Environmental Engineering professor Ben Livneh has co-authored a very informative study published by the journal Nature Communications about predicting whether precipitation will be rainfall or snowfall. Meteorological forecasting and climate simulations are commonly informed by the 32 degree Fahrenheit threshold: rain is expected above the threshold, and snow is expected below it. However, this CU Boulder study, which made millions of precipitation observations in over 100 countries across the Northern Hemisphere, found that the threshold is not static. Rather, it is a function of relative humidity and surface pressure, making it so coastal areas have a cooler threshold and inland and mountain areas a higher threshold; below-freezing temperatures could mean rain along the United States’ southeastern coast, while above-freezing temperatures could mean snow in the Rockies.
Why is snowfall versus rainfall important? Prof. Livneh explains, “Snow and rain differ greatly in the ways they affect climate. Snow acts as a water reservoir and reflects incoming sunlight, whereas if the same amount of precipitation falls as rain, that can dramatically change water resource management decisions.” For example, increased rain-on-snow events raise the risk of flooding. Snowfall versus rainfall predictions are thus especially important in areas crucial for freshwater, agriculture, and biodiversity. The map this study created of the Northern Hemisphere showing how location and humidity can affect precipitation is hoped to be improved upon by future research incorporating more data points from around the world.
Prof. Livneh has a PhD in Civil Engineering with an emphasis in hydrology from the University of Washington-Seattle. His research interests include the impacts of changing land cover and climate on water resources, land surface modeling and the terrestrial water balance, and regionalization of land surface processes and parameters. At CU Boulder, he has a joint appointment with the Cooperative Institute for Research in Environmental Studies (CIRES), and he teaches Modeling Hydrologic Systems and Engineering Hydrology.
Spatial variation of the rain–snow temperature threshold across the Northern Hemisphere
Keith S. Jennings, Taylor S. Winchell, Ben Livneh & Noah P. Molotch