Freezing rain develops as falling snow encounters a layer of warm air deep enough for the snow to completely melt and become rain. As the rain continues to fall, it passes through a thin layer of cold air just above the surface and cools to a temperature below freezing.
http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/cld/prcp/zr/frz.rxml

