the late afternoon and may not dissipate until well after sunrise. It never forms over a water surface. Radiation fog usually covers a wide area.

After sunset, Earth receives no heat from the Sun, but its surface continues to reradiate heat. The surface begins to cool because of this heat loss, As Earth cools, the layer of air adjacent to the sur-face is cooled by conduction (the transfer of heat from warmer to colder matter by contact). This causes the layer near Earth to be cooler than the air immediately above it, a condition called an in-version. If the air beneath the inversion layer is sufficiently moist and cools to its dew point, fog forms. (See fig. 5-1-2.) In case of a calm wind, this cooling by conduction affects only a very shallow layer (a few inches deep), since air is a poor con-ductor of heat. Wind of low speed (3 to 5 knots) causes slight, turbulent currents. This turbulence is enough to spread the fog through deeper layers. As the nocturnal cooling continues, the air tem-perature drops further, more moisture is con-densed, and the fog becomes deeper and denser. If winds increase to 5 to 10 knots, the fog will usually thicken vertically. Winds greater than 10 knots usually result in the formation of low scud, stratus, or stratocumulus.

After the Sun rises, Earth is heated. Radia-tion from the warming surface heats the lower air, causing evaporation of the lower part of the fog, thereby giving the appearance of lifting. Before noon, heat radiated from the warming surface of Earth destroys the inversion and the fog evaporates into the warmed air. Radiation fog is common in high-pressure areas where the wind speed is usually low (less than 5 knots) and clear skies are frequent. These conditions permit 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 temperature 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. 5-1-3.) It is easy to locate areas of temperature contrast on the weather map, as they are usually found along coastlines or between snow-covered and bare ground.

Sea Fog.—Sea fog is always of the advection type and occurs when the wind brings moist,

Figure 5-1-2.—Radiation fog.

Figure 5-1-3.—Advection fog.

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 night. Some wind is necessary, not only to provide some vertical mixing, but 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 persist 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 tem-perature of the ocean surface changes very little during the day, it is not surprising to hear of sea fogs which have lasted for weeks. A good example of sea fog is that found off the coast of Newfoundland.

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