Freezing rain is liquid precipitation that falls through the atmosphere as water droplets and reaches the ground while still liquid, but then freezes when it contacts surfaces that are at or below the freezing point. The droplets are often supercooled, meaning that they remain liquid despite having temperatures below zero degrees Celsius. Upon striking roads, trees, power lines, buildings, vehicles or other cold objects, the droplets freeze and form a layer of ice known as glaze.
Freezing rain is therefore fundamentally different from ice pellets, because the precipitation does not refreeze into solid particles before reaching the surface. It is an important winter weather hazard because even a relatively small amount of freezing rain can produce dangerous surface icing and substantial disruption.
The atmospheric structure required for freezing rain commonly involves snow forming in a cold upper layer, followed by a warm layer where the snow melts completely into rain, and then a shallow cold layer near the ground. The warm layer must be sufficiently deep to melt the falling ice particles. The lower cold layer is generally not deep or cold enough to refreeze the liquid droplets completely before they reach the surface.
The droplets therefore remain liquid while passing through the near surface cold air. When they encounter cold surfaces, the liquid freezes rapidly. This vertical temperature arrangement distinguishes freezing rain from snow and ice pellets and explains why the precipitation type can change over short distances as the atmospheric profile changes.
Freezing rain is closely associated with temperature inversions and frontal weather systems. In a typical winter setup, cold dense air remains near the surface while warmer air is lifted over it. Precipitation generated in the colder cloud environment falls into the warmer air and melts, then enters the shallow cold layer trapped near the ground. The surface itself can remain below zero even when the air temperature at a few hundred metres altitude is above freezing. This means that reports of above freezing temperatures aloft do not rule out freezing rain. Meteorologists therefore use radiosonde observations, numerical weather models, radar and surface measurements to determine the vertical temperature structure when forecasting freezing precipitation.
The principal hazard from freezing rain is the formation of glaze ice. Because the droplets freeze directly onto exposed surfaces, they can create a smooth or irregular coating that rapidly reduces friction on roads, pavements, bridges and steps. Ice accumulation on tree branches can add substantial weight, causing branches to break. Power lines and utility structures can also accumulate enough ice to sag, break or collapse, producing widespread power outages. The hazard can be amplified by strong winds, which increase stress on ice loaded trees and infrastructure. Freezing rain can also make aircraft runways, vehicles and other transportation surfaces dangerous, requiring de icing and other operational measures.
Freezing rain differs from ordinary rain primarily because of surface temperature. Ordinary rain remains liquid after reaching the ground when the receiving surface is above freezing or when conditions do not permit significant freezing. Freezing rain reaches surfaces that are sufficiently cold for the water to freeze. It also differs from sleet or ice pellets because those particles freeze in the air and arrive as solid pellets.
Hail is different again because it forms inside strong convective clouds through repeated growth of ice particles. Snow remains frozen throughout the relevant part of its descent. These distinctions are based on the physical state and formation pathway of the precipitation rather than merely on how cold the weather feels.
The amount of freezing rain can be described by precipitation accumulation and by the resulting ice accretion. A small liquid equivalent of precipitation can produce significant glaze under suitable conditions, although the thickness of ice formed depends on droplet size, surface temperature, wind, surface geometry and the efficiency with which droplets freeze.
Ice accretion is not necessarily equal to the liquid precipitation depth because some water may run off before freezing, especially on smooth surfaces or during periods of changing temperature. Forecasting therefore considers both the expected precipitation and the likelihood of freezing at exposed surfaces. Roads can become hazardous before thick visible ice develops because a thin glaze can substantially reduce tyre traction.
Freezing rain is especially common in regions where cold air can become trapped near the surface beneath warmer air. Examples include parts of eastern North America and other temperate regions with winter frontal systems, although the phenomenon can occur wherever the required atmospheric profile develops. Valleys and low lying areas can sometimes retain cold air while warmer air moves above them, increasing the possibility of freezing precipitation. Coastal and continental weather patterns can also create suitable conditions. The exact frequency varies geographically and seasonally. Climate changes can alter the geographical and seasonal distribution of freezing rain through changes in temperature profiles, storm tracks and the duration of conditions near the freezing threshold.
Forecasting freezing rain requires more than checking whether the surface air temperature is below zero degrees Celsius. Meteorologists examine the complete vertical temperature profile, including the altitude and thickness of warm and cold layers. Radar can show precipitation type indirectly, while weather balloons and aircraft measurements can provide direct atmospheric profiles. Numerical weather prediction models estimate the evolution of temperature, humidity and precipitation processes.
Because small errors near the freezing threshold can change the predicted precipitation type from snow to sleet, freezing rain or rain, forecasts can contain substantial uncertainty in marginal situations. Freezing rain is consequently a technically important example of how precipitation classification depends on atmospheric structure rather than a single surface measurement.
