Figuring out the dividing line between rain and snow has long flummoxed forecasters, especially in places like the high country of the American West, where complex topography and dramatic elevation differences shape the weather. … To gain a clearer picture of the rain-snow transition and its impact on the water cycle, scientists have been using a free phone app and data from thousands of volunteer observers who provide real-time reports of what precipitation type they’re seeing. The observations from the NASA-funded citizen science project—known as Mountain Rain or Snow—have highlighted the shortcomings of existing approaches to differentiating the phases of precipitation, according to a study published in Geophysical Research Letters in December.