Climate change, humidity, and the future of snowSource: Mountain West News
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In a new study published today in Proceedings of the National Academy of Sciences, University of Utah professor Paul Brooks and University of Nevada Reno professor Adrian Harpold show that changes in humidity may determine how the contribution of snowpack to streams, lakes and groundwater changes as the climate warms. Surprisingly, cloudy, gray and humid winter days can actually cause the snowpack to warm faster, increasing the likelihood of melt during winter months when the snowpack should be growing, the authors report. In contrast, under clear skies and low humidity the snow can become colder than the air, preserving the snowpack until spring.
Scientists know that there are various forms of energy, including sensible heat (which we measure as temperature), radiant energy (like what we feel from the sun), and latent heat. Latent heat is stealthier — it’s released and absorbed as water changes phase, for example between ice and liquid water. You experience the power of latent heat on a sweaty summer day. As the sweat on your skin evaporates, it absorbs heat in the transition from liquid water to water vapor, cooling you off in the process.
So how does this apply to snow melting? Snow’s brilliant white results from snow crystals reflecting incoming solar radiation. This minimizes energy input to the snow, and also leads to the sunburn so common when skiing on sunny winter days. The molecular structure of snow crystals also emits energy back into the sky on clear nights — which serves to cool the snowpack. Also, snow on dry days can “sublimate,” or change directly from a solid to vapor. This process, just like evaporation, absorbs heat and further cools the snow.
“That is one of the reasons skiing in Utah, Colorado, New Mexico or the Eastern Sierra can be so fun!” Brooks says. “The snow stays cold and dry and powdery, while the sun warms us as we ski or sit on a deck and admire the view — especially if we wear dark colors.”
Cloudy, humid days reverse the cooling from both radiation and sublimation — cloud cover prevents snow from emitting energy, and condensation of water vapor on the snow releases latent heat, warming the snow. That is why a couple of humid days with temperatures right around freezing result in large melt events and even minor flooding. An extreme case of this can come on foggy days, Brooks says. “We often say ‘fog eats snow.’”