Acid rain is rain that is unusually acidic due to the presence of oxides of sulfur and nitrogen.
Acid rain usually has a pH of less than 5.6.
It is harmful in many ways as it dissolves the nutrients for plants, causes yellowing of monuments, causes respiratory ailments, etc.
Acid rain is formed due to emissions of Sulphur dioxide(SO2) and Nitrogen dioxide(NO2), which react with water molecules in the atmosphere to produce acids.