What is the fog?
Fog is a meteorological phenomenon that the vapor in the air condenses close to the ground and floats in the air in the form of water droplets, reducing visibility to less than 1 km.
Because fog occurs in the boundary layer, it is affected by surface’s humidity, temperature and wind, and det
ermined by the amount and type of condensation nucleus. Fog is generated by evaporation and cooling.
Also they are classified as advection fog, radiation fog, frontal fog etc. depending on the cause of the occurrence. In practice, usually multiple types of fogs appear at one case.
The conditions for Fog occurrence are
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Stable Atmosphere and weak wind for preventing vertical air mixing
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Enough water vapor supply
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Saturation of air near the surface
Among these, (1) is an essential for fog occurrence. This condition is usually fulfilled at center or boundary of high pressure. It is rare fog to occur when low pressure approaches, except fog by front. Depending on how the conditions (2) and (3) appear, there are various forms of fog may occur.
Fogs can be classified as follows, depending on the synoptic conditions that have major impacts on the occurrence.
Fogs formed by cooling
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Upslope Fog
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Cause: It is produced as relatively moist air travels along gently sloped plains or ascends along steep mountain slopes.
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When Earth's atmospheric pressure decreases at higher altitudes, the rising air expands and consequently leads to cooling. Therefore, when the humid air rises up the slope of the mountain, the air cools and saturates to form fog.
2. Radiation fog
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Cause: When the ground temperature drops by radiational cooling on a clear day, the air around the ground becomes saturated and condenses excessive water vapor.
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Radiant cooling is active in winter, as the nights are longer than summer. However, as the temperature is low in winter, the absolute amount of water vapor in the atmosphere is smaller than summer, resulting in heavy fog in spring and autumn than in winter.
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If the surface heating due to solar radiation is weak or if it rains a little and clears quickly from the night with plenty of water on the ground, the possibility of radiant fog is increased the next morning.
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Radiation fog is highly dependent on the nature of the ground, so the fog is often fragmented locally. Radiation fog is more likely to occur, especially in areas where water vapor can be ejected at night; radiation fog easily appears around lakes.
3. Advection Fog
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Feature: When warm, humid air passes over a cold surface, it contacts the cold surface and cools. The wind is somewhat lower in the lower layer, which is advantageous when there is turbulence from the sea. Thus, in the thermodynamic diagram, the lower average wind heads inland from the sea, and there is a shear relaxation.
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When air of different properties is mixed, the heat and water vapor of the two air are mechanically mixed. Sometimes different air mixtures can lead to saturation.
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When warm air passes over a cold surface or water surface, supersaturation occurs as the temperature decreases on the border.
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Particularly where there is a large regional differences in sea level temperature at sea, air masses formed on warm sea water produce thick sea fog as they move to relatively cold sea levels.
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If the wind blows more than 2-3 m/s in the synoptic pattern of the advection fog, the possibility of fog rain increases.
4. Advection-Radiation Fog
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It occurs when after abundant water vapor is pushed inland or water vapor evaporating from the lake move to the lower side and undergo radiational cooling at night.
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The wind is weak and the sedimentary reversal layer is clear, which gives a strong radiational cooling effect at night.
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Advection supplies water vapor, and radiational cooling creates condensation conditions.
Evaporation Fog: fog whose main supply is water vapor
5. Frontal Fog
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Feature: In the inversion layer formed on the frontal surface, when rain falls from the clouds formed in the warm air mass on the front, it falls to the cold base under the inversion layer; in extreme cases, the air in the lower layer will be supersaturated, causing fog.
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The base layer is generally saturated, showing a conductive inversion layer structure.
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It often occurs when the wind is weak on slow moving cold fronts or stagnant fronts.
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Fog tends to set in even when the temperature of the cold air in front of it is lower than the dew point of the warm air that rides on the warm front.
6. Steam Fog
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Cold air passes over warm ocean or wet ground, causing water vapor to generate fog. Also occurs when cold, moist air passes over warm ocean.
There are Seasonal Features of Korean peninsula's Fog
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Spring: With the weakening of the Siberian air mass, low pressures developed in China's southeast, winter solstice, and the sea affect the Korean Peninsula, often causing persistent, warm-line fog with mild rainfall.
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Summer: On the east coast, warm turbulence and the winds of the south wind predominate, forming advection fog in the colder Korean wave region in the north, which leads to rapidly increasing number of foggy days on the east coast and Ulleungdo.
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Autumn : Autumn fog occurs frequently throughout the Korean peninsula except on the east coast. Inland areas are active in radiant cooling of the ground, resulting in many radiation fog due to huge daily temperature range, while coastal fog has high seawater temperature, which evaporates well from the surface, and wet air moves to land and cools at night, forming frequent advection fog.
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Winter: Winter fog occurs mainly in inland areas near the west coast and basin areas where cold air accumulates, and is generally less frequent but longer. When the northwest monsoon is weak, the ground reversal is low due to radiational cooling of the ground or the stagnation of cold air. Especially, in the metropolitan area, many pollutants such as smoke and dust accumulate under the urban reversal layer, frequently causing a slight fog.