The warm layer
- If Tw of warm layer exceeds 3 to 4o C, snow melts completely resulting in rain or freezing rain.
- If Tw is less than 1o C, only partial melting occurs and snow will usually refreeze.
- If Tw is 1-3o C usually results in partial melting of snowflakes but then usually refreezes into sleet (or a mixture of sleet and freezing rain depending on the depth of the warm layer.
The cold layer
- If temperature is less than --8oC, and freezing nuclei are sufficiently abundant and enough time is spent in the cold layer, either snow or sleet can occur.
- If cold layer is warmer than -8oC, droplets remain super cooled if the snow was completely melted (favors freezing rain).
- Depth of cold layer is not nearly as important as the temperature of the cold layer.
From Stewart and King, 1987