The dirty truth of snowmelt

Researchers study effects of dust on melting snow

By Marshall Swearingen

There's something dirty in the air causing snowmelt, and it's not ozone.

As summer approaches and the winter's snow begins to melt in the mountains, stream runoff across much of the western U.S. will be earlier and faster due to the presence of something ordinary and seemingly harmless: dust.

When fine particles of soil are carried by spring winds from the dry Southwest to the high elevations of the Rocky Mountains, they settle on the snow surface and increase absorption of sunlight, causing the snow to warm up more than it would otherwise.

"It's like putting on a black T-shirt on a sunny day," McKenzie Skiles, a researcher who visited Montana State University recently to study the phenomenon, said. Skiles, an assistant professor in the Geography Department at University of Utah, is partnering with MSU snow researcher Kevin Hammonds on the work. "Darkening snow with dust is a really effective way to melt snow faster," she said.

 University of Utah researcher McKenzie Skiles, right, and MSU researcher Kevin Hammonds handle snow samples in MSU's Subzero lab as part of a study about dust's effects on snowmelt. MSU photo by Kelly Gorham

Dust is so effective, in fact, that Skiles and other scientists concluded in a 2018 paper that dust was the single biggest factor — greater even than air temperature — affecting the timing of runoff in the Colorado River basin, which provides water for nearly 40 million people. That makes understanding dust-snow interaction a major concern for those who manage reservoirs and make other decisions about the West's agricultural and municipal water use.

But while dust's impact has become increasingly clear, questions remain about how exactly the tiny particles alter normal snowmelt. That's why, for two weeks in May, Skiles visited MSU to seek answers in the unique cold facilities of the Subzero Research Laboratory.

"There are some processes we can't study in nature, so it's really helpful to be able to do experiments in the lab like this," said Skiles, who has studied the phenomenon for years at field sites in Utah and western Colorado. "There aren't many places in the world that have this kind of setup."

In a specialized cold room at the Subzero lab, Skiles and Hammonds, who directs the lab and is an assistant professor in the Department of Civil Engineering in MSU's Norm Asbjornson College of Engineering, re-created how dust would affect a melting snowpack. They made snow using the lab's snow-maker and sprinkled varying amounts of dust over portions of the surface, then used a special lamp to simulate sunlight. With a variety of cameras, including some that sense invisible infrared light, they measured changes in the snow's properties.

According to Skiles, the melting effect of dust is more complicated than it might appear. When the darkened snow absorbs extra sunlight, the heated surface causes the snow grains to grow. The larger crystals, in turn, absorb more of the sun's radiation, further increasing the melting. It's thought that the dust particles themselves may even help the crystals grow.

"We don't understand that very well," Skiles said.

That's where one of the Subzero lab's most powerful instruments, called the microCT, comes in. A smaller version of the X-ray devices used to make non-invasive CAT scans of the body, the microCT allows the researchers to see what is happening with individual snow grains and precisely measure their size and structure.

Hammonds, who specializes in studying the microstructure of snow, said the experiment is one of the first of its kind to examine the dust-snow interaction at the microscopic level in a lab. "It's a project that's relevant to a lot of important questions," with implications for climate science and hydrological forecasting, he said.

Because other regions of the world experience a similar dusty phenomenon, the results of the experiment could have far-reaching impact. Ideally, Skiles and Hammonds said, insights generated by the study could help improve how satellite imaging and other remote sensing are used to measure dust's presence in snow and forecast the effects on runoff. The researchers expect to publish their findings later this summer or early this fall.