What causes freezing rain to occur?
The usual setup that leads to freezing rain is simply a layer of above freezing air above a sub freezing layer. This is not unusual during very cold arctic air outbreaks, where the very cold and heavy arctic air slips under a warmer airmass. Precipitation is produced and falls through the warmer air aloft and melts (if it was frozen to begin), then as it falls through the sub-freezing layer at and just above the surface, the rain is chilled and freezes on impact with the sub-freezing surfaces. The cold surface airmass must not be too deep, otherwise the chilled raindrops can freeze before reaching the surface, resulting in sleet.
