Urban and industrial air pollution has been shown qualitatively to suppress rain and snow. Here we quantify the precipitation losses over topographical barriers downwind of major coastal urban areas in California and in the land of Israel, which amounts to 15-25 percent of the annual precipitation. The suppression occurs mainly in the relatively shallow orographic clouds within the cold air mass of cyclones. The suppression that occurs over the upslope side is coupled with similar percentage enhancement on the much drier downslope side of the hills. The evidence includes significant decreasing trends of the ratio of hill / coast precipitation during the 20th Century in polluted areas in line with the increasing emissions during the same period, whereas no trends are observed in similar nearby pristine areas.
The evidence suggests that air pollution aerosols that are incorporated in orographic clouds slow down cloud-drop coalescence and riming on ice precipitation, hence delaying the conversion of cloud water into precipitation. This explains the pattern of greatest loss of precipitation at the mid-level of the upwind slopes, smaller losses at the crest, and enhancement at the downslope side of the hills.
William Woodley has extensive research experience in the dynamics of clouds and precipitation with emphasis on cloud seeding and aerosol effects. Dr. Woodley is a long-time collaborator with Dr. Daniel Rosenfeld of the Institute of Earth Sciences, Hebrew University of Jerusalem, who leads the research team whose work is the topic of this seminar. Dr. Woodley is a private consultant, president of Woodley Weather Consultants in Littleton, Colorado.
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