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!). Relative humidity expresses the degree of saturation. As a rule cold air holds fewer water molecules than warmer air holds. If air is completely saturated with water molecules the humidity is 100%.

In relationship to the humidity is dew point. 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. Weather reports for pilots usually include the dew point as well as the temperature. When the two are compared, the difference reveals to the pilot how close the air is to being 100% saturated. This difference is called the temperature-dew point spread.

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. Fog is most likely when the temperature-dew point spread is 5 F or less and decreasing. The fog would be lifting when the temperature-dew point spreadbegins increasing. Fog usually forms when the dew point and the temperature are within a few degrees of each other. The air temperature being lowered to the dew point, or the dew point being increased to the air temperature causes fog formation. Air temperature can be lowered as the air crosses over a colder surface like cold lake waters or a snow-covered area. Increasing the atmospheric moisture occurs when air flows from a water source (large lake, ocean) then moves over land. Pilots need to be mindful of the conditions which cause radiation fog and advection fog. Of the two types of fog, radiation fog does not hang around as long, is less hazardous and more localized. This means that when flying at low altitudes, a pilot will encounter patches of it and be able to fly through it quickly.

Radiation fog (also known as ground fog) occurs most often during clear, cool autumn nights while the Earth’s surface is rapidly cooling. It may hang in the air through the morning, but dissipates a few hours after sunrise. Advection fog however, forms when air laden with moisture from a maritime area moves from the water area over higher terrain while gradually cooling. As the air temperature is reduced to the dew point advection fog forms. This happens most often during the winter months over the eastern half of the United States as moist air flows northward from the Gulf of Mexico across the land increasing in elevation and cooling as it moves. This same phenomenon occurs along the coastal region of California as warm winds blow across the chilled California Current resulting in advection fog that can stretch from San Francisco to San Diego.