Sleet is a type of frozen precipitation composed of small, transparent ice pellets. It is often confused with snow, freezing rain, or hail. This article clarifies what sleet is, how it forms, its characteristics, and how it differs from other winter precipitation types.
How Sleet Forms
Sleet forms through distinct temperature layers in the atmosphere. Snowflakes fall from clouds, where temperatures are at or below freezing. As these snowflakes descend, they encounter a warmer air layer, typically located between 1,500 and 3,000 meters (5,000 and 10,000 feet) above the ground, causing them to melt into raindrops.
Below this warm layer, a deep layer of sub-freezing air, at or below 0°C (32°F), extends to the Earth’s surface. As the melted precipitation falls through this cold layer, the raindrops refreeze into small, solid ice pellets before they reach the ground. This layered temperature profile, with warm air sandwiched between two colder layers, is important for sleet formation.
Key Characteristics of Sleet
Sleet appears as small, translucent ice pellets, typically ranging from 0.1 to 0.5 inches (2.5 to 13 mm) in diameter. These ice pellets are generally spherical. When sleet strikes hard surfaces, it produces a distinctive rattling or pinging sound, much like small beads hitting a window or car windshield.
Upon impact, sleet pellets tend to bounce, rather than splat or stick. While sleet can accumulate on the ground, sometimes several inches deep, it does not typically stick or glaze surfaces like freezing rain. The accumulation often resembles tiny frozen raindrops or small, coarse pellets.
Sleet Compared to Other Winter Precipitation
Differentiating sleet from other winter precipitation is important. Snow, for instance, remains frozen from the cloud to the ground. Snowflakes are ice crystals that aggregate as they fall, maintaining their crystalline structure. Sleet, conversely, involves a melting and refreezing process as it traverses different temperature layers.
Freezing rain is another distinct phenomenon, characterized by liquid rain that falls through a thin layer of sub-freezing air near the surface. These supercooled raindrops freeze upon contact with surfaces, forming a smooth, transparent ice glaze. Unlike sleet, which is already frozen into pellets before impact, freezing rain forms ice on surfaces, creating hazardous conditions such as ice-coated trees and power lines.
Hail forms under entirely different meteorological circumstances, typically within strong thunderstorms with powerful updrafts. Water droplets are carried high into the atmosphere, where they freeze and grow into larger, often irregularly shaped ice balls through repeated cycles of accretion and freezing. Sleet, in contrast, forms in layered temperature inversions and consists of smaller, more uniform ice pellets, making it distinct from the larger, often more damaging hailstones.