Particulate air pollution has been suggested as the cause of the recently observed decreasing trends of 10 to 25% in the ratio between hilly and upwind lowland precipitation, downwind of urban and industrial areas. We quantified the dependence of this ratio of the orographic-precipitation enhancement factor on the amounts of aerosols composed mostly of pollution in the free troposphere, based on measurements at Mt. Hua near Xi'an, in central China. The hilly precipitation can be decreased by 30 to 50% during hazy conditions, with visibility of less than 8 kilometers at the mountaintop. This trend shows the role of air pollution in the loss of significant water resources in hilly areas, which is a major problem in China and many other areas of the world.