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 rban 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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