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.
