Pilots need to be mindful of the conditions which cause radiation fog and advection fog. Increasing the atmospheric moisture occurs when air flows from a water source (large lake, ocean) then moves over land. Air temperature can be lowered as the air crosses over a colder surface like cold lake waters or a snow-covered area. The air temperature being lowered to the dew point, or the dew point being increased to the air temperature causes fog formation. Fog usually forms when the dew point and the temperature are within a few degrees of each other. The fog would be lifting when the temperature-dew point spread begins increasing. Fog is most likely when the temperature-dew point spread is 5 F or less and decreasing. On a clear night when the dew point colder than 32 F and the temperature-dew point spread is 5 F or less and decreasing, then frost will form. This difference is called the temperature-dew point spread.
When the two are compared, the difference reveals to the pilot how close the air is to being 100% saturated. Weather reports for pilots usually include the dew point as well as the temperature. Dew point is the temperature (in degrees) to which air must be cooled in order to be saturated with water vapor already in the air. In relationship to the humidity is dew point. If air is completely saturated with water molecules the humidity is 100%. As a rule cold air holds fewer water molecules than warmer air holds. Relative humidity expresses the degree of saturation. Relative humidity relates the actual amount of moisture in the air (in the form of a percent) to what total amount of moisture could be held in the air (That means it is also a ratio!). Knowing the conditions during which water changes state also helps pilots to avoid moisture-related problems during flight. Water in the atmosphere is measured by relative humidity and dew point accompanied by a temperature-dew point spread.
Moisture in the air creates more hazards during flight than any other weather phenomenon. Flying would be much easier if moisture were not such an influential component found in the atmosphere.
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