Laser discrimination between rain and snow

In mountainous environments where rainfall or mixed rain and snow comprise significant portions of the winter precipitation, warm-storm and rain-induced peak streamflow discharges can cause flooding and reduce the efficiency of reservoir operations. Most snow accumulation and melt and streamflow forecasting models use air temperature-based procedures to differentiate between rain and snowfall. The precipitation type-air temperature relationship is not completely deterministic and the scarcity of meteorological stations in mountainous areas often necessitates assumptions on temperature change with elevation. A prototype rain-snowfall discriminator is described. The laser/photo-detector unit monitors the fall velocity and indexes the shape of particles passing vertically through a 3-meter horizontal beam. In a laboratory environment the device discriminates between drop former 'rainfall'' and two types of small paper punches. Future research needs include the development and evaluation of a probabalistic algorithm for distinguishing between the three precipitate types, a low power light-emitting diode laser, precipitates more closely resembling snow, and a field prototype gauge.