Winter precipitation often comes in varied forms, making it difficult to distinguish between types of frozen or partially frozen moisture. Understanding the difference between these types of precipitation is more than just a matter of terminology; it directly relates to travel safety and the potential for infrastructure damage. This article aims to provide a clear, precise definition of sleet by examining its meteorological classification, its formation process in the atmosphere, and how it differs from other winter hazards like freezing rain and hail.
The Meteorological Definition of Sleet
Sleet, as defined by the National Weather Service (NWS) in the United States, is precipitation that reaches the ground as ice pellets. These pellets are small, translucent balls of ice that are typically smaller than five millimeters in diameter. When sleet hits the ground, it makes a distinct rattling sound and bounces upon impact because it is already frozen solid. Accumulation of sleet can happen much like snow, and it is measured for depth using a ruler.
How Sleet Forms in the Atmosphere
The formation of sleet requires a specific, layered temperature structure in the atmosphere, often called a temperature inversion. The process begins high in the cloud layer where the temperature is below freezing, allowing for the initial formation of snow or ice crystals. As the frozen precipitation falls, it descends into a layer of air that is above the freezing point, causing the snow to melt completely or partially into liquid raindrops. For sleet to form, these liquid drops must then fall through a final, deep layer of freezing air positioned near the Earth’s surface. This cold layer must be thick enough to allow the drops to refreeze fully into solid ice pellets before they strike the ground. The depth of this final cold layer is the determining factor that separates sleet from other winter precipitation types like freezing rain.
Distinguishing Sleet from Freezing Rain and Hail
Sleet vs. Freezing Rain
Sleet is frequently confused with freezing rain, but their difference is determined by their state upon impact with the surface. Freezing rain also requires a temperature inversion, but it occurs when the final layer of freezing air near the ground is too shallow to allow the liquid drops to refreeze completely. This results in “supercooled” liquid water hitting surfaces that are below freezing, where the liquid instantly freezes on contact, creating a glaze of ice. Sleet, conversely, is already a solid pellet when it lands, bouncing rather than coating surfaces in a sheet of ice.
Sleet vs. Hail
Sleet is distinct from hail, which is a summer or warm-weather phenomenon. Hailstones form within the powerful updrafts of intense thunderstorms. Water droplets are repeatedly carried high into the sub-freezing part of the cloud, gaining layers of ice until they become too heavy. Unlike sleet, which is a winter occurrence that starts as snow, hail is produced by vigorous convection and can grow much larger than the ice pellets. Sleet is defined by a specific vertical temperature profile, whereas hail is defined by its formation within a thunderstorm’s strong vertical air movement.