Cloud Condensation Nuclei and their Possible Influence on Precipitation

Abstract

Measurements of cloud condensation nuclei (CCN) conducted both from the ground and from an instrumented aircraft during two winter seasons, utilizing a thermal diffusion chamber, suggest that. CCN spectra prove to always have, in both rainy and dry weather, continental-like characteristics. Their concentrations were shown to depend on wind direction and found to be surprisingly high, even in air masses with a partial maritime trajectory. The existence of local sources appears to have a limited and a generally minor effect on the characteristics of CCN spectra. On the average, CCN concentrations tend to decrease by a factor of 2 during the first three days of a rainy spell. Consequently, cloud droplet concentrations, computed for cumulus clouds assumed to have base updraft velocities of 250 cm sec?1, seem to show a similar trend but are always within the range of 180?950 cm?2. Since these variations do not seem to correlate with rainfall, they are not dominant in their effect on the relative efficiency of the rain-forming processes. However, the ever-present high concentrations of CCN, and thus, of cloud droplets, must have a significant effect on the nature of these processes in view of the resulting high colloidal stability of local clouds.