Acid rain is a form of pollution which became a serious environmental problem in the large urban and industrial areas of North America, Europe and Asia.
Any precipitation that contains a heavy concentration of Sulphuric Acid (H2SO4) and Nitric Acid (HNO3) is generally called acid rain.
Automobile industries, electrical power plants and some other industrial operations that burn fossil fuels and emit the gases like sulphur dioxide (SO2) and nitrogen oxide (NO2) into the environment. In the air the gases meet with water vapour available in clouds to reform Sulphuric Acid and Nitric Acid.
This acid rain or highly acidic precipitation from those clouds may contaminate lakes and streams, thereby, harming the fishes and other aquatic species. It also causes a great harm to vegetation including agricultural crops and plants and trees.
Now-a-days acid rain or acid precipitation has become a threatening problem for the environment and humankind.