Rain with pH values < about 5; commonly results from acids formed from pollutants. 'Pure' rain water equilibrated with atmospheric CO2
and naturally occurring acids in relatively clean air usually has a pH > 5.
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2170 (https://doi.org/10.1351/pac199062112167)