Acid rain forms through a chemical process that begins when sulfur dioxide (SO2) and nitrogen oxides (NOx) are released into the atmosphere, primarily from human activities such as burning fossil fuels in power plants, vehicles, and industrial processes

. These gases rise high into the atmosphere, where they react with water vapor, oxygen, and other chemicals to form sulfuric acid and nitric acid

. These acids then mix with precipitation-rain, snow, fog, or hail-and fall to the ground as acid rain or acid deposition

. Natural sources like volcanic eruptions and lightning also release some sulfur dioxide and nitrogen oxides, but human activities are the dominant contributors

. The acidic precipitation typically has a pH between 4.2 and 4.4, which is more acidic than normal rain (pH about 5.6)

. In summary, acid rain forms when sulfur dioxide and nitrogen oxides emitted into the air chemically react with water and oxygen to produce acidic compounds that fall to the earth in wet or dry forms