When winter weather arrives, the terminology for frozen precipitation often causes confusion. Sleet and freezing rain are distinct meteorological events with different properties and impacts. The difference lies in the specific arrangement of warm and cold air layers the precipitation passes through on its way to the ground. Understanding these separate processes provides clarity on what to expect when a winter storm approaches.
The Definition of Sleet
Sleet, which meteorologists call ice pellets, is precipitation that is completely frozen before reaching the surface. It typically begins high in the atmosphere as snow, then encounters a layer of warmer air above the surface that causes it to melt into raindrops.
These liquid drops then fall into a deep layer of sub-freezing air situated just above the ground. This extended exposure allows the water droplets enough time to completely refreeze into small, translucent balls of ice. When sleet hits the ground or other objects, it makes a distinctive tapping sound and often bounces upon impact. The accumulation of these frozen pellets can resemble snow, though it is denser and often creates a slushy layer underfoot.
The Definition of Freezing Rain
Freezing rain is precipitation that falls as liquid water but is supercooled, meaning it is below the freezing point of 32 degrees Fahrenheit (0 degrees Celsius). Unlike sleet, the precipitation remains entirely liquid as it travels through the atmosphere and freezes instantly only upon contact with surfaces.
When these supercooled droplets strike an object—such as a road, tree branch, or power line—that is at or below freezing, they spread out before immediately crystallizing. This process creates a smooth, hard coating of ice known as glaze ice. Even a light accumulation of freezing rain results in extremely slick conditions, making driving and walking hazardous.
The danger is significantly higher than with sleet because the glaze ice adheres firmly to objects. Heavy accumulations, sometimes called an ice storm, can add considerable weight to infrastructure, causing widespread power outages from downed trees and utility lines.
How Temperature Layers Determine the Outcome
The difference between sleet and freezing rain is controlled by the vertical temperature profile of the atmosphere. Both types of precipitation require a temperature inversion, a situation where a layer of warm air rests above a layer of colder air near the surface. The initial precipitation starts as snow or ice crystals in the cold upper atmosphere.
For both sleet and freezing rain to occur, the snow must first fall through a warmer layer, where temperatures are above 32°F (0°C), causing it to melt into rain. The outcome depends entirely on the depth of the subsequent sub-freezing layer near the Earth’s surface.
A deep cold layer, extending hundreds or thousands of feet above the ground, gives the melted raindrops sufficient time to refreeze completely into solid ice pellets, resulting in sleet. In contrast, freezing rain occurs when the sub-freezing layer near the ground is shallow. The liquid raindrops cool down to below freezing as they pass through this shallow layer, becoming supercooled, but they do not have enough time to nucleate and freeze into solid pellets before impact.