Amir Givati and Daniel Rosenfeld The Hebrew University of Jerusalem, Jerusalem, Israel
Previous studies had showed qualitatively that urban and industrial air pollution suppresses the cloud drop coalescence and so reduces the precipitation from the polluted clouds. Here we present the first study that attempts to quantify these effects based on time series of rain gauge data during the last century. The most vulnerable areas are coastal cities, which receive most of their precipitation from marine convective clouds that flow from sea inland. Precipitation is suppressed in the new clouds that are formed in the urban polluted air downwind over orographic barriers, such as the mountains to the east of Los Angeles and Tel Aviv. Control areas are selected from nearby rural side wind coast-hill precipitation analysis. The results from both California and Israel areas show significant decrease (up to 25%) in the ratio between the mountain and coastal stations with the increase of air pollution emissions along the years. All the mountain/coast pairs in San Diego, Los Angeles and San Francisco Metropolis areas had the same trends – decrease in the ratio along the years, with the sharpest decrease between the 40’s to the 70’s. In Israel this trend was found in Judea and Samaria hills that are being located down wind from the Tel Aviv Metropolis. On the other hand, no trend was found in the “clean” control areas side wind. The ratio between the mountain stations to the coast remains stable along the years.