From Outside Magazine (Katarina Zimmer):
As the climate warms, Colorado may see fewer days of this light powder and more of the heavy, moist stuff, according to Noah Molotch, a professor of snow hydrology who directs the Center for Water, Earth Science, and Technology at the University of Colorado, Boulder. “One of the first signals that I think we’ll see related to climate change in Colorado is an increase in snowfall density,” Molotch says. “As it gets warmer, that snow will be less fluffy and heavier.”
The reason lies in how snowflakes form, thousands of feet above the ground. Each snowflake begins when water vapor in clouds condenses around particulates—like pollen or dust—creating ice crystals, which begin to grow outward. Because of the unique features of water vapor movement at icy temperatures, vapor will condense only onto the very tips of the crystals, forming six arms, each splitting into many branches—ultimately, a snowflake. True to the cliché, each one is indeed unique.
For the quintessential, picture-book snowflake to form, the temperature must be between minus 8 and 15 degrees Fahrenheit, with a relatively high level of moisture in the air. Different shapes arise at temperatures slightly warmer than 15 degrees but still below freezing: columns, prisms, and needles that don’t look anything like traditional snowflakes. “If one were to cast judgment on the beauty of snowflakes, these would not win the beauty contest,” Molotch says.
When the air is warmer, the movement of water vapor inside clouds will slow down. Instead of condensing onto the outermost tips of ice crystals, water vapor will build all around it, rendering its nascent six-sided structure indistinguishable. Ultimately, this creates thick blobs that often collide with water droplets and other flakes in the air and reach the ground as dense, heavy snow. David Robinson, a snow scientist at New Jersey’s Rutgers University, calls it “packing snow,” which is great for crushing into snowballs and snowmen but makes for a more arduous skiing experience.
No studies have attempted to document an increase in snow density over time, Molotch says. Gathering that data isn’t something scientists can easily determine from satellites or airplanes and would therefore require a lot of work on a large scale. But based on well-understood physical principles about how snow forms under different temperatures in the atmosphere, it’s likely that Colorado skiers may gradually see less light powder as the climate warms—although, Molotch adds, the state’s snow still has a way to go until it becomes as heavy as snow in the Sierra.