Measurements of droplet size and visibility have been made at different levels above the base of all types of cumulus clouds. The evaluation of these data permits calculations of corresponding values of water content and droplet concentration. It has been found that the droplet spectrum in fair-weather cumulus is narrow, whereas the spectra in cumulus congestus and cumulonimbus are very broad with droplet radii as large as 100µ. The droplet concentration decreases with height above base, suggesting the action of an efficient process of coalescence. At a height of about 2000 m above base, minimum values of visibility have been found. It appears that coalescence of cloud droplets is an important process in the formation of droplet spectra.