ICE FOG: LOW TEMPERATURE AIR POLLUTION DEFINED WITH FAIRBANKS, ALASKA AS TYPE LOCALITY

Abstract

Abstract : Stable pressure systems over interior Alaska sometimes produce prolonged, extreme (below -40C) cold spells at the surface. The meteorological conditions responsible for two such cold spells are discussed in detail, where it is shown that the rate of radiative cooling of the air is enhanced by suspended ice crystals which are themselves a result of the initial cooling. Radiation fogs formed during the onset of cold spells are generally of short duration because the air soon becomes desiccated. These fogs consist of supercooled water droplets until the air temperature goes below the 'spontaneous freezing point' for water droplets (about -40C); the fog then becomes an ice crystal fog, or simply 'Ice Fog'. During the cooling cycle water is gradually condensed out of the air until the droplets freeze. At this point there is a sharp, discontinuous decrease in the saturation vapor pressure of the air because it must be reckoned over ice rather than over water. The polluted air over Fairbanks allows droplets to begin freezing at the relatively high temperature of -35C. Between -35 and -40C the amount of water vapor condensed by freezing of supercooled water droplets is 3 to 5 times greater than the amount condensed by 1C of cooling at these temperatures. This results in rapid and widespread formation of ice fog which persists in the Fairbanks area as long as the cold spell lasts. The persistence of Fairbanks ice fog depends on a continual source of moisture (4.1 x 1,000,000 Kg H2O per day) from human activities within the fog.