Saharan, Asian Dust Contribute to Ice Particles that Lead to Precipitation Over U.S., Says New Colorado State University Study in Science

Note to Reporters: Photos of Paul DeMott are available with the news release at

They’re the unseen air particles way up in the atmosphere, but they’re important: Even the ones that come from the other side of the planet are catalysts that help determine rain and snowfall levels.

Tiny specs of dust particles measured in the air over the Sierra Nevada mountains, that come from as far away as the Saharan desert, are vital to precipitation development, according to a new groundbreaking study in the journal Science this week led in part by a Colorado State University atmospheric chemist.

The research was led by Kimberly Prather of the University of California-San Diego and funded largely by the California Energy Commission, while the National Oceanic and Atmospheric Administration provided ground-based radar measurements and meteorological support. Measurements in clouds were done using a Department of Energy aircraft. A key contributor to the airborne measurements was Paul DeMott, a senior research scientist in the Department of Atmospheric Science at Colorado State University, who measures the process that enables ice to form in the atmosphere.

Formation of ice crystals is necessary for precipitation formation in many clouds, and DeMott has previously shown that the numbers of crystals formed is linked to the abundance of larger aerosol particles in the atmosphere. This latest study compares measurements of precipitation rates and chemistry taken on the ground and detailed studies of particles present in clouds. That’s DeMott’s expertise – capturing samples of air from higher points in the atmosphere in specially equipped planes.

“Although it takes water vapor being lifted over the mountains to make snow, the precipitation process is affected by seeds that help start the snow,” DeMott said. “You do need some things in the air that will help to make ice, and their abundance can vary. One of the more important factors is the transportation of dust. It’s a natural source that can be affected by humans and changes in climate.”

“It wouldn’t be something you’d recognize from the ground, but it’s there floating in the air and triggering a more efficient freezing process in clouds that would otherwise want to stay supercooled,” DeMott said. “We had clouds as cold as minus-20 degrees centrigrade that had very little ice in them. When dust layers were present, this would create the trigger that would lead to more efficient precipitation.”

Urban air that’s heavy with pollution has been associated with decreasing the amount of precipitation from clouds.

“Ultimately, we’re trying to understand the totality of aerosol effects,” DeMott said. “What types of particles are taking part in that process of making ice in the clouds?”

How capturing air from a plane works: CSU scientists take air samples into a small chamber through a special port on the side of the aircraft. A diffusion chamber cools and humidifies the air and particles between two plates of ice toward conditions where ice forms — essentially "growing" clouds by simulating the conditions in the atmosphere. Researchers then evaluate how many particles will form ice crystals for specific cloud conditions. In some studies, the plane passes through clouds to measure, with other instruments, how much ice really forms.

Scientists also used specialized instruments to determine the chemical makeup of the particulates forming ice.

DeMott, a member of the CSU general faculty since 1986, co-chairs the International Committee on Nucleation and Atmospheric Aerosols, which will host its 19th annual conference June 23-28 at Colorado State.