Here's why:

* Normal rain is slightly acidic: Even "clean" rain is slightly acidic, with a pH of around 5.6. This is due to the natural presence of carbon dioxide in the atmosphere, which dissolves into water to form carbonic acid.

* Acid rain is more acidic: Acid rain has a pH lower than 5.6. It gets its acidity from pollutants like sulfur dioxide and nitrogen oxides, which are released into the atmosphere by burning fossil fuels and other industrial processes. These pollutants react with water, oxygen, and other substances to form sulfuric acid and nitric acid, which then fall to the ground as acid rain.

The lower the pH, the more acidic the solution. So, acid rain is more acidic than regular rain, and its low pH can have damaging effects on the environment.