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.
From Stewart and King, 1987