Observations of the Geographical Variation of Cloud Nuclei

Airborne measurements at flight level of cloud nuclei have been carried out during approximately 100,000mi of flight over wide areas of the world. A thermal diffusion chamber was used to obtain cloud-nucleispectra which present the concentration of active nuclei as a function of supersaturation over the rangeof supersaturation of interest in cloud formation. The results confirm previous conclusions that continentalair masses are systematically richer in cloud nuclei and indicate that the median of cloud nuclei over theopen ocean is reasonably predictable and varied little from region to region. Over continents the medianspectrum does not seem to vary greatly even when North America was compared to Africa and Australia;however, the variability about the median is much greater over land when compared to ocean areas. Theresults indicate that the lifetime of cloud nuclei, at least over the oceans, is about 3 days. The results requirea widespread and relatively uniform source of cloud nuclei both over the oceans and over the land, and itis indicated that the sources of nuclei are not largely dependent on industrial or other man-made pollution.