Acid from the clouds Lake Barkevatn in Aust-Agder county used to have healthy stocks of trout and perch. As a result of acid rain, the trout stock died out in the mid-1970s and the perch stock at the beginning of the 1980s. · Precipitation that is polluted by sulphur dioxide (SO2) and nitrogen oxides (NOX) is known as acid rain. · Acidity is measured in pH units: the lower the pH, the more acidic the substance. Unpolluted rain has a pH of roughly 5.6, but there are natural variations. Acid rain and snow, with a pH of about 4.4, has fallen in southern Norway for many years, causing serious environmental damage