Freezing rain is precipitation that falls as liquid but freezes upon contact with surfaces. It occurs under specific atmospheric temperature conditions, making it distinct from other winter weather. This can lead to dangerous accumulations of ice, often called glaze ice, impacting travel and infrastructure.
The Ground-Level Temperature Threshold
For freezing rain to occur, the temperature at the Earth’s surface and a shallow layer just above it must be at or below 32°F (0°C). Supercooled rain droplets, which remain liquid below freezing, instantly freeze upon striking surfaces like roads, trees, or power lines. This forms a clear, smooth layer of ice. Bridges and overpasses often freeze first, cooling more rapidly than ground surfaces due to exposure to cold air from both above and below.
Understanding Atmospheric Temperature Layers
The formation of freezing rain depends on a specific vertical temperature profile in the atmosphere, characterized by a temperature inversion. This means a layer of warmer air sits above a layer of colder air near the surface. Precipitation originates as snow or ice crystals high in the atmosphere.
As these frozen particles fall, they encounter a deep layer of air where temperatures are above freezing, causing them to melt completely into raindrops. These raindrops then pass through a shallow layer of air at or below 32°F (0°C) just before reaching the ground. This cold layer is not deep enough for the raindrops to refreeze into ice pellets. Instead, the water droplets become supercooled, meaning they remain in a liquid state even though their temperature is below freezing. The presence of this warm layer aloft, sandwiched between two subfreezing layers, is the primary meteorological setup for freezing rain.
Freezing Rain Versus Other Winter Precipitation
The specific temperature profile throughout the atmosphere dictates whether precipitation falls as snow, sleet, freezing rain, or plain rain. Snow forms when the entire column of air from the clouds to the ground remains at or below freezing, allowing ice crystals to fall without melting.
Sleet, also known as ice pellets, occurs when snow falls through a relatively shallow warm layer, partially melts, and then re-enters a deeper freezing layer closer to the surface. This deeper cold layer allows the partially melted precipitation to refreeze into small ice pellets before reaching the ground.
In contrast, plain rain forms when temperatures remain above freezing from the clouds all the way to the Earth’s surface, preventing any ice formation or refreezing. Freezing rain’s unique characteristic is the specific shallow cold layer at the surface that causes supercooled liquid droplets to freeze on impact, distinguishing it from these other precipitation types.