Over the northern Atlantic Ocean, clouds that often produce heavy rain
storms grew taller and were more frequent when plumes of pollution from
North America or dust from Africa's Sahara Desert were present.

However, when smoke from large fires billowed into the sky over South
America's Amazon River basin, clouds were consistently fewer than when
the air was relatively clear.

Aerosols sometimes stop clouds from forming and in other cases increase
cloud cover. Clouds deliver water around the globe and they also help
regulate how much of the sun's warmth the planet holds.

The capacity of air pollution to absorb energy from the sun is the key.

Separating the real effects of the aerosols from the coincidental
effect of the meteorology was a hard problem to solve. In addition, the
impact of aerosols is difficult to observe because aerosols only stay
airborne for about one week

Observations suggest it is the darkness or brightness of aerosol
pollution and not weather factors that cause pollution to act as a
cloud killer or a cloud maker.

http://www.nasa.gov/vision/earth/env...on_clouds.html