Acid rain (or acid deposition, as it's called in technical circles) is produced by the burning of fossil fuels. It is formed when emissions of sulfur dioxide and nitrogen oxides react in the atmosphere with water, oxygen and oxidants to form various acidic compounds. These compounds then fall to the ground in either wet or dry form.
Acid rain acidifies lakes and streams and contributes to damage of trees at high elevations. (Check out Mount Mitchell in North Carolina if you want a graphic example of tree damage.) Hundreds of lakes in the Adirondacks have become too acidic to support sensitive fish species. In addition, acid rain accelerates the decay of paints and buildings.
Electric power plants account for about 70 percent of sulfur dioxide emissions about 30 percent of nitrogen oxides emissions. Cars, trucks and buses also are major sources of nitrogen oxides.