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a cloud is at least 50 feet above the surface. Fog may be distinguished from haze by its dampness and gray color. It is hazardous during takeoffs and landings, as well as the inflight process, because it restricts surface visibility. Knowledge of fog formation and dissipation processes, as well as types of fog help the balloon pilot plan a flight more accurately.Fog Formation
Since neither condensation nor sublimation occurs unless the relative humidity is near 100 percent, a high relative humidity is of prime importance in the formation of fog. The natural conditions which bring about a high relative humidity (saturation) are also fog-producing processes, such as the evaporation of additional moisture into the air or cooling of the air to its dew point temperature. A high relative humidity can be estimated, from hourly sequence reports, by determining the spread (difference in degrees) between the temperature and dew point. Fog rarely occurs when the spread is more than 2.2 °C. It is most frequent when the spread is less than 1.1 °C.
A light wind is generally favorable for fog formation. It causes a gentle mixing action, which spreads surface cooling through a deeper layer of air and increases the thickness of the fog.
Although most regions of the Earth have sufficient condensation nuclei to permit fog formation, the amount of smoke particles and sulphur compounds in the vicinity of industrial areas is pronounced. In these regions, persistent fog may occur with above average temperature-dew point spreads.
Fog tends to dissipate when the relative humidity decreases. During this decrease, the water droplets evaporate or ice crystals undergo sublimation, and the moisture is no longer visible. Either strong winds or heating processes may cause the decrease in relative humidity.
Some Fog Types and CharacteristicsRadiation Fog
Radiation fog forms after the Earth has radiated back to the atmosphere the heat gained during daylight hours. By early morning, the temperature at the surface may drop more than 11 °C. Since the dew point temperature (moisture content) of the air normally changes only a few degrees during the night, the temperature-dew point spread will decrease as the air is cooled by contact with the cold surface. If the radiational cooling is sufficient, and other conditions are favorable, radiation fog will form. Radiation fog is most likely when the:
• Sky is clear (maximum radiational cooling).
• Moisture content is high (narrow temperature-dew point spread).
• Wind is light (less than 7 knots).Advection Fog
The movement of warm moist air over a colder surface creates advection fog which is common along coastal regions where the temperature of the land surface and the water surface contrasts. The southeastern area of the United States provides ideal conditions for advection fog formation during the winter months. If air flows (advection) from the Gulf of Mexico or the Atlantic Ocean over the colder continent, this warm air is cooled by contact with the cold ground. If the temperature of the air is lowered to the dew point temperature, fog will form. Advection fog, forming under these conditions, may extend over larger areas of the nation east of the Rockies. It may persist throughout the day or night until replaced by a drier air mass.
If advection fog forms over water, it is often referred to as sea fog. Cold ocean currents, such as those off the coast of California, may cool and saturate moist air coming from the warmer areas of the open sea. Sea fog is often dense offshore, as well as onshore.
As advection fog moves inland during the winter, the colder land surface often causes sufficient contact cooling to keep the air saturated. The fog may then persist during the day or with a wind speed of 10 to 15 knots.Valley Fog
During the evening hours, cold dense air will drain from areas of higher elevation into low areas of valleys. As the cool air accumulates in the valleys, the air temperature may decrease to the dew point temperature, causing a dense formation of valley fog. While higher elevations may often remain clear throughout the night, the ceiling and visibility become restricted in the valley.Evaporation Fog
Fog formed by the addition of moisture to the air is called evaporation fog. The major types of evaporation fog are frontal fog and steam fog.Frontal fog is normally associated with slow-moving winter frontal systems. Frontal fog forms when liquid precipitation, falling from the maritime tropical air above the frontal surface, evaporates in the polar air below the frontal surface. Evaporation from the falling drops may add sufficient water vapor to the cold air to raise the dew point temperature to the temperature of the air. The cold air will then be saturated, and frontal fog will form. Frontal fog is common with active warm fronts during all seasons. It occurs ahead of the surface front in an area approximately 100 miles wide. It is, therefore, frequently mixed with intermittent rain or drizzle. When fog forms ahead of the warm front, it is called prefrontal fog. A similar fog formation may occur in the polar air along a
　
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