Observations of the vertical structure of a stratus cloud and radiation fogs in relation to the mechanism of drizzle formation

Observations of the vertical structure of radiation fogs and a stratus cloud were made by means of a rotary droplet sampler and other techniques. The results of the surveys of four radiation fogs and one stratus cloud are presented. It is found that the upper part of a deep fog or a stratus cloud has a similar physical structure to a cumulus cloud. The observation reveals that ordinary fog droplets grow mainly by condensation. It is also found that the presence of large droplets of radii about 60 microns or more near the top of the fog or stratus cloud is a necessary condition for the production of drizzle. The rate of growth of the large droplets is approximately explained by the Langmuir’s accretion theory.