Freezing rain is liquid precipitation that instantly solidifies when it contacts surfaces at or below the freezing point. This phenomenon leads to the rapid formation of slick, clear ice, known as glaze, which can accumulate to dangerous thicknesses on infrastructure and roads. The specific atmospheric conditions that prevent this precipitation from falling as snow or sleet require a precise vertical layering of warm and cold air masses. This delicate balance creates a scenario where water remains liquid at temperatures below freezing until the moment it strikes an object.
The Critical Melting Layer
The process that leads to freezing rain always begins high in the atmosphere with frozen precipitation, typically snow or ice crystals. As these frozen particles descend, they encounter a layer of air elevated above the ground that is warmer than 0° Celsius. This warm air pocket, which meteorologists call the melting layer, is often associated with the overriding of cold air by a warm front. The temperature within this layer must be sufficiently high and the layer itself deep enough to ensure that the descending ice crystals completely melt.
Once the snow melts, the precipitation continues its downward journey entirely as liquid raindrops. This complete transformation from solid ice to liquid water is a fundamental requirement for freezing rain. If the warm layer were too shallow or too cool, the ice crystals would only partially melt. The presence of this melting layer high above the ground is the first step in creating the complex vertical temperature profile necessary for freezing rain.
The Unique Temperature Sandwich
The transformation into freezing rain is completed by a second layer of air, which is a shallow blanket of sub-freezing air situated very close to the earth’s surface. The raindrops, having melted fully in the warm air aloft, now fall through this final, cold layer. This arrangement creates a unique atmospheric “sandwich,” with a warm layer trapped between two cold layers.
The critical factor is that this cold layer near the surface is quite shallow, typically less than 1,500 to 2,000 feet deep. This limited depth means the raindrops do not spend enough time in the sub-freezing air to completely solidify before reaching the ground. Instead, the water droplets cool to a temperature below 0° Celsius but remain in a liquid state, a phenomenon known as supercooling.
When these supercooled raindrops strike any object—such as a tree branch, power line, or pavement—that is also at or below freezing, the impact provides the necessary nucleation site. The liquid water instantly freezes upon contact, rapidly forming a layer of hard, clear ice called glaze. This immediate solidification upon impact is the defining characteristic of freezing rain.
Why Not Snow or Sleet?
The subtle differences in the vertical temperature profile explain why freezing rain occurs instead of other common forms of winter precipitation like snow or sleet. Snow requires the simplest profile: the air temperature must remain continuously at or below freezing from the cloud layer all the way to the ground. If this condition is met, the original ice crystals will never melt and will fall as snowflakes. The presence of the elevated warm layer that creates the liquid rain is the exact reason snow is prevented.
Sleet, which is precipitation that hits the ground as ice pellets, forms under a temperature profile very similar to that of freezing rain, but with one important modification. For sleet, the sub-freezing layer of air near the surface must be much deeper. This greater depth provides the fully melted raindrops with enough time to spend in the cold air to completely refreeze into solid ice pellets before they strike the ground.
Therefore, freezing rain is the result of a perfectly balanced, shallow cold layer near the surface. The rain melts aloft and then cools below freezing, but the cold layer is not deep enough to allow the droplets to transition from supercooled liquid to solid ice pellets.