“Freezing rain” and “sleet” are frequently mentioned in our beloved “Winter Weather Update” emails. Although these terms are familiar, they are often confused with each other or misunderstood. At Blythewood, our awareness of their differences ensures that we are timely and efficient in servicing winter conditions.
Before explaining the significant differences of sleet and freezing rain, let’s start with their similarities. Both cause particularly slick conditions on streets, especially on bridges and any elevated roadways which are surrounded by colder air. Both begin as snowflakes, pass through a warm front and melt into raindrops. As raindrops, both pass through subfreezing layers and cool rapidly just before hitting the ground surface.
Freezing rain delivers arguably the most dangerous road conditions, as it is hardly visible to the human eye. As mentioned, it begins as snowflakes that begin melt when they pass through a warm front. As the liquid droplets approach the ground surface, they pass through a thin layer of subfreezing air. This layer is extremely cold, but not quite thick enough to freeze the droplets, rather they quickly become supercooled to below freezing and land as liquid. Upon reaching the surface, the droplets form a glaze of ice. This glaze spreads fairly evenly, but wind and gravity can cause one side of a surface to build a thicker coat of ice than the other. Of course, this is the creation of extremely dangerous road conditions that are in need of de-icing treatments. Freezing rain can also weigh on plants and other surfaces, damaging them. When significant accumulation of freezing rain lasts for several hours or more, it is considered an “ice storm.”
Now, let’s talk sleet. During sleet formation, snowflakes partially melt as they pass through warmer temperatures. After making it through the warmer layer, the droplets enter a subfreezing layer just above the ground surface where they refreeze fully. This refreeze is possible because, unlike with freezing rain, the subfreezing layer above the surface is thick enough to do so. The droplets, now frozen solid, reach the ground as as pellets of ice. Of course, this also lends dangerous road conditions, but is slightly more visible than freezing rain. Sleet can also accumulate similar to snow, depending on the conditions.
In essence, sleet reaches the ground frozen, while freezing rain reaches the ground in liquid form and then freezes.