Fog Formation Valley Fog Evaporation Fog Some Fog Types and Characteristics Radiation Fog Advection Fog  •  Sky is clear (maximum radiational cooling). •  Moisture content is high (narrow temperature-dew  point spread). •  Wind is light (less than 7 knots). 4-19 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  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. 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:  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. 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. 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. 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. 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. 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. 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. 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. 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.