Some fog droplet size distributions obtained by an impaction method

Abstract A cascade impactor technique based on the methods of previous workers has been used to study the size distribution of water droplets in several fogs. Nomarski interference contrast, used as a method of microscopic examination of the samples revealed large numbers of small droplets which were not visible under phase contrast. In about one third of the fogs studied these droplets, smaller than 2 μm diameter, made a large contribution to the optical extinction coefficient. The use of interference contrast for assessment of samples has resulted in some reduction of the large deficit, frequently found by previous workers, in the extinction coefficient calculated from the droplet size distribution, as compared with that measured optically. The remaining discrepancy, however, is significant and sometimes exceeds a factor of two. The small droplets are probably the result of condensation of water on hygroscopic nuclei whose critical supersaturation has not been exceeded. Their implications regarding the rate of clearance of fog are considered.