Acid rain is caused by the mixing of moisture in the atmosphere with sulfur dioxide and nitrous oxides, forming sulfuric acid and nitric acid in the precipitation that falls to the earth. In the Northern Hemisphere, the most common source of these chemicals in the atmosphere is air pollution from industry, generation of electricity and automobiles.
Volcanoes and forest fires also release oxides into the air that cause acid rain, but these do not match the proportions caused by man-made pollution. Acid rain can cause damage to buildings, vehicles and natural formations. The acids dissolve limestone and other minerals, accelerating natural weathering of stone. The low pH of the rain affects plant growth and pollutes surface and ground water. About.com reports that over half of the forests in Germany and Poland are affected by acid rain.
Acid rain dilutes as it mixes with water. However, it accrues over time as water evaporates from bodies of water in the water cycle. In addition, acid rain causes clay soils to release metals, including aluminum and magnesium, which further lower the pH of the water. Approximately 50,000 lakes in the United States alone have a pH below normal, and several hundred have fallen to pH levels that no longer support life.