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Jun 9 th, 2013
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What Is Fog? What Causes Fog?

What is fog? How do fog form? What are the causes and kinds (types) of fog? Information about fog.

fog-cloudsFog; is a cloud at the surface of the earth. A fog is formed by the proximity of a warm to a cool surface. It is composed of minute water droplets—less than about 0.1 millimeter (0.004 inch) in diameter—suspended in the air, and it reduces visibility in the atmosphere to less than 1 kilometer (0.6 mile). When the visibility is greater than 1 kilometer, the obstruction to vision is known technically as mist. In the United States, however, mist popularly means drizzle-that is, precipitation in the form of very small drops.

Fog is hazardous to transportation. Many shipwrecks have been caused by the conditions of poor visibility experienced in fogs, despite aids to navigation such as radar and fog horns. Highway accidents frequently occur under the same conditions, and fog on airport runways may prevent landing and takeoff of aircraft for hours. Therefore, in selecting sites for highways and airports, engineers must consider the frequency and intensity of fog at the sites.

Cause of Fog: Fog forms when the relative humidity of the air approaches 100%, but its formation depends on whether there are sufficient condensation nuclei present. In the absence of such nuclei the relative humidity would have to be much greater than 100%—that is, the air would have to be supersaturated—before condensation could occur. Fog is especially likely to form in the presence of hygroscopic nuclei. Such nuclei are very small, soluble particles that absorb water vapor from the air at relative humidities as low as 75%.

Kinds of Fog: There are five principal circumstances under which fog occurs.

Radiation fog forms at night when the surface air is cooled by the loss of heat through radiation. Ground fog is a radiation fog and tends to occur in patches and in low-lying areas.

Advection fog is caused by the cooling of warm, moist air as it passes over a cold surface. Such fogs are frequent over the Grand Banks off the coast of Newfoundland, for example, where air that has been warmed and moistened by passing over the Gulf Stream is cooled by the cold Labrador Current well below its condensation temperature.

Frontal fog is caused by evaporation from relatively warm raindrops that fall from clouds along a frontal surface (between air masses) into colder air below. A frontal fog is similar to the fogginess caused by a hot shower in a cold bathroom.

Steam fog is an unstable fog found where cold air moves across a wann, moist surface. There is a rapid condensation of the water vapor in the air.

Upslope fog results from the cooling of moist air as it moves to higher elevations along a slope of the earth’s surface. The cooling and subsequent condensation are caused by expansion of the air as it encounters lower air pressures at higher altitudes.

Fog Dispersal Efforts: Scientists have tried several methods of dispersing fog on airport runways and on highways. Thus, burners placed alongside a runway can clear fog by evaporating the droplets, but the great amount of fuel required makes this method expensive. Seeding a supercooled fog (which consists of water droplets at temperatures below freezing) with dry ice is fairly effective, and spraying water drops into a fog has some effect in causing the droplets to coalesce into drops large enough to fall to the ground. Another promising method is the use of a helicopter to stir the air, mixing saturated with drier air and causing evaporation.

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