STEAMBOAT SPRINGS, Colo. — Every snowflake tells a story to the scientists who sift for clues among the ice crystals.
And the one being told by the flakes that fall each winter near the summit of Mt. Werner here sounds ominous for the future of water in the drought-parched West.
Along the western slope of the Rockies, in a laboratory 10,500 feet above sea level, a team of atmospheric researchers has spent the past decade deciphering a deeper meaning from the blizzards that blanket the Steamboat ski resort in its famously pillowy "champagne powder."
They have skimmed snow clouds with screens to size up their icy content. They have zapped falling flakes with lasers to record digital images of the hexagonal shapes. They have captured crystals in a contraption that melts them with a heat gun and measures the mass of the water droplets.
And they have come to a provocative conclusion: Air pollution is reducing mountain snowfall, the wellspring of drinking water for Los Angeles, Las Vegas and much of the urban West.
Storm clouds packed with microscopic particles from diesel trucks, coal-burning power plants and cow manure produce far less snow than clouds comparatively free of pollution, the scientists from the University of Nevada's Desert Research Institute have determined.
In a study of two otherwise identical storms published earlier this year -- one dirty and one clean -- they found that the system sullied by specks of air pollution snowed 50% less. And the snow that did fall contained 25% less water.
"The difference can be as much as 50% in the mass of a snowflake," said Douglas Lowenthal, who conducted the study with colleague Randy Borys. "When you aggregate all those snowflakes, you can have a pretty significant effect."
All clouds are full of aerosols, or tiny particles suspended in the air. Most of the little bits are tossed into the air naturally, kicked up by sea spray, dust storms, wildfires and volcanoes.
But human activities are thickening the particle stew, particularly in the air above industrialized nations.
Ten percent of the particles now in the air came from man-made causes such as the burning of fossil fuels, scientists estimate. The majority of the particles are so small that they are not restricted by any environmental regulations.
Numerous scientific researchers are showing that pollution particles in clouds can reduce precipitation. But the Colorado snowflake studies provide some of the most detailed evidence yet that it is happening.
Many atmospheric experts believe the phenomenon could prove to be one of the most significant ways in which human beings are altering the Earth's weather patterns.
Yet it has received little public attention compared with a more controversial aspect of climate change, global warming, which many experts already consider a threat to Western water supplies.
Ironically, the same pollution particles may also help reduce the hothouse effects of global warming by reflecting some of the sun's rays back into space.
Indeed, without any particles, clouds would not form rain or snow.
Inside clouds, water condenses, or changes from vapor to liquid, around individual particles, forming water droplets. The droplets collide with one another and coalesce into larger drops, or if it's cold enough, ice crystals. Repeated collisions result in drops so heavy that gravity pulls them to the ground. Along the way, they bump into other drops, forming larger raindrops and snowflakes.
In polluted clouds, there are more particles, and as condensation occurs, smaller droplets form.
Even in cold winter skies, much of the moisture evaporates before reaching the ground as snow. The snowflakes that do fall to earth tend to be scrawnier, with less moisture content.
Digital photographs from the two storms the Desert Research Institute scientists studied show clear differences in the shape and size of the snowflakes.
The pollution-laden storm produced the stereotypically pretty snowflakes one is likely to see on a Christmas card: clear and thin, with intricate geometric patterns.
The clean storm produced the kind of snowflakes that in reality are far more common: opaque and lumpy, less dainty because they are more waterlogged.
How Much Moisture?
From years of storm studies, Borys and Lowenthal knew that clouds sometimes contained excessive amounts of particles from air pollution.
The answer they sought from the snowflakes was whether the clouds were releasing more or less moisture as a result of the pollution. They suspected it was less. And from their Storm Peak Laboratory atop Mt. Werner, they knew they had the perfect vantage point to find out.
In jean jacket weather on a gentle summer night recently, the laboratory offered panoramic views of the Rockies and the starlit western sky. But in winter, it resembles a scene from the movie "Ice Station Zebra."