Acid rain describes any form of precipitation with high levels of nitric and sulphuric acids. When humans burn fossil fuels, sulphur dioxide and nitrogen oxides get released into the atmosphere. These gases rise very high into the atmosphere, react with water, oxygen and other substances to form mild solutions of sulphuric and nitric acid. These mix with rainwater and fall down to the earth as acid rain. Acid rain is caused by large scale emission of nitrogen oxides, sulphur dioxide and other pollutants from thermal power plants, industries and automobiles.