The physics of snow

In a fortuitous turn of events, the North Tahoe-Truckee region was blanketed in snow two weeks ago — the same day a presentation was scheduled on the physics of snow.

Ironically, in response to the foot-plus of snow that fell at lake level on Feb. 18, people wondered if the lecture at the UC Davis Tahoe Environmental Research Center in Incline Village would still occur.

“Some people are afraid to drive in the snow to come talk about snow,” said Heather Segale, TERC education and outreach coordinator, garnering laughs from the crowd at the start of the lecture that went on as scheduled. “I got a lot of phone calls — ‘Is the snow lecture still on?’”

Similarly, lecture presenter Bernhard “Benny” Bach, with the University of Nevada, Reno, said UNR colleagues asked if we would be able to make it up the mountain for the presentation.

“We live in the mountains,” he said. “We can make it — so thanks for showing up.”

More than 70 people were in attendance at that Thursday evening lecture, having traveled on roadways and in vehicles that only that morning were covered in snow.

Now, with a series of winter storms forecast to bring heavy precipitation — and potentially multiple inches of snow — to the Tahoe-Truckee region this weekend and into next, according to the National Weather Service, it’s timely for Tahoe residents and visitors to understand the physics of snow.

Snow Formation

For snow to be present on the ground, a process needs to occur above in the sky. When a westerly breeze off the Pacific Ocean hits the Sierra Nevada, it lifts and expands, cooling as it does so, Bach explained.

Eventually, the moist air reaches a point where it’s saturated, meaning it can’t hold any additional water vapor, and it starts to condense, forming a cloud.

A snow cloud has two layers — one where ice condenses, and the other one above where water condenses.

The ice condenses on a particle such as pollen or dust, and as it’s lifted through the cloud, it starts collecting the water vapor around it, he said.

“As it (climbs through the cloud), it continues to scavenge water vapor, and it continues to grow and continues to grow,” Bach said. “Eventually it’ll get heavy enough that it can no longer lift. ... It’ll start falling, but as that snowflake falls, it continues to scavenge water vapor out of the air.”

Should a snowflake hit warm air temperatures as it falls, then it melts and becomes rain at ground level, he explained in response to an audience question.

“When do we pick up most of our snow?” Bach posed. “It’s in the winter, but what temperature is it? It’s right around freezing. We don’t get a lot of snow when it’s below freezing; we don’t get any snow when it’s above freezing, but right around freezing is usually when you get your snowpack.

“So imagine if the Earth’s temperature came up by a degree. Are you going to get snow, or are you going to get rain? You’re going to get rain.”

Loss of snow, and Tahoe impacts

Whether it’s snowpack loss in the Sierra or receding glaciers in the Andes — where Bach has collected data as a member of the American Climber Science Program — there are humanitarian impacts, he said.

“All these valleys (at the base of Cordillera Blanca, part of the larger Andes range) are filled with livestock, like California,” Bach said, reflecting on a Cordillera Blanca expedition he took with ACSP. “All these guys, they don’t have reservoir; they are using the snowpack as their reservoir.

“If this starts to go away, it’s going to have a negative impact on their quality of life. … Their agriculture is completely dependent on this snow reservoir just like California has been, so that’s going to be an issue.”

Other impacts the Lake Tahoe region experienced during this most recent drought due to four consecutive mild winters include recreation, business and wildfire danger.

“ ... The more we understand about our environment, the better stewards we can be of the natural environment that we live in and all value,” said Crystal Bay resident Dennis Breen, who attended the snow lecture. “We have to preserve this.”

This winter started off strong with multiple big snowstorms — but February was relatively dry, with exception of that Feb. 18 early morning snowstorm.

However, with more snow in the forecast for this weekend and next week, it could be a wet start to March for Lake Tahoe.

“We’re coming out of a drought, hopefully, maybe, and we want as much snow as we can get, so understanding it makes it easier to live with it when you have to shovel it,” Breen said.