Rain containing higher than normal amounts of nitric and sulfuric acids. Acid rain formation results from both natural sources, such as volcanoes and decaying vegetation, and man-made sources, primarily emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) resulting from fossil fuel combustion.
Adapted from "What Is Acid Rain?" US Environmental Protection Agency. Web. 24 Nov. 2015. <http://www3.epa.gov/acidrain/what/>.