Freezing Rain - Mechanism

Mechanism

Usually freezing rain is associated with the approach of a warm front when cold air, at or below freezing temperature, is trapped in the lower levels of the atmosphere as warmth streams in aloft. This happens, for instance, when a low pressure system moves from the Mississippi River Valley toward the Appalachian Mountains and the Saint Lawrence River Valley of North America, in the cold season, and there is a strong high pressure system sitting further east. The warm air from the Gulf of Mexico is often the fuel for freezing precipitation.

Freezing rain develops as falling snow encounters a layer of warm air usually around 800 mbar (800 hPa) level, then the snow completely melts and becomes 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 (0 °C or 32 °F). However, the drops themselves do not freeze, a phenomenon called supercooling (or forming "supercooled drops"). When the supercooled drops strike ground, power lines, tree branches, aircraft, or anything else below 0 °C (32 °F), they instantly freeze, forming a thin film of ice, hence freezing rain.