Clouds of smoke don't necessarily bring clouds of rain. Researchers have found that heavy smoke over the Amazon River Basin interferes with the formation of clouds. This can reduce or delay rainfall, two teams report in the 27 February issue of Science, and it can make the storms that do occur more violent.
All clouds need a certain amount of smoke and dust in order for water droplets to condense. To understand the effects of heavy smoke over the Amazon, two groups compared satellite pictures of Amazon fires with measurements of smoke, rain, and water droplets in clouds. Meinrat Andreae of the Max Planck Institute for Chemistry in Mainz, Germany, and colleagues report that a surfeit of smoke particles can delay rainfall, because small droplets form instead of fewer, larger ones. These small droplets don't fall as rain but rise higher up in the atmosphere, releasing energy as they cool. When the rain finally does fall, that extra energy can create especially violent thunderstorms. The thunderstorms in turn can suck even more smoke and heat energy into the upper atmosphere, creating a cycle of violent storms.
The other group describes an even more insidious effect of smoke: the prevention of cloud formation. The veil of smoke hanging over the forest reflects a lot of sunlight, and according to Yoram Kaufman of NASA's Goddard Space Flight Center in Greenbelt, Maryland, and colleagues, up to 20% of the days during the Amazon's dry season have smoke so thick that only a third of the sun's energy passes to the ground. This cools the ground, preventing moist air from warming and rising into the atmosphere to form clouds. Fewer clouds mean less sunlight is reflected away from Earth, and the atmosphere warms. Perversely, the smoke that cools the ground can have a significant warming effect on the planet, says Kaufman.
The studies "show clearly how burning can actually affect clouds," says Mario Molina of the Massachusetts Institute of Technology in Cambridge. Fires are burning all over the tropics, and the centuries-old agricultural practice is unlikely to end soon. Smoke from these fires has a significant, if complex effect on local climate and global warming, according to Molina. "The bottom line" about smoke's impact, he says, is that "it's very strong."