Acid rain is formed by the atmospheric presence of sulfur oxides and nitrogen oxides. These gases react with water in the air and form strong acids that fall to the earth as precipitation.

The so called acid in acid rain is usually sulfur dioxide and nitrogen oxides that have been released into the air due to the burning of fossil fuels. As these gaseous elements react with moisture in the atmosphere, they are absorbed by the water droplets and fall back to earth as acid rain.