pH of Acid Rain over a Period of 50 years

These maps show how the areas of lower pH have spread in a 30 years 1955-1988. Darkest area is lowest pH. Since the Clean Air Act Amendments of 1990, there have been significant decreases in the amount of sulfur oxides escaping from the electric power plants. As a result there has been a measurable reduction in the amount of acid rain, which is actually translated as an increase in the pH levels (higher pH means less acid).