Standing too close to a smoky fire can make your eyes water. But for some clouds, smoke has the opposite effect. Satellite observations have shown for the first time that smoke from forest fires can shut off rainfall from storm clouds in the tropics, according to a report in today's issue of Geophysical Research Letters. If urban air pollution turns the same trick, then rainfall near large cities may be suppressed markedly.
Smoke wafting into a cloud provides countless tiny particles upon which water vapor can condense. Atmospheric scientists believe that such particles make rain less likely, because they spread the available water among many smaller droplets that stay afloat in the cloud. During the last 30 years, researchers have noted an apparent drop in precipitation behind exhaust-belching ocean vessels and downwind from burning agricultural fields. However, they had no direct evidence that tied those drier patches to smoke and not some other factor.
A joint NASA-Japanese satellite filled that gap by measuring the sizes of water droplets and detecting rainfall amounts in tropical clouds. The satellite, called the Tropical Rainfall Measuring Mission  (TRMM), flew over Indonesia in March 1998 during massive forest fires there. At one point, much of the island of Borneo was shrouded by a thick pall, while part of it was still clear of smoke. Radar and infrared data from TRMM showed that clouds within the two regions grew to similar sizes and heights. Clouds in the smoke-free area dumped normal levels of rain, but no water fell from the smoke-laden clouds, says atmospheric scientist Daniel Rosenfeld of the Hebrew University of Jerusalem in Israel, the paper's author.
Vast plumes of smoke from forest fires and agricultural burning are common in the Amazon Basin and elsewhere in the tropics, Rosenfeld notes. "This effect may have a significant impact on global tropical rainfall," he says. But outside the tropics, he observes, smoke is less likely to reduce rainfall. Many clouds at mid-latitudes make rain by freezing water into ice crystals (which fall from the cloud then melt before they hit the ground), rather than by coalescing warm water droplets together. Smoke does not appear to limit the growth of ice crystals, although it may trigger more lightning in such storms (Science, 2 October 1998, p. 21 ).
"We've suspected this for a long time, but [Rosenfeld's work] is the smoking gun that we were missing," says TRMM project scientist Chris Kummerow of NASA's Goddard Space Flight Center in Greenbelt, Maryland. Kummerow thinks industrial pollution has a similar drying influence on clouds. "There's not much difference between urban pollution and smoke," he says. Indeed, Rosenfeld says data suggest that such an "urban rain shadow" exists near Manila in the Philippines.