Earth & Sky

Air pollution can change cloud cover, say scientists

DB: This is Earth & Sky. Air pollution might enhance or stunt the formation of clouds.

JB: It all depends on whether the pollution particles are relatively light or dark in color, according to research by the late atmospheric scientist Yorum Kaufman and Ilan Koren of the Weizman Institute of Science in Israel.

DB: These floating particles of pollution – called aerosols – are the seeds around which clouds form. The particles get cold as they ride high in the sky, and water vapor condenses onto them, like dew on morning grass. Koren told Earth & Sky that more pollution particles don’t always mean more clouds.

Ilan Koren: Sometimes, more pollution would correlate with more clouds. And sometimes, for a different type of pollution, or for a different type of aerosol, more pollution would choke, or would suppress clouds.

JB: In other words, Koren and Kaufman found that darker particles – such as soot from wildfires – soak up sunlight and heat up. That prevents water vapor from condensing around them, so clouds don’t form.

DB: But particles from fossil fuel burning reflect sunlight. They’re cooler and do form clouds. But these particles may be too small to easily cluster into droplets big enough to fall as rain. And that might leave areas downwind of a city high and dry. Our thanks today to NASA. We’re Block and Byrd for Earth and Sky.