How Does Sleet Form?

How and Why Do Sleet & Freezing Rain Form?

Sleet consists of clear or translucent particles of ice that form when ice crystals melt partially or totally and become more like raindrops and then re-freeze within a colder layer of air near the surface. The thickness of this cold layer is very important because it is what distinguishes between sleet and freezing rain.

If the cold layer is sufficiently thick, the raindrop will re-freeze and fall to the surface as a raindrop sized piece of ice called sleet.

If the cold layer is not thick enough and it is confined to the lowest level near the ground, the raindrops will not freeze until they have splashed onto the ground, thus covering the surface with a glaze of ice; this is called freezing rain. [If you recall, colder air is more dense and tends to sink toward the lowest levels of the atmosphere, thus colder air may remain near the surface keeping temperatures on the ground below freezing].

Below are two soundings that illustrate Sleet and Freezing Rain scenarios within the atmosphere. Can you guess which one is the typical sleet sounding and which is the typical freezing rain sounging?

Base Diagram taken from PSC website.

Base diagram taken from PSC website.

A. is a typical profile for sleet. You can see how the layer of colder air goes up higher in the atmosphere than it does in B, which is a typical profile for freezing rain. The layer of cold air in B is much more shallow and therefore, for a falling rain droplet, it would not have time to freeze while it is still in the air, but instead would freeze as it makes contact with the below freezing surface of the earth.

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