Spatial variation of the rain-snow temperature threshold across the Northern Hemisphere

We present the first continuous map of rain-snow air temperature thresholds over the Northern Hemisphere land surface, underlining the spatial variability of precipitation phase partitioning. Land surface models typically discriminate between rain and snow using a simple, spatially uniform air temperature threshold, but observations indicate the threshold is not static. Our analysis of a 29-year observational dataset (n = 17.8 million) shows the threshold varies significantly, averaging 1.0°C and ranging from -0.4°C to 2.4°C for 95% of Northern Hemisphere stations, with continental climates exhibiting the warmest thresholds and maritime the coolest. Relative humidity exerts the strongest control on phase partitioning, while surface pressure plays a secondary role. Simulations indicate the selection of a rain-snow threshold introduces significant uncertainty to snowfall frequency and that including relative humidity as a predictor variable provides the greatest improvement to precipitation phase prediction between 0.6°C and 3.4°C, the interval where phase partitioning is most uncertain.