Lower cold layer
If temperature is less than -10oC, 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