Droplet Sampling in Cumulus Clouds

An analysis is made of 150 cloud droplet samples obtained by the impaction method in cumulus clouds varying in vertical depth from 750 to 7000 ft.

From a consideration of the spectra relating numbers of droplets with droplet diameter, a mode diameter of about 8 µ, is found. A secondary mode diameter of about 15 µ is in evidence in some clouds. In the main numbers of droplets are between 100 and 300 per cm³ but extreme values of about 20 and 1000 occur.

Spectra relating water content with droplet diameter show that in clouds between about 750 and 2500 ft thick the mode diameter is about 8 µ, whereas in clouds between about 4000 and 7000 ft thick the mode diameter is about 18 µ. The highest value of water content found is about 5 g m⁻³.

Evidence is found that mean volume diameter tends to increase with height above cloud base and that high values of water content are due to larger droplets rather than to greater numbers of droplets per cm³.

A droplet distribution formula given by Best is examined and the constants involved are found to depend on the largest size of droplet present.