LOS ANGELES (AP) —
One of the driest spots on Earth — the Sahara desert — is increasingly
responsible for snow and rain half a world away in the western U.S., a
new study released Thursday found.
It's no secret that winds
carrying dust, soot and even germs make transcontinental journeys
through the upper atmosphere that can affect the weather thousands of
miles away. Yet little is known about the impact of foreign pollutants
on the West Coast, which relies on mountain snowmelt for its water
needs.
Previous studies hinted these jet-setting particles may
retard rainfall in the Sierra Nevada mountains in Northern California by
reducing the size of water droplets in clouds. But scientists who flew
through storm clouds in an aircraft, measured rain and snow and analyzed
satellite imagery found the opposite: Far-flung dust and germs can help
stimulate precipitation.
During the 2011 winter, a team from the University of California, San Diego
and National Oceanic and Atmospheric Administration traced particles
suspended in clouds over the Sierra to distant origins — from the skies
over the arid Sahara that later mingled with other pollutants in China
and Mongolia before crossing the Pacific.
The days with the most
particles in the clouds were also "days when we see the most snow on the
ground," said study leader Kimberly Prather, an atmospheric chemistry
professor at UC San Diego, whose study was published online Thursday in the journal Science.
Scientists
believe wafting dust, grit and microbes — including bacteria and
viruses — can spur the formation of ice crystals in clouds that in turn
can influence how much rain or snow falls.
For years, governments
and utilities in California and other Western states have used cloud
seeding, in which a chemical vapor is sprayed into clouds, in a bid to
increase rainfall.
The new study shows how "Mother Nature has
figured out how to give us more precipitation" and that may lead to
changes in cloud-seeding efforts, which can be hit-or-miss, Prather
said.
David J. Smith at the NASA Kennedy Space Center said it was
refreshing to see measurements from the ground, air and orbit to tackle
how airborne particles affected Northern California snowfall.
"Such
a comprehensive approach is the only way to thoroughly examine global
transport" of particles, Smith, who had no role in the research, said in
an email.
Copyright 2013 The Associated Press.
This 2011 image provided by the National Oceanic and Atmospheric Administration shows a field survey site in California's Sierra Nevada mountains.
