Rain-on-snow

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During rain-on-snow periods, it is assumed that melt is occurring at the snow surface.
Snowmelt during rain-on-snow periods is separated in the model from melt during non-rain periods because the magnitudes of the various energy transfer processes tend to be different during the two types of periods and the dominant energy transfer processes during rain-on-snow periods are known.

A critical first step in any snow cover model is to determine whether any new precipitation is in the form of rain or snow.

Operationally, a variety of information such as upper air data, ground level temperatures, radar returns, and visual observations are available to assist the user in determining the form of the precipitation. During calibration, a simple criterion, based on the available data, is needed so that the form of the precipitation can be classified correctly most of the time. The snow cover model uses the air temperature to estimate the form of precipitation. The model parameter PXTEMP (oC) indicates the temperature which delineates rain from snow.