On the measurement of drop size and liquid water content in fogs and clouds

Abstract

A short critical review of possible methods for the measurement of the size of fog particles is presented. It is concluded that the only suitable method of obtaining the distribution of drop sizes present in a given fog consists in the microscopic measurement of large numbers of drops which have been collected on a properly surfaced slide. A method for surfacing microscope slides with a thin, uniform layer of petroleum grease is described. The important problem of obtaining a representative sample of drops on a slide is next considered. Experimental results indicate that slides no larger than 5 mm square will collect satisfactory samples if exposed facing the wind. Larger slides are found to discriminate against the smaller drops. Special fog microscopes which have been constructed for observing droplet samples are described, and typical results obtained in natural fogs are presented. Although forty sets of data have been procured in sixteen different fogs, it has not been possible to correlate the drop size data with any of the accompanying meteorological conditions. There is no evidence of mass grouping, such as Köhler observed in clouds; however, definite conclusions cannot be drawn from such a relatively small amount of data. The usefulness of fog water data is indicated and possible methods of procuring them are reviewed. An investigation of the sampling problem encountered in the operation of most fog measuring instruments is described. The method of avoiding sampling diffculties in a new fog water instrument is explained and the constructional features and operation of the apparatus are discussed.