Rain or any form of precipitation that is unusually acidic is known as acid rain.
Acid rain occurs when sulfur dioxide and nitrogen oxide gases react in the atmosphere with water, oxygen, and other chemicals to form various acidic compounds such as sulfuric acid and nitric acid.
Acid rain causing gases:
Sulfur dioxide
Nitrogen oxide
These gases are introduced into the atmosphere from natural resources (volcanoes) and man-made resources (fossil fuel combustion).