Precipitation type?

• 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

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