Freezing rain is defined as rain that falls while remaining entirely liquid, even when its temperature is below the freezing point of water. It is one of the most dangerous forms of winter weather because it results in a clear, hard glaze of ice upon contact with any exposed object, making roads, sidewalks, and infrastructure extremely slick and heavy. The formation of this precipitation requires a very specific balance of atmospheric temperatures throughout the air column.
The Crucial Atmospheric Temperature Profile
The formation of freezing rain depends on a precise, layered thermal structure in the atmosphere, often referred to as a temperature inversion. The process begins high in the atmosphere, where precipitation starts as ice crystals or snow in a layer well below \(0^\circ\text{C}\) (\(32^\circ\text{F}\)). As these ice particles descend, they encounter a thick, elevated layer of air where the temperature rises significantly above freezing. This warm layer must be deep enough to completely melt the falling snow, turning the ice crystals entirely into liquid raindrops.
The liquid raindrops continue their descent until they reach a final, shallow layer of air at the surface that is again below freezing. This cold layer near the ground is the final and most defining part of the temperature profile. For freezing rain to occur, this sub-freezing layer must be thin enough that the liquid droplet does not have sufficient time to refreeze completely before reaching the ground.
The Mechanism of Supercooling and Impact Freezing
The liquid state of water below its standard freezing point is known as supercooling, a condition that is central to the formation of freezing rain. Water droplets can become supercooled because they lack a solid particle, or nucleation site, around which the water molecules can organize and crystallize into ice. As the liquid rain passes through the shallow cold layer near the surface, it cools, often reaching temperatures a few degrees below \(0^\circ\text{C}\) while still remaining liquid.
These supercooled droplets are thermodynamically unstable. When a supercooled droplet impacts a solid surface, such as a road, a tree branch, or a power line, the mechanical shock and contact with the cold surface provide the necessary nucleation point. This contact instantly triggers the rapid phase change from liquid water to solid ice. The freezing occurs nearly instantaneously, spreading a thin sheet of clear, hard ice known as glaze ice across the surface.
The rapid phase change is destructive, as the weight of the accumulating ice can quickly down power lines and tree limbs. The resulting glaze is transparent and smooth, offering little traction and making affected surfaces exceptionally slick.
How Freezing Rain Differs from Other Winter Precipitation
The specific temperature profile needed for freezing rain differentiates it from other common forms of winter precipitation, namely snow and sleet. Snow requires the most straightforward profile, where the entire column of air from the cloud base to the ground remains at or below \(0^\circ\text{C}\). In this scenario, the ice crystals that form high up continue to fall as snow throughout their entire descent.
Sleet, which is precipitation that reaches the ground as ice pellets, also involves a layered temperature profile, but with a structural difference. Sleet-forming conditions include the warm layer aloft that melts the snow, but they require a deeper sub-freezing layer near the ground. This deeper cold layer provides enough time and distance for the supercooled liquid droplet to refreeze completely into a small, hard pellet before it strikes the surface. These pellets bounce when they hit the ground, unlike the liquid drops of freezing rain.
Regular liquid rain occurs when the surface temperature is above freezing. If the warm air layer extends down to the surface, the liquid drops will reach the ground without freezing on impact. Therefore, the critical factor distinguishing freezing rain is the presence of a shallow, sub-freezing layer of air at the surface that is not deep enough to turn the liquid back into ice pellets.