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During July and August, 1971, measurements were made of cloud condensation nuclei (CCN), supersaturation spectra, and cloud-base microstructure upwind and downwind of St. Louis. The supersaturation spectrum of CCN in 19 upwind-downwind pairs of air samples collected at an altitude of ∼600 m MSL was measured with a thermal diffusion chamber-type cloud nucleus counter. The average downwind increase in the concentration of CCN active at supersaturations of 0.17 and 1.0% was found to be 52 and 94%, respectively. Measurements of the droplet size distribution in small cumulus and stratocumulus clouds on 27 and 30 August revealed that downwind clouds were composed of smaller and more numerous droplets and had a narrower dispersion of droplet sizes than upwind clouds. The observed differences in droplet concentration between upwind and downwind clouds are somewhat greater than the predicted effect of the upwind-downwind differences in CCN spectra.
Fitzgerald et al. (Sun,) studied this question.