The greater the air pollution over hilly land in semi-arid regions, the less rain the area will get, according to an Israel-Chinese research team led by Prof. Daniel Rosenfeld of The Hebrew University of Jerusalem's department of atmospheric sciences, whose study has just been published in Science. This phenomenon, the scientists said, has dire consequences for limited water resources in the Middle East and many other parts of the world. The research study, called "Inverse Relations between Amounts of Air Pollution and Orographic Precipitation," was coauthored by Rosenfeld, Jin Dai et al from the Meteorological Institute of Shaanxi Province in China, and Zhanyu Yao of the Chinese Academy of Meteorological Science. The researchers, working on Mount Hua near Xian in central China, showed that the mountain's average precipitation has decreased by one-fifth over the last 50 years as air pollution increased. The precipitation loss doubled on days that had the poorest visibility due to pollution particles in the air. This explains widely observed trends of less rain in hilly areas relative to rainfall in nearby densely populated lowlands, which until now had not been directly ascribed to air pollution. The researchers studied records dating back to 1954 of precipitation and visibility at Mount Hua's peak and linked the decreasing visibility at its over-two-km.-high summit to increasing air pollution particles that reach the clouds. The team was able to show that the higher concentrations of these fine, airborne aerosols were responsible for the decrease in mountain precipitation. This is the first time that the connection has been demonstrated so conclusively. Precipitation is inhibited as water vapor condenses on the pollution particles and creates a cloud containing a large number of drops that are so small that they float with the air and are slow to coalesce into raindrops or to freeze into sleet and snowflakes. This translates into a net loss of precipitation when the "lifetime" of the cloud is shorter than the time necessary for it to release its water, as happens when clouds form as they ascend across a ridge and then descend and evaporate downwind. By making use of precipitation and visibility records showing a direct causal link between airborne particle pollution and the loss in mountain precipitation, the study can bear out other hypotheses on the effects of pollution on rainfall. These findings highlight the threat to water resources in polluted regions of the world where hilly-area precipitation makes a significant contribution to the regional water supply, as in the southwestern US, central and northern China, and the Middle East. Rosenfeld has already conducted such research in hilly regions with similar pollutive conditions, including California and much of the western US. Similar trends have already been reported for Israel and observed in South Africa, Portugal, France, Switzerland, Morocco, Canada, Greece, and Spain.