Acid Rain usually refers to precipitation with pH less than:

Normal, unpolluted rainwater is already slightly acidic, with a pH of around 5.6. This natural acidity occurs because carbon dioxide ($CO_2$) in the atmosphere dissolves into the water droplets to form a weak acid called carbonic acid ($H_2CO_3$). Therefore, precipitation is officially classified as acid rain only when its pH drops below this natural baseline of 5.6.How Acid Rain FormsAcid rain is primarily caused by the emission of Sulfur Dioxide ($SO_2$) and Nitrogen Oxides ($NO_x$) into the atmosphere, largely from burning fossil fuels (like coal in power plants) and vehicle exhaust.These gases react with water oxygen, and other chemicals in the atmosphere to form sulfuric acid ($H_2SO_4$) and nitric acid ($HNO_3$), which then fall to the ground as wet or dry deposition.