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Page Title:
Formation and Dissipation of Stratus and Fog |
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Formation and Dissipation of Stratus and Fog When an air mass cannot hold all of the water vapor contained in it, the excess vapor condenses out in the form of small droplets. Air can be brought to saturation by (1) cooling, (2) the addition of water vapor, or (3) the mixing of one air mass with another. Sometimes more than one process is involved, either simultaneously or one after another.1. The cooling of an air mass may be caused by one or more of the following processes:Cooling of Earths surface by outgoing nocturnal radiation, with resultant cooling of the lower layer of air Advection of air over a colder surface Adiabatic cooling of air by orographic, frontal, or turbulent lifting Cooling of air by the evaporation of precipitation falling through it Radiation from the air or the cloud to the environment (usually a slow and unimportant factor in fog) 2. The addition of moisture to an air mass may be brought about by one or more of the following processes:Evaporation from falling precipitation (under certain conditions) Evaporation from a wet surface, either land or sea Moisture resulting from the combustion of hydrocarbon fuels, such as gasoline and oil. Turbulent transfer of moisture upward (when a suitable vertical gradient of vapor pressure exists) 3. Increasing the temperature of the air can be caused by one or more of the following processes:Contact warming of the ground layer by incoming solar radiation during the day Advection over a warmer surface. Adiabatic warming of the air by subsidence, downslope motion, or the turbulent transfer of heat downward. Turbulent mixing of the fog layer with adjacent warmer air (above). Fog and clouds can dissipate by either of two processesthe heating of air or the removal of moisture. A decrease in moisture content can be brought about in the lower layers by turbulent transfer of moisture upward, by turbulent mixing of the fog layer with adjacent drier air, or by the condensing-out of water vapor in the form of dew or frost. |
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