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inquinamento atmosferico interferisce con precipitazioni
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.
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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