form of precipitation containing a heavy concentration of sulfuric and nitric acids. The term is also commonly applied to snow, sleet, and hail that manifest similar acidification. Such precipitation has become an increasingly serious environmental problem in many areas of North America, Europe, and Asia (see video-->). Although this form of pollution is most severe in and around large urban and industrial areas, substantial amounts of acid precipitation may be transported great distances.

The process that results in the formation of acid rain generally begins with emissions into the atmosphere of sulfur dioxide and nitrogen oxide. These gases are released by automobiles, certain industrial operations (e.g., smelting and refining), and electric power plants that burn fossil fuels such as coal and oil. The gases combine with water vapour in clouds to form sulfuric and nitric acids. When precipitation falls from the clouds, it is highly acidic, having a pH value of about 5.6 or lower. (The term pH is defined as the negative logarithm of the hydrogen ion concentration in kilograms per cubic metre. The pH scale ranges from 0 to 14, with lower numbers indicating increased acidity.) At several locations in the eastern United States and western Europe, pH values between 2 and 3 have been recorded. In areas such as Los Angeles, San Francisco, and Whiteface Mountain in New York, fog is often 10 or more times as acidic as the local precipitation.