Sleet is a form of wintry precipitation that often causes confusion among different types of ice and snow. Meteorologically defined in the United States as ice pellets, sleet forms when liquid water freezes into solid grains before reaching the ground.
The Visual Characteristics of Sleet
Sleet appears as small, translucent, or clear ice pellets, typically smaller than a pea. These pellets are essentially frozen raindrops, which gives them a spherical or slightly irregular shape. They are hard and do not easily crush or clump together like snowflakes or soft hail. A tell-tale sign of sleet is the distinct, sharp tapping or rattling noise it produces when striking hard surfaces, differentiating it from the quiet accumulation of snow. When sleet hits the ground, it often bounces rather than sticking, creating a crunchy layer that can be slick for walking or driving.
The Atmospheric Process of Sleet Formation
Sleet formation requires a specific vertical temperature profile in the atmosphere. The process begins high up where the temperature is below freezing, allowing precipitation to start as ice crystals or snowflakes. As these snowflakes fall, they encounter a layer of air that is above freezing, which is warm enough to completely melt the ice crystals into liquid raindrops. The raindrops continue their descent until they pass through a deep layer of sub-freezing air located close to the Earth’s surface. This lower cold layer must be substantial enough to allow the liquid drops to refreeze completely into solid ice pellets before they hit the ground.
Distinguishing Sleet from Freezing Rain and Hail
The defining difference between sleet and freezing rain lies in the state of the water upon impact with the surface. Sleet is fully frozen into ice pellets before it reaches the ground, causing it to bounce off objects. Freezing rain, by contrast, is liquid water when it falls, but it supercools in a shallow layer of sub-freezing air near the surface and then freezes instantly upon contact with cold objects, creating a glaze of ice. The atmospheric profile for freezing rain features a shallower cold layer near the ground, which is not deep enough to allow the raindrops to refreeze into pellets. Sleet is also distinct from hail, which is much larger and forms in the strong updrafts of powerful thunderstorms, typically during warmer months, unlike sleet’s smaller, uniform pellets.