Fog is a cloud with the base near or near the surface. There is no physical difference between fog and a cloud because they have the same look and structure. The essential difference is the method of development and where the training takes place.
Fog forms when air cooling or the addition of water vapor by evaporation causes saturation. Fog is generally considered a danger from the atmosphere. When fog is light, visibility is reduced to 2 or 3 kilometers.
When it is dense, visibility can be reduced to 12 meters or less, making land transport not only difficult but also dangerous. Weather stations report fog only when visibility is reduced to 1 kilometer or less.
Condensation produces a fog when the temperature of a surface air layer falls below its dew point (dew point or saturation point is the temperature of the air below which the water vapor present becomes liquid in the form of small drops). Depending on prevailing conditions, the surface may be obscured by various types of fog.
Radiation fog is the result of radiation cooling of the surface and surrounding air. This is a night phenomenon and requires clear skies and high humidity values. Under these conditions, the surface and surrounding air will be cooled rapidly. Because of the high humidity, only a little cold will lower the temperature to the dew point.
If the air is calm, the fog may be uneven, extending vertically less than one meter. Light winds of 3 to 4 km / h can cause turbulence and allow the fog to extend intact from 10 to 30 meters. Since the air containing the fog is relatively cold and dense, it descends into the mountainous terrain. For this reason, the radiation fog is denser in the valleys, while the surrounding mountains are clear.
Sometimes people say that the fog rises, but this is not correct. At dawn, solar radiation penetrates the fog and warms the surface, causing the surrounding air temperature to rise. Some of the fog droplets evaporate into the warmer air and allow more solar radiation to reach the surface.
Surface air heating causes the fog to quickly evaporate and disappear completely. If the fog layer is thicker, it may not dissipate and a Stratus cloud layer may cover the region. Sometimes this type of fog is called high fog.
When hot, humid air moves over a sufficiently cooler surface, it can cool to the dew point and form the advection fog (or transport fog). Advection fog is the consequence of air releasing heat to the surface below during horizontal movement. A good example is frequent advection fogs around the Golden Gate Bridge in San Fransisco, USA.
Turbulence (usually 10 to 30 km / h winds) is required for proper development. Turbulence facilitates cooling through a denser layer of air, and also lifts the fog to high altitudes. In contrast to radiation fogs, advection fogs are persistent and typically extend from 300 to 600 meters.
Rising mountain fog forms when relatively humid air rises on the slope of a plain, hill or mountain. The rising air expands and cools. An extensive fog layer may form if the air temperature reaches the dew point. Rising mountain fogs may persist for many days.
Cape Dissapointment in Washington, DC, is probably the fogiest place in the world. The weather station there records an annual average of 2552 hours (106 days) of fog. These fogs are produced during the summer and early fall, when the warm, humid air of the Pacific Ocean moves over the cold California Stream.
When fresh air moves over hot water, a sufficient amount of moisture can evaporate from the water surface to saturate the air above. As soon as rising water vapor meets the cold air, it recovers and rises with the heated air from below.
This phenomenon is called steam fog because it is similar to steam rising above a water surface. Steam fog is common over ponds and rivers in the fall and early winter, when the water can be relatively hot but the air is cold.
Occasionally, condensed vapor columns may rise from the fog layer and form swirls of steam, similar to those of the devil's dust on earth. The vapor mist is sometimes low because when the air rises it evaporates again into the unsaturated air above.
Steam fog can be dense, especially during winter, when cold air moves from continents to the ocean. This air may be 20 to 30 ° C colder than water. Because of the appearance, this fog of vapor over the ocean in polar regions is called arctic sea fire. In extremely cold arctic air, ice crystals form rather than water droplets, and produce ice fog.
Front fog is the result of frontal lifting when warm air rises over colder air, producing clouds and precipitation. If the air below is cold and near the dew point, sufficient rainfall can evaporate and produce fog. The result is more or less a continuous zone of condensed water droplets striking the surface and extending upward through the clouds. Generally, this fog develops in the lower layer of cold air in front of a cold front or behind a warm front.