When the pH of rainwater falls below 5.6, it is commonly referred to as:

When the pH of rainwater falls below 5.6, it is commonly referred to as acid rain. Normal rainwater has a pH of around 5.6 due to the natural presence of carbon dioxide forming weak carbonic acid. When the pH drops below 5.6, due to pollutants like sulfur dioxide and nitrogen oxides mixing with rainwater, it becomes more acidic.