Acid rain refers to any form of precipitation (rain, snow, sleet, or hail) that has a higher acidity than normal, typically with a pH level below 5.6. It is primarily formed when sulfur dioxide (SO2) and nitrogen oxides (NOx) are released into the atmosphere, often from industrial processes, vehicle emissions, and the burning of fossil fuels. These pollutants react with water vapor, oxygen, and other chemicals in the atmosphere to produce sulfuric and nitric acids. When these acids mix with rainwater, they create acid rain, which can have harmful effects on the environment, human health, and infrastructure.