Sometimes the weather forecast warns of “sleet,” rather than snow. When meteorologists in the United States use this term, they are referring to tiny ice pellets (the size of a pea, at most) formed when falling snow melts then quickly refreezes. (In the United Kingdom, sleet usually refers to a wintry mix). These pellets typically bounce as they hit the ground. Sleet can be dangerous, quickly coating the surface of roads and making driving hazardous.

Sleet Forms in Layers of Air (Warm above Cold)

To understand how sleet forms, it helps to know how snow forms.

The air closest to Earth’s surface—the layer where weather happens—is called the troposphere. In general, the higher you go in the troposphere, the colder the air becomes.

During precipitation formation, if temperatures are at or below freezing, 0°C (32°F), at cloud level, water in the air freezes into ice crystals, and the crystals stick together to make snow. The snow starts to fall, and if the air column is freezing cold all the way down from the clouds to the ground, the precipitation stays frozen. It simply falls as snow.

Sometimes, however, a temperature inversion occurs. Normally, the temperature decreases with increasing altitude. A temperature inversion is when a layer of warm air intrudes between the ground and the clouds.

Under these conditions, when the falling snow reaches the layer of warm air, it melts. Then it hits the layer of cold air just above Earth’s surface and refreezes. This all happens very fast, and the result is tiny ice pellets called sleet.

Sleet, Freezing Rain, Hail ... What Is the Difference?

The conditions that lead to freezing rain are similar to those for sleet: Snow falls through a layer of warm air and melts into raindrops, then is intercepted by a layer of freezing cold air just above Earth’s surface. When that bottom-most layer of cold air is thin, the melted snow does not have time refreeze as it falls through. It hits the ground as liquid water—rain—then freezes as it touches a freezing cold surface, such as a tree branch, a road, or a bridge.

Hail also consists of ice pellets, but hailstones are larger than the tiny pellets that make up sleet. Hailstones form when the updrafts generated by thunderstorms (which are more common in spring and summer than winter) quickly lift water droplets high in the troposphere, where they freeze at very low temperatures, then fall.