The Cause and Forecasting of Ice Fogs *

Studies carried out in Alaska and Canada have shown that fog is a relatively rare phenomenon at temperatures between 0° and − 30°F, with a minimum frequency between − 20° and −30°. At still lower temperatures, however, the frequency of fog increases rapidly. This effect is noted only in the immediate vicinity of inhabited areas, such as towns and airfields. The reason for the sudden increase in fog frequency at these temperatures, and the rarity or lack of fog at the higher temperatures, has not been heretofore explained. In a recent study on aircraft condensation trails, it was shown that if the temperature is sufficiently low (between − 20 and − 40°F, depending on the relative humidity), the burning of hydrocarbon fuels, such as would occur in towns and at airfields, easily results in supersaturation of the air and a “surface contrail” or ice fog. At higher temperatures, on the other hand, combustion actually reduces the relative humidity of the atmosphere, hindering the formation of fog. In this paper it is shown that low-temperature (ice) fogs form as a result of the combustion process, and curves are presented showing the temperature-dew-point relationship necessary for the formation of such fogs.