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inquinamento atmosferico interferisce con precipitazioni

Cari tutti,
l'inquinamento atmosferico puo' interferire con le precipitazioni globali, 
influenzando le dimensioni delle goccioline di pioggia nelle nuvole, e 
diminuendone il volume e peso medi.
Alessandro Gimona

American Association for the Advancement of Science
Air pollution can prevent rainfall, reported in the 10 March 2000 issue of
Washington D.C. - Urban and industrial air pollution can stifle rain and 
snowfall, a new study shows, because the pollution particles prevent cloud 
water from condensing into raindrops and snowflakes. These findings are 
reported in the 10 March issue of Science.
The new study, by Daniel Rosenfeld, of the Hebrew University of Jerusalem, 
presents satellite images and measurements of "pollution tracks" downstream 
from major urban areas and air pollution sources such as power plants, lead 
smelters, and oil refineries. The tracks consist of polluted clouds that 
have shut off virtually all precipitation because they contain abnormally 
small water droplets.
The droplets' small size is caused by pollution particles that act as 
"seeding" sites around which cloud moisture condenses. Approximately one 
million small droplets must collide and coalesce in order to make a 
precipitation-sized drop-that is, one large enough to fall below the cloud 
base and reach the ground before evaporating. In polluted clouds, there are 
too many small droplets and not enough larger ones. These small droplets 
float in the air with low probability of bumping into each other and merging 
into raindrops. The smaller droplets are also slower to freeze into ice 
crystals, resulting in less sleet and snowfall.
Because urban and industrial air pollution is a significant problem in many 
regions of the world, Rosenfeld's findings suggest that human activity may 
be affecting rainfall patterns on a global scale.
These data are the first direct evidence of how urban and industrial 
pollution affects rainfall levels, a question scientists have debated for 
several decades. In fact, some previous studies have concluded that air 
pollution might increase rainfall, but the debate has continued due to a 
lack of convincing data.
"In the past, scientists had to collect information by poking little holes 
in clouds from airplanes, or using statistics about rainfall patterns 
because you can't replicate rain-clouds in the lab. Now, new satellite 
instruments allow us to have a comprehensive look at the problem. For the 
first time, we can measure cloud precipitation and microstructure 
simultaneously over large areas," Rosenfeld said.
In his Science paper, Rosenfeld presents the first images of pollution 
tracks over land. The images, taken over regions in Turkey, Canada, and 
Australia, all contain known sources of industrial or urban air pollution.  
The tracks stream away from these pollution sources in long narrow plumes.
Rosenfeld took yet a closer look at the pollution tracks in Australia, where 
the plumes were particularly striking. Further measurements from a bevy of 
satellite instruments showed that precipitation of both raindrops and ice 
crystals-which was occurring in the unpolluted clouds-was practically shut 
off in the clouds within the pollution tracks. However, the total amount of 
moisture in the polluted clouds was sufficient to produce rain and snow.
Rosenfeld also notes that in other parts of the world air pollution is more 
widespread and not as easy to distinguish as it is against the relatively 
clean Australian atmosphere. Thus the well-defined tracks identified in the 
study "serve as a Rosetta stone for the potential impact of more widely 
distributed aerosol pollution on clouds," writes Owen Toon, of the 
University of Colorado at Boulder, in a related commentary article.

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