Defining Freezing Rain
Freezing rain is a type of precipitation that falls as liquid rain but freezes upon contact with surfaces that are at or below 0°C (32°F). Unlike sleet (ice pellets), which freezes in the atmosphere before reaching the ground, freezing rain remains liquid until it strikes an object, creating a smooth, clear, and often thick layer of ice known as glaze.
The Atmospheric Conditions for Freezing Rain
The formation of freezing rain requires a specific temperature profile in the atmosphere. There must be a layer of warm air aloft (above freezing) that melts falling snow or ice pellets into raindrops. Crucially, below this warm layer, there must be a shallow layer of sub-freezing air near the surface (at or below 0°C). If this cold layer is too deep, the raindrops will refreeze into sleet before reaching the ground.
Impact and Hazards
Freezing rain creates highly dangerous conditions. The glaze ice makes roads, sidewalks, and elevated surfaces extremely slick, leading to hazardous driving and walking. Furthermore, the weight of accumulating ice can cause tree branches to snap and power lines to break, leading to widespread power outages and significant infrastructure damage. Even a thin layer can be problematic, while thicker accumulations are devastating.
Distinguishing from Other Winter Precipitation
It's important to differentiate freezing rain from other winter precipitation. Snow consists of ice crystals that never melt. Sleet (ice pellets) forms when raindrops refreeze into small, solid ice pellets before hitting the ground. Freezing drizzle is similar to freezing rain but involves smaller liquid droplets. The key characteristic of freezing rain is its liquid state upon falling and subsequent freezing on impact.