Why Raindrops Keep Falling . . . Or Do Not

Research published in the Sept. 5 issue of Science puts two of the biggest problems in climate modelingâ''the respective roles of clouds and aerosolsâ''in a new perspective. Of the greenhouse gases in the atmosphere, the one that has the biggest overall effects is a natural one, water vapor. Yet the impacts of clouds on temperature are incredibly complicated and still quite poorly understood: depending on their height, density, and other factors, they can either trap radiation or reflect it back into space. Aerosolsâ''tiny particles or drops of liquid, suspended in a gasâ''also have big effects on climate, and those effects also are complicated and ambiguous; black carbon particles, for example, reflect radiation and dampen warming locally or regionally, and yet also dry out the land they blanket, aggravating droughts.

The article published today in Science reviews the scientific literature on the relationship between aerosols and rainfall and comes to a striking conclusion. The authors find that rainfall is greatest when aerosols levels are intermediate, not too big and not too small, but just right. Water precipitates out too fastâ''may we say too precipitously?â''from clouds in clean air with little aerosol content, so that the really big clouds that produce heavy rainfall never form. On the other hand, clouds in heavily polluted air get so warm that most water evaporates out of them before having the opportunity to form raindrops.

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