Freezing rain occurs due to a specific, layered temperature structure in the atmosphere. It is a unique phenomenon where precipitation falls as liquid water but instantly turns to solid ice upon making contact with any surface that is at or below the freezing point. This process creates a smooth, transparent coating of ice known as glaze. Unlike snow, the immediate freezing of the liquid rain forms a dense, heavy ice layer that adheres strongly to everything it touches, making it highly destructive.
The Necessary Atmospheric Layers
The formation of freezing rain requires a distinct three-layer temperature profile. The process begins high up in the atmosphere with a cold layer where precipitation first forms as ice crystals or snowflakes. As the snow falls, it descends into a thick, intermediate layer of warm air that is well above freezing. This warm layer completely melts the snowflakes, transforming them into liquid raindrops. The final component is a shallow layer of cold air situated right at the surface, where temperatures are at or below 32°F (0°C). This arrangement is often associated with a temperature inversion, where warm air is trapped between two colder layers. This surface layer must be cold enough to cool the liquid raindrops below freezing, yet too thin for the drops to refreeze into ice pellets before impact.
The Physics of Supercooling
The liquid nature of the raindrops as they fall through the below-freezing air is explained by supercooling. Supercooling is the state where liquid water remains liquid even when its temperature drops below 32°F (0°C). The droplets remain liquid while suspended in the air primarily due to a lack of nucleation sites, which are impurities or solid particles needed to initiate crystallization. When the supercooled liquid water impacts a solid surface, such as a road or power line, the surface acts as the necessary nucleation site. This contact causes the water to rapidly freeze, forming the hard, clear layer of glaze ice.
The Difference Between Freezing Rain Sleet and Snow
The specific vertical temperature profile dictates whether the precipitation will be snow, sleet, or freezing rain. Snow occurs when the temperature remains at or below freezing throughout the entire column of air. The ice crystals that form high up never encounter a warm layer and reach the surface as snowflakes.
Sleet, which strikes the ground as ice pellets, requires the same three-layer temperature inversion as freezing rain. The difference lies in the depth of the cold surface layer. For sleet, the cold air near the ground is deep enough, usually 1,500 feet or more, allowing the supercooled raindrops sufficient time to completely refreeze into small, bouncing ice pellets before impact.
The distinction is that the liquid drops in freezing rain hit the ground and then freeze, while the ice pellets in sleet are already solid when they arrive.
Practical Hazards and Safety
The glaze ice created by freezing rain is dangerous because of its smooth, heavy, and transparent nature. This dense ice accumulates rapidly, adding hundreds of pounds of weight to objects like tree limbs and power lines. Just a quarter-inch of accumulation can add approximately 500 pounds of weight per span of tree branch, often causing widespread power outages as infrastructure collapses. The ice coating on roadways and walkways is extremely slick, offering virtually no traction for vehicles or pedestrians. This slick layer is often nearly invisible, sometimes referred to as black ice, catching drivers and walkers off guard. Avoiding travel during a freezing rain event is the primary safety measure.