There's no argument that air pollution makes rain acidic, but there's been a lot of debate on whether pollution affects how much it rains.

Now some new research suggests that pollution actually REDUCES precipitation. Here's how it works: for clouds to form, you need microscopic particles in the air that water vapor can condense onto. This condensation forms tiny cloud drops, and one way to make rain is for lots of these cloud drops to collide and stick together.

The new research shows that when extra particles of pollution flood the air, you get more of the tiny cloud droplets than usual. But with all the competition, most never grow heavy enough to fall out of the cloud. The overall effect is for slightly less precipitation in areas where wind blows the pollution.

If this new research holds up, it adds a new chapter to the ways in which we're altering the environment.