Sleet and freezing rain are distinct meteorological events. Both begin as snow high in the atmosphere, but variations in the column of air they fall through determine their final form. While both create hazardous winter conditions, the physical structure of the resulting ice is entirely different, leading to a major disparity in their danger levels.
The Crucial Role of Atmospheric Temperature Layers
The formation of both sleet and freezing rain depends on a specific temperature profile, often described as a warm air sandwich. This profile starts with a cold layer aloft where snow forms, followed by a middle layer of warmer air above the freezing point, and finally a cold layer near the Earth’s surface. The snow melts into rain as it passes through the warm layer, typically found between 5,000 and 10,000 feet above the ground.
The thickness of the final, sub-freezing air layer near the ground is the determining factor separating sleet from freezing rain. If this layer is deep enough, the liquid water droplet has sufficient time to completely freeze back into a solid form before impact. If the layer is too thin, the droplet does not have enough time to freeze fully.
Sleet: The Mechanism of Ice Pellet Formation
Sleet occurs when the final layer of sub-freezing air near the surface is deep enough to refreeze the melted raindrops. The liquid water droplet falls into this thick layer of air (at or below \(32^\circ\text{F}\) or \(0^\circ\text{C}\)). The duration of its descent allows the droplet to fully solidify into a small ice pellet.
These ice pellets are translucent, small, and hard. Upon reaching the ground, they are already frozen and tend to bounce off hard surfaces, creating a distinctive tapping or rattling sound. Sleet accumulates much like a light, grainy snow, creating a slippery layer of tiny, icy ball bearings.
Freezing Rain: The Creation of Supercooled Liquid
Freezing rain forms when the final cold layer of air right above the ground is very shallow. The melted raindrop passes through this thin, sub-freezing layer, but lacks the time to completely freeze into a solid pellet. Instead, the water is cooled below its freezing point while remaining liquid, a phenomenon known as supercooling.
These supercooled liquid droplets maintain their liquid form even at temperatures slightly below freezing. The droplet freezes instantly when it makes contact with any surface that is at or below \(32^\circ\text{F}\). This immediate freezing upon impact creates a coating of ice known as glaze ice.
Practical Differences in Surface Impact and Hazard
The most noticeable difference between the two is their surface impact, which translates directly into hazard level. Sleet pellets accumulate on roads and sidewalks, making travel slick but often manageable, similar to granular ice. Because the pellets are already frozen, they do not adhere immediately to surfaces and can sometimes be cleared with plows or shovels.
Freezing rain is significantly more destructive due to the adhesive quality of the glaze ice it creates. The liquid water instantly freezes onto every exposed object, forming a dense, clear, and extremely heavy coating. Even a fraction of an inch of this glaze ice can add immense weight to infrastructure, causing tree limbs to snap and power lines to break, leading to widespread power outages.