Freezing rain is an interesting winter event that can cause tremendous amounts of damage. Streets and sidewalks become skating rinks. In addition, ice collecting on trees and power-lines result in power outages that can take days to repair. Freezing rain occurs when warm moist air overrides cold air at the surface. Precipitation falls as liquid, then freezes on contact. Sometimes, the cold air is sitting on top of warm ground, so you get icing on trees, fences, and elevated surfaces but little collection on roads. How cold can it be and still have liquid precipitation? On December 21, 1990 Des Moines, Iowa reported freezing drizzle and a temperature of -2 oF.
Mary Knapp, Weather Data Library