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Forest fire smoke inhibits rainfall, study says

Can fire
New data proves that smoke from forest fires inhibits rainfall.  

October 7, 1999
Web posted at: 2:49 p.m. EDT (1849 GMT)


Smoke from forest fires reduces rainfall, according to data collected by NASA.

Scientists have known for a while that smoke from burning vegetation led to decreased rainfall but this is the first direct evidence of the phenomenon, said Dr. Daniel Rosenfeld, a scientist at the Institute of Earth Sciences, Hebrew University of Jerusalem.

Raindrops in the atmosphere form in two ways — a warm rain process and one involving ice. In the warm rain process, a few cloud drops get large enough to start falling. As they fall, they pick up other cloud drops until they become large enough to fall to Earth as rain.

The newly analyzed data shows that when tropical clouds are saturated with heavy smoke from forest fires, the warm rain process is almost completely shut off. The smoke-saturated clouds must grow above the freezing level of the atmosphere, which is 16,000 feet above sea level, in order for the clouds to start producing rain by the alternative mechanism of ice.

TRMM image of the island country Borneo shows precipitation in dark blue, rain droplet size in light blue (brighter is larger) and fires in red.  

In clean air, rain forms in significantly smaller clouds without ice.

The data on smoke was collected by a satellite on March 1, 1998, over Borneo (Kalimantan), Indonesia. At the time, the southeastern portion of the island was engulfed in smoke, while the northwestern portion was relatively smoke free. The spacecraft's radar detected precipitation in smoke-free clouds, but almost none in the smoke-infested clouds, showing the impact of smoke from fires on precipitation over the rain forest.

"It's important to note that this is not a unique case," said Rosenfeld. "We observed and documented several other cases that showed similar behavior. In some instances, even less severe smoke concentration was found to have comparable impacts on clouds."

"Findings such as these are making the first inroads into the difficult problem of understanding humanity's impacts on global precipitation," said Dr. Christian Kummerow, TRMM project scientist at NASA's Goddard Space Flight Center.

Tropical rainfall, which falls within 35 degrees north and south of the equator, comprises more than two-thirds of the Earth's rainfall.

The data was collected as part of a Tropical Rainfall Measuring Mission, a United States/Japanese mission and part of NASA's Earth Science Enterprise, a long-term research program designed to study the Earth's land, oceans, air, ice and life as a total system. The study, entitled "TRMM Observed First Direct Evidence of Smoke from Forest Fires Inhibiting Rainfall," will be published in the Oct. 15 issue of Geophysical Research Letters.

Copyright 1999, Environmental News Network, All Rights Reserved

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Hebrew University of Jerusalem - Institute Earth Sciences
NASA Goddard Space Flight Center
Geophysical Research Letters
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