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Étude sur l'effet de pollution sur les orages

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t's raining again? It must be Wednesday

Summertime storms get boost from workweek pollution, study suggests

Last Updated: Monday, February 4, 2008 | 12:05 PM ET

CBC News


More rain falls midweek than on weekends in the southeastern United States, say NASA atmospheric scientists, who claim the disparity is likely a result of air pollution from humans.


Rainfall data acquired from NASA's Tropical Rainfall Measuring Mission (TRMM) satellite found midweek storms tend to be stronger, drop more rain and cover larger areas in the southeast than weekend storms, NASA said on Friday.


Thomas Bell, an atmospheric scientist at NASA's Goddard Space Flight Center, suggests in a statement that the timing of the fiercer storms coincides with peaks in atmospheric pollution.


"It's eerie to think that we're affecting the weather," said Bell, lead author of the study published online last week in the American Geophysical Union's Journal of Geophysical Research. "It appears that we're making storms more violent."


The researchers found, on average, that it rained more between Tuesday and Thursday than it did from Saturday through Monday.


The findings, taken from data from 1998 to 2005, seem to run counter to previous studies that suggested an increase in airborne particulates could reduce rainfall.


But Bell and his research group suggest the two findings are not totally at odds. Both theories, he says, have their root in the process by which airborne particles interact with the water and ice found in clouds to form water droplets or ice crystals.


Researchers who suggest pollution leads to a drop in precipitation say the increase in particulates makes the droplets and crystals smaller, and thus more likely to dissipate as gas instead of as rain or snow.


But Bell's team suggests such a process can be thwarted when conditions are favourable to larger storms, with updrafts carrying the pollution-seeded raindrops higher into the atmosphere, where they condense and freeze rather than dissipate. Data from the TRMM satellites support this theory, showing midweek storms climbed to higher altitudes than weekend storms.


Bell suggested the new data might help improve the accuracy of weather forecasts, which he suggested "probably under-predict rain during the week and over-predict rain on weekends."

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