Nonprecipitating clouds leave a distinctive fingerprint on the aerosol particles that cycle through them by segregating aerosol particles into two populations, those which are incorporated into cloud droplets and those which are not. This leads to a bimodal or double-peaked character in the aerosol number distribution. If some reasonable assumptions are made, cloud microphysical properties can be inferred from the bimodal aerosol distributions. We have collected over 1700 bimodal distributions from five stations in North America and inferred cloud droplet concentration and maximum supersaturation in the clouds which processed the aerosol particles. Average cloud droplet concentrations are 100--200 cm-3 at background stations, while at the polluted site, cloud droplet concentrations were as high as 3000 cm-3. Inferred values of maximum supersaturation ranged from 0.3% at clean sites to 0.1% at the polluted site. Cloud droplet concentration and maximum supersaturation were usually inversely correlated. Cloud droplet concentration and geometric mean diameter of the cloud-processed mode in the aerosol number distribution were also inversely correlated. These two relationships can be understood by comparison with a simple model of cloud activation. ¿ 1999 American Geophysical Union