Definition
Precipitation in the form of small, translucent ice pellets that form when raindrops fall through a layer of below-freezing air and freeze before reaching the ground. Sleet indicates a temperature inversion, with warmer air above a layer of colder air near the surface.
Plain English
Rain that turns into small ice pellets on the way down because it falls through a cold layer of air near the ground. The pellets bounce when they hit the surface.
Context Anchor
Seen in warm-front weather, where warmer air can ride over cold air near the ground and cause falling precipitation to melt, then freeze again.
Derivation
From Middle English 'slete,' meaning a mix of rain and snow or icy precipitation. The word has carried this weather meaning for centuries, so the everyday and aviation senses are closely related -- though aviation defines it more precisely as ice pellets, not a rain/snow mix.
Why Pilots Care
Sleet indicates conditions favorable for airframe icing and can rapidly reduce visibility and braking action on runways.
Grounding Statement
Picture raindrops falling into a shallow layer of freezing air near the ground and turning into tiny ice beads before they land.
Intuition Check
Sleet does not mean any cold, messy winter weather. In aviation weather, it specifically means small ice pellets falling from the sky.
Example Sentence 1
The METAR reported sleet at the destination, which told the pilot a warmer layer aloft was likely producing freezing rain in the climb.
Example Sentence 2
The forecast called for sleet mixed with rain, so the flight was delayed to avoid potential icing.