Acid rain

Acid rain is rain that is more acidic than in its natural state, and is usually considered to have a pH of less than 5 (Natural rain water is slightly acidic due to the presence of dissolved carbon dioxide). Acid rain forms when sulfur dioxide dissolves in water to make sulfurous acid. The sulfur dioxide can also be oxidized to sulfur trioxide and form sulfuric acid upon reaction with water.

Acid rain can cause damage to structures and statues, and also plant life. (For example, the Black forest of Germany has been heavily damaged by acid rain.)