Acid rain is defined as rainwater with a pH of less than:

1. Ordinary rain is already slightly acidic from dissolved carbon dioxide. 2. The definition therefore sits below that natural baseline. 3. Acid rain is rainwater that has a pH of less than 5. 4. It arises from nonmetal oxides including sulfur dioxide, sulfur trioxide and the nitrogen oxides dissolving and forming sulfuric and nitric acids. _Source: OpenStax Chemistry 2e (CC BY 4.0), Ch 14 'Acid-Base Equilibria', sections 14.1-14.7_