From AMS Glossary
Rain that falls in liquid form but freezes upon impact to form a coating of glaze upon the ground and on exposed objects.
In aviation weather observations, this hydrometeor is encoded ZR. While the temperature of the ground surface and glazed objects is typically near or below freezing (0°C or 32°F), it is necessary that the water drops be supercooled before striking. Freezing rain can sometimes occur on surfaces exposed to the air (such as tree limbs) with air temperatures slightly above freezing in strong winds. Local evaporational cooling may result in freezing. Freezing rain frequently occurs, therefore, as a transient condition between the occurrence of rain and ice pellets (sleet). When encountered by an aircraft in flight, freezing rain can cause a dangerous accretion of clear icing.