Dr. Ryan Sullivan, Carnegie Mellon University – Origin of California Snow

Jul 10, 2013

In today’s Academic Minute, Dr. Ryan Sullivan of Carnegie Mellon University explains why California snow is dependent on dust from across the Pacific.

Ryan Sullivan is an assistant professor of mechanical engineering and chemistry at Carnegie Mellon University and a faculty member in the Center for Atmospheric Particle Studies. His research interests include the development of aircraft-deployable analytical instrumentation to characterize individual atmospheric particles in real-time. He holds a Ph.D. from the University of California, San Diego.

About Dr. Sullivan

Dr. Ryan Sullivan – Origin of California Snow

Snow in the Sierra Nevada Mountains is an important source of water and hydroelectric power for the state of California. Knowing when it will snow, and how much snow to expect, are crucial questions. But, as anyone who has ever relied on a weather forecast knows, predicting rain or snow can be difficult.

In this part of the Calwater study, we wanted to investigate what triggers large snowstorms in the Sierra Nevadas. To do this we sampled the clouds with a specially equipped aircraft, and took atmospheric measurements from ground stations, and satellites.
Clouds are made up of water droplets and/or ice crystals. When the liquid droplets freeze, they steal water from other remaining droplets. This causes the ice crystals to grow quickly and eventually fall to the Earth in the form of snow.
But, this freezing process is not simple. Cloud droplets do not actually freeze at 0 degrees Celsius like you might expect, but must cool to below negative 38 degrees Celcius. This is colder than most clouds. So for cloud droplets to freeze, they need help. This help comes from unique atmospheric particles called ice nuclei. Ice nuclei are rare, about 1 particle out of a million in the atmosphere, and are mostly made up of mineral dust and biological particles, such as bacteria.
We found that during snowstorms, the clouds above the mountains were often seeded with mineral dust and biological particles that had travelled across the Pacific Ocean from Asia. When the clouds didn’t have these particles, they were more likely to produce rain instead of snow.

This shows that natural particles from far-away places like Asia can impact clouds and precipitation in the Western U.S. By knowing this, we can begin to improve weather and winter storm predictions. We can also better understand how things like changing wind patterns might impact our climate.


Production support for the Academic Minute comes from Newman’s Own, giving all profits to charity and pursuing the common good for over 30 years, and from Mount Holyoke College.