In the United States, sleet means ice pellets: small grains of ice, usually transparent or translucent and about 0.2 inch (5 mm) across or smaller, that form when raindrops or largely melted snowflakes refreeze before reaching the ground. Because sleet is already frozen when it lands, it bounces off hard surfaces instead of freezing on them as a glaze.
How it forms
Snow falls from cold air aloft into a layer warmer than freezing and partly or mostly melts. The slushy drops then fall through a deep layer of below-freezing air near the surface and freeze back into pellets.
Sleet and freezing rain come from similar temperature profiles. The main differences are how deep and how warm the warm layer is and how deep the cold layer below it is.
Sleet or freezing rain
One National Weather Service (NWS) training rule of thumb favors sleet over freezing rain when the warm layer peaks below about 36°F (2°C) and is less than about 1,000 feet deep. Weather Prediction Center guidance similarly notes that when the warm layer's wet-bulb temperature stays below about 34°F (1°C), snow only partly melts and usually refreezes.
Not the same as graupel
Graupel, also called snow pellets, is white, opaque and easy to crush. It forms when supercooled droplets freeze onto a falling ice crystal as rime, so it starts as snow that gathers ice rather than a drop that refreezes.
A word with two meanings
In British terminology, and colloquially in parts of the United States, sleet means a mix of rain and snow. US forecast products use it to mean ice pellets.
NWS winter products treat sleet as one of the wintry elements. A Winter Weather Advisory, for example, can be issued for snow, sleet, ice or a combination that falls short of warning criteria.