Freezing Rain

Freezing rain (FZRA) is precipitation that falls as liquid rain but freezes on contact with any surface at or below 0°C, forming a glaze of clear ice. It is one of the most hazardous weather phenomena in Canadian aviation, responsible for flight delays, diversions, and accidents. Reported in METARs as FZRA.

Cross-section diagram showing freezing rain formation: warm air aloft melts snow, supercooled rain falls through shallow below-freezing layer and freezes on contact with cold surface
Freezing rain formation mechanism. Source: NAV CANADA Aviation Meteorology Reference

How Freezing Rain Forms

Freezing rain forms when a warm, above-freezing layer of air (typically 1,000–8,000 ft) overlies a shallow sub-freezing layer at the surface. Snow falling from above melts as it enters the warm layer, becoming liquid rain droplets. These droplets fall through the shallow sub-freezing layer and arrive at the surface as supercooled liquid — immediately freezing on contact with any cold surface.

Where Freezing Rain Occurs in Canada

Freezing rain is most commonly associated with warm fronts and trowals (troughs of warm air aloft). In Canada, the Ottawa and St. Lawrence valleys, southern Ontario, and Atlantic Canada are particularly susceptible due to the topographic channeling of cold air. The Great Ice Storm of 1998 that devastated southern Ontario and Quebec was one of the most severe freezing rain events in Canadian history.

Aviation Hazards

  • Severe clear icing: Freezing rain deposits clear, dense glaze ice rapidly on all exposed surfaces — potentially at rates exceeding de-icing system capability
  • Ground icing: Runway and taxiway surfaces become extremely slippery; braking and directional control severely compromised
  • Aircraft contamination: Aircraft on the ramp require thorough de-icing before any departure; the clean aircraft concept applies absolutely
  • In-flight encounter: FZRA encountered in flight is a genuine emergency — the ice accretion rate in heavy FZRA can exceed the capacity of any anti-icing system

Forecast Products

  • TAF: FZRA in any TAF group is a serious warning — request forecaster notes if available
  • GFA: Freezing precipitation is depicted on the clouds and weather panel and the icing and turbulence panel
  • SIGMET: Severe icing from FZRA may be the subject of a SIGMET
  • PIREPs: Airborne observations from other pilots are the most current information available

On the TC Written Exam

Freezing rain is a guaranteed exam topic. Know: it forms when rain falls through a shallow sub-freezing layer; it produces clear/glaze ice; it is associated with warm fronts and trowals; severe FZRA is effectively unsurvivable for unprotected aircraft.

Related Terms

See also: Freezing Drizzle, Icing, Warm Front, Trowal, Aircraft Critical Surface Contamination.