Measurements of Cloud-Droplet-Size Distributions in Polluted and Unpolluted Stratiform Clouds

Abstract

A number of cloud physics investigations were made within stratiform clouds in the areas around Denver, Colorado, and Kansas City, Missouri, during the winter months of 1989/90. There significant difference in the cloud-droplet-size distributions within the different cloud systems that appear to be associated with the urban plume of them metropolitan areas. This association has been made by looking at the trajectories of the air making up the cloud in different parts of the cloud and by concomitant measurements of NO2 within the cloud, using the presence or absence of NO2 as an indicator of an urbanization effect. In general the effect of the urban area is to significantly increase the number of droplets and decrease the median volume diameter (MVD) and the variance of the droplet diameters. In the clouds formed in unpolluted air, the MVD averaged 21.9 µm with a standard deviation of 7.4 µm, while the clouds formed in polluted air had an average MVD of 13.6 µm and a standard deviation of 1.9 µm.