Acid rain is any form of precipitation — rain, snow, sleet or fog — with a pH lower than 5.6.
Normal rain is already slightly acidic because atmospheric CO₂ dissolves in water to form weak carbonic acid (H₂CO₃). However, when factories, power plants and vehicles release large amounts of sulfur dioxide (SO₂) and nitrogen oxides (NOₓ), these gases react with water vapour in clouds to form much stronger acids.
The result? Rain pH can plunge to 4.0 or even lower — more than 10× more acidic than normal rain.
Key atmospheric reactions that form acid rain:
These strong acids dramatically lower the pH of rainwater, making it highly corrosive to living organisms and materials alike.