Forecasting the Dissipation of Fog and Stratus Clouds

Abstract

F the time at which fog or stratus clouds will break has become a particularly important problem in recent years with the development of commercial and military aviation and the improved methods of flying which have resulted. Many times the completion of an airline schedule is dependent upon the dissipation of a fog or low cloud deck within a given time. Even the rate at which the ceiling is rising becomes important, for may flights could be dispatched toward fog-bound terminals without long delays if a definite indication of sufficient improvement were available. At present, forecasts involving these elements are largely a function of the forecaster's experience, and are not always extremely accurate or reliable. Various empirical methods employing such elements as the thickness of the cloud, the height of the cloud base, and the fact that terrestrial radiation is completely absorbed by the lower levels of the clouds, appear to have formed the principal aids to forecasting the breaking of fog and stratus systems. The absorption of terrestrial radiation has been generally credited as the source of heat providing for the dissipation of a cloud, since the process normally takes place upward from the cloud base. A moment's thought will convince one that this factor cannot be of very great importance. Brunt has shown that the radiative transfer of heat in the atmosphere in regions of the spectrum where water vapor is the effective absorbing agent, is negligible. This means that any net gain of heat at a cloud base by radiation must originate in the transparent regions of the spectrum, a series of bands accounting for only about 25% of the total. This gain is further reduced by the fact that a cloud acts as a black body, and therefore the sky radiation on a cloudy day cannot differ greatly from that originating at the earth's surface, especially if the cloud is low and its temperature nearly the same as that of the ground beneath. Hence, it seems illogical to assume that cloud decks several thousand feet in thickness may be dissipated within a few hours by heat gained from the absorption of terrestrial radiation.