Here's why:

* Carbon Dioxide (CO2) in the Atmosphere: Rainwater absorbs carbon dioxide from the atmosphere. CO2 dissolves in water to form carbonic acid (H2CO3), which then releases hydrogen ions (H+), making the water acidic.

* Natural Variations: The actual pH of rainwater can vary depending on factors like:

* Location: Industrial areas or areas with high levels of air pollution might have more acidic rain.

* Time of Year: Rainwater can be slightly more acidic during drier periods when there is less water to dilute pollutants.

It's important to note that "acid rain" refers to rain with a pH below 5.0, significantly more acidic than typical rainwater. Acid rain is primarily caused by human activities that release sulfur dioxide and nitrogen oxides into the atmosphere.