Freezing rain is precipitation that falls as liquid water but freezes immediately upon contact with surfaces below \(32^\circ\text{F}\) (\(0^\circ\text{C}\)). Snow, in contrast, remains frozen from the cloud base to the surface. The defining factor determining the final form of winter precipitation is the specific temperature structure of the atmosphere from where the precipitation originates down to the earth.
The Role of Atmospheric Temperature Profiles
Meteorologists use a technique known as an atmospheric “sounding” to measure how temperature changes with altitude. This profile is essentially a vertical slice of the atmosphere, revealing the temperature at various heights above the surface. Precipitation always begins high in the atmosphere as tiny ice crystals or snowflakes, since temperatures are almost always well below freezing at cloud level.
The final state of the precipitation—whether it is snow, sleet, or freezing rain—depends entirely on the temperatures the ice crystals encounter as they descend. A single degree or a slight variation in the depth of a warm or cold layer can cause the precipitation type to change completely. The \(32^\circ\text{F}\) (\(0^\circ\text{C}\)) threshold acts as the critical boundary, determining if the ice melts, stays frozen, or refreezes.
The Atmospheric Conditions That Produce Snow
The temperature profile required for snow is the most straightforward of all winter precipitation types. For snow to reach the ground, the air temperature must remain consistently at or below the \(32^\circ\text{F}\) (\(0^\circ\text{C}\)) threshold from the cloud base down to the surface. As the initial ice crystals fall through this uniformly cold column of air, they continue to grow and aggregate into snowflakes.
The snowflake never passes through a layer warm enough or deep enough to cause significant melting, maintaining its solid structure throughout its descent. Snow can sometimes occur even if a very shallow layer near the surface is slightly above freezing, provided the overall profile ensures melting is minimal.
The Atmospheric Conditions That Produce Freezing Rain
Freezing rain is produced by a complex temperature structure known as a temperature inversion, involving three distinct layers. The precipitation begins as snow in a layer of sub-freezing air high above the surface. The snow then falls into a deep layer of warm air, where temperatures rise significantly above \(32^\circ\text{F}\) (\(0^\circ\text{C}\)).
This deep warm layer completely melts the snowflakes, turning them entirely into liquid raindrops. As these raindrops continue their descent, they encounter a third, shallow layer of air at the surface that is again below freezing. The raindrops cool rapidly in this sub-freezing air, reaching temperatures below \(32^\circ\text{F}\) (\(0^\circ\text{C}\)) without turning into ice.
This phenomenon is called supercooling, where the water remains liquid despite being below its freezing point because it lacks a nucleus to start crystallization. Because the cold layer near the ground is very shallow, the supercooled drops do not have enough time to freeze before they strike an object. Once the supercooled liquid contacts a cold surface, such as a road or power line, it instantly freezes into a smooth, clear coating of ice called glaze.
Distinguishing Freezing Rain from Sleet (Ice Pellets)
Both freezing rain and sleet require the same three-layer atmospheric profile: a cold layer aloft, a warm layer in the middle, and a cold layer near the surface. The difference between the two precipitation types is determined by the depth of that final sub-freezing layer near the ground.
For sleet to occur, the final cold layer must be sufficiently deep. This extra depth gives the melted raindrops enough time to fully freeze solid into ice pellets before they reach the ground. These pellets are often heard bouncing off surfaces upon impact. Conversely, freezing rain occurs when the final cold layer is very shallow, preventing the liquid drop from solidifying before impact.