Crystal Clear Warnings
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.
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Detailed Evidence
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.
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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.”
The summit is surrounded by storm clouds that dump 25 feet of snow a year. The wind chill can send temperatures plummeting to 50 degrees below zero. Visibility drops to 30 feet.
Snowstorms envelop the mountaintop roughly 30% of the time, and as skiers glide down the black-diamond trails below, a team of scientists studies snowflakes around the clock, waking in shifts from triple-decker bunk beds to check conditions through the night.
The altitude and precise position of the mountain provide scientists with ideal conditions to study falling snow as storms assault the summit. It’s even better than flying planes into the clouds.
Snow is a far better research subject than rain, because the shape and size of each crystal explains where and how it was created in the atmosphere.
“These crystals tell you a story, and they’re very specific,” Borys said. “Snow is unique in that it records a lot of processes. It gives you a lot of information.”
To reach their conclusions, the scientists examined tens of thousands of snowflakes, using a torpedo-shaped laser monitor and a device known as a “snow video spectrometer” that recorded pictures of individual ice crystals.
On the roof of the laboratory, they mounted screens that measured the amount of condensation remaining in the clouds that passed overhead. The moisture turned to ice as soon as it struck them, and they resembled cotton candy.
Using satellite images of other storms, Daniel J. Rosenfeld, an atmospheric scientist at Israel’s Hebrew University, was among the first to make a persuasive case that air pollution stunts rain and snow.
One of his studies, published in the journal Science in 2000, followed plumes of exhaust from a coal-fired power plant in Australia, factories in Turkey and a smelter in Canada.
It found that the polluting industries affected weather hundreds of miles away.
Earlier this year, Rosenfeld published a study of precipitation patterns downwind of urban centers in California and Israel. He found that storm systems that had passed over heavily polluted areas such as Los Angeles and the Bay Area snowed less when they emptied out over the Sierra Nevada.
Similar storm systems that passed over the less industrialized North Coast released more snow in the Sierra. “But when you go north to Seattle, it occurs again,” Rosenfeld said. “Give me another weather pattern that works like that.”
J. Marshall Shepherd, a scientist with the National Aeronautics and Space Administration who studies global rain patterns, agreed that Rosenfeld and other scientists are showing that pollution can reduce precipitation in high altitudes.
But other studies, including research by Shepherd on thunderstorms downwind of Houston, suggest that some of the same factors can actually increase the intensity of rain in lower-lying areas, where clouds hold together longer before finally releasing torrential downpours.
Many human activities appear to be altering storm clouds, including the hotter air that emanates from dense urban centers, known as the heat island effect. It will probably take scientists decades to fully understand what is happening, and how water supplies in different parts of the country are being affected, Shepherd said.
“There is certainly convincing evidence that aerosols are having an effect on precipitation,” he said, “but there are other factors, including the urban heat island effect, and we are not sure how it all works just yet.”
In the water-starved American West, where mountain snows nourish vast croplands and fast-growing cities, any loss of snow due to air pollution could be deeply felt. California has built a vast network of reservoirs and aqueducts to capture and collect water as the Sierra snowpack melts during spring and summer.
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Alarm, and Skepticism
The idea that air pollution could be reducing the snowpack has alarmed some water experts, who note that population growth is already straining resources and that global warming already appears to be diminishing snowfall. A study by 19 scientists released last month concluded that global warming could reduce the Sierra snowpack as much as 89% by the next century.
Maurice Roos, California’s chief hydrologist, said he is skeptical of the scientists’ claims.
Roos has reviewed historical records, and concluded that though the snowpack has clearly diminished over the last 40 years, the trend is more likely linked to a slight uptick in temperatures.
If air pollution was causing the serious effects Rosenfeld and others have suggested, the state should be losing far more water, Roos said.
“I would not be surprised if there was some influence” from air pollution, said Roos, who has spent 47 years as a state water official. “But we, happily, do not see the major changes in stream runoff that these guys are predicting.”
The Desert Research Institute’s scientists are the first to caution that the extent of the effect remains unknown.
They did not attempt to trace the origins of the pollution during their recent comparison of two snowstorms. But they had done a previous study that identified pollution in Utah, Nevada, and California as a major source of the particles that eventually wind up in the air above Mt. Werner. They were able to ferret out the culprit because the sulfate particles in the air had a distinctive fingerprint.
The scientists do not claim that air pollution is the cause of the drought that has afflicted much of the West over the last five years.
They attribute that to larger, more complex changes in global weather patterns. But they do suspect that air pollution is making the Western drought worse.
How serious are the snowfall losses? They plan to look at many more snowflakes in hopes of finding out.
“We think we have proved our hypothesis,” Borys said. “The next question becomes, how does this pan out over an entire season?”
