Normal unpolluted rain water is acidic because it contains carbonic acid, according to the Virtual Chembook of Elmhurst College. The pH or acidity of rainfall is typically less than 6, with 7.0 being neutral.
Carbonic acid forms when carbon dioxide in the atmosphere dissolves in water droplets. The clouds containing these droplets produce rainfall or other precipitation with carbonic acid. Human activities can further acidify precipitation.
The term "acid rain" refers to rain that contains acids that form from man-made pollutants, including nitrogen and sulphur oxides. The pH of acid rain can range from about 4 to 5.5. Lower acidity is associated with more industry, power plants and cars, according to the U.S. Geological Survey.
Rain samples are collected around the country and their pH is measured to assess the distribution of acidity. Natural sources such as volcanoes, organic decay and phytoplankton also produce compounds like sulphur dioxide that can acidify rain, according to Tropical-Rainforest-Animals.com. The source of acid in rainfall is sometimes impossible to determine.