Natural rains are generally considered as weakly acidic. Long-term measurement in the Tibetan capital city, Lhasa, reveals that alkaline rain is also natural. For the last 3 years the volume-weighted mean pH of rainwater is 7.5. Earlier observation shows even higher average pH values, such as 8.36 in the 1987--1988 period. The major cause of alkaline rain is the alkaline and soil-borne continental dusts in this semiarid area. Bicarbonate is the dominant anion in the water samples. The analysis also shows that the rainwater in this city contains few pollutants, compared with other places in the world. Measurements carried out in two additional industrial cities on the northern and northeastern Tibetan Plateau, Xining and Germu, demonstrate how fast human activities such as industrial development may increase rainwater acidity. In a period of 13 years the rainfall pH value of Germu has dropped from 8.03 to 6.8, representing a manyfold increase of the H+ concentration. Such an increase was caused by rising contents of NO-3 and SO-24 in the atmosphere. On the basis of the measurements on the Tibetan Plateau, evidence from other places around the world, and the experiments and calculation, the authors believe that the original pH of natural rainwater in arid and semiarid areas on this planet should be weakly alkaline because of the influence of alkaline dusts.