The traditional measure of the natural pH of rainwater is around 5.7. However, considering all chemical compounds in the environment leads to modern scientists deducing that the pH is in the region of 5 to 7.
The pH of pure water is 7; however, there are always impurities in rainwater because it stays in equilibrium with carbon dioxide in the atmosphere. Carbon dioxide is weakly acidic, so it causes rainwater to be naturally acidic as well. This leads to a calculation of a pH of around 5.7 for rainwater, according to Harvard University.
In addition to carbon dioxide, there are other acids in the atmosphere, such as sulfuric acid resulting from emissions from the biosphere and volcanoes, and nitric acid from fires and lightning. There is some degree of balancing out of these acids in rainwater with natural substances in the atmosphere, such as calcium carbonate from soil dust and ammonia from biosphere emissions. Considering all of these chemical compounds results in the pH of rainwater actually falling between 5 and 7.
If the pH of rainwater drops below 5, it is called acid rain. This generally only occurs as a result of extreme levels of environmental pollution caused by human activities.