Acid rain occurs with sulfur and nitrogen and released into the air. Factories that burn coal and oil, release sulfur and nitrogen. They become acidic as they come back to earth. There is a Clean Air Act in the U.S. that limits the amount of sulfur that can be released in the air. This has helped reduce the amount of acid rain in the U.S.

Acid rain forms when water vapor and certain elements chemically combine with natural and man-made pollutants-such as sulfur dioxide or nitrogen oxides-in the stratosphere, forming sulfuric and nitric acids. Acids have a pH of 6 or less; acid rain includes precipitation with a pH lower than 5 to 5.6,though it has been recorded as low as pH 2.4 to 2.8. Acid rain can fall as wet deposits in the form of rain, fog, or snow, or as dry deposits in the form of gases and particles.