Freezing Rain

What is Freezing Rain?

Freezing rain occurs when rain, formed in a warm layer aloft, falls into a shallow layer of air at or below 0°C near the surface. The raindrops remain liquid (supercooled) as they descend and only freeze when they strike surfaces colder than freezing, such as aircraft skin, runways, or trees.

Formation Mechanism

• Precipitation starts as snow or ice crystals high in the cloud.
• As these fall through a warm layer, they melt into rain.
• The rain then passes through a colder sub-zero layer near the ground, becoming supercooled.
• On impact with any surface below 0°C, the droplets freeze instantly, forming a hard, clear ice layer.

Weather Conditions and Synoptic Context

• Most commonly found ahead of warm fronts in winter, where a warm, moist air mass overrides a shallow cold layer at the surface.
• Can also occur beneath surface inversions or in cold high-pressure areas when rain falls into subfreezing air.
• Less common but possible near cold fronts, though the affected area is usually smaller.

Hazards to Aviation

• Freezing rain is one of the most dangerous icing conditions for aircraft, rapidly producing thick, hard-to-remove clear ice.
• It can severely reduce visibility by coating windscreens and degrade aircraft performance by adding weight and disrupting airflow over wings and control surfaces.
• On the ground, it creates extremely slippery conditions on runways and taxiways, increasing the risk of accidents during taxi, take-off, and landing.

Freezing Rain vs. Other Precipitation

• Freezing rain differs from snow (which forms and remains solid) and freezing drizzle (which involves smaller droplets but similar freezing behaviour).
• Unlike rime ice (from small supercooled droplets in cloud), freezing rain produces dense, clear ice from larger droplets.

Operational Strategies

• Avoidance is the only safe option; no light aircraft de-icing system can cope with freezing rain.
• Climbing above the warm front or descending below the freezing layer (if terrain and traffic permit) are potential escape maneuvers.
• Monitor METARs and forecasts for FZRA or FZDZ codes, which indicate freezing rain or drizzle hazards.