Sahara desert dust stimulates snow, rainfall in US, study says

One of the driest spots on Earth - the Sahara Desert - is increasingly responsible for snow and rain half a world away in the western United States, a new study found.

It is no secret that winds carrying dust, soot and even germs make transcontinental journeys through the upper atmosphere that can affect the weather thousands of kilometres 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 analysed 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 on Thursday in the journal Science.

The new study shows how nature "has figured out how to give us more precipitation" and that may lead to changes in cloud-seeding efforts, she said.