The phrase “ice rain” is a common term for winter precipitation that results in a coating of ice, but it is not a technical meteorological term. Scientists use specific language to distinguish between different types of icy weather based on how the precipitation forms. The two primary phenomena people refer to as “ice rain” are Freezing Rain and Sleet (also known as Ice Pellets). Understanding the distinction is necessary because they are created by different atmospheric structures and present unique hazards.
Freezing Rain: The Liquid That Freezes on Contact
Freezing rain is precipitation that falls entirely as liquid water droplets but instantly freezes when it strikes surfaces at or below 32°F (0°C). This phenomenon requires a complex atmospheric inversion, often starting with snow. The snow falls through a deep layer of air that is above freezing, causing the ice crystals to melt completely into raindrops.
The liquid then descends into a subsequent layer of cold air near the surface that remains below freezing. Since this sub-freezing layer near the ground is too shallow, the raindrops do not have enough time to freeze fully. The liquid drops become “supercooled,” meaning their temperature is below the freezing point, yet they remain liquid.
These supercooled drops are highly unstable and freeze upon contact with solid objects like roads, trees, or power lines. The resulting accumulation is a smooth, hard, and transparent layer of ice called glaze ice. Since the water adheres to surfaces before freezing, it can accumulate to significant thicknesses, posing a major threat to infrastructure.
Sleet (Ice Pellets): Precipitation That Freezes Before Impact
Sleet, officially called ice pellets by the U.S. National Weather Service, is precipitation that is solid before it hits the ground. Like freezing rain, sleet begins as snow that melts into a liquid raindrop after passing through an elevated layer of warm air. The key difference lies in the depth of the cold air layer near the surface.
For sleet to form, the sub-freezing layer of air must be deep enough to allow the melted raindrops time to refreeze. As the drops fall through this deep, cold layer, they turn into small, translucent ice spheres. These pellets are typically less than 5 millimeters in diameter.
When ice pellets strike the ground, they often make a distinct tapping sound and bounce. Sleet is essentially frozen raindrops. This complete freezing process before hitting the surface is what differentiates sleet from the supercooled liquid state of freezing rain.
Understanding the Difference: Glaze Ice vs. Ice Pellets
The difference between freezing rain and sleet translates into significantly different impacts on the ground, making the distinction important for public safety. Freezing rain creates glaze ice, a smooth, continuous sheet that conforms to every surface it touches. This glaze ice is dangerous on roads and walkways because it provides virtually no traction and is often difficult to see.
The accumulation of glaze ice is the primary cause of widespread damage during an ice storm. When the ice accretion exceeds about a quarter-inch (0.6 cm), the weight can cause tree limbs to snap and power lines to collapse. This structural damage is why freezing rain is more hazardous to infrastructure than sleet.
Sleet, in contrast, results in a layer of small, loose ice pellets that can be shoveled or swept. While it creates hazardous travel conditions, the pellets do not accrete heavily onto vertical infrastructure like trees and power lines. Therefore, ice pellets rarely cause the widespread damage and power outages associated with severe freezing rain events.