Sleet (also called aguanieve in Spanish) refers to a mixture of rain and snow, or to small, translucent ice pellets that bounce on impact. The term has different meanings in British and American English: in the UK, sleet is a rain-snow mix; in the US, sleet specifically means ice pellets. Both forms are winter precipitation types associated with the transition zone around the warm front of a depression.
Ice pellets form when snowflakes fall through a warm layer aloft (above 0 °C) that partially melts them, followed by a cold layer near the surface (below 0 °C) that refreezes the drops into small balls of ice before they hit the ground. The key difference from freezing rain is the depth of the cold surface layer: if it is deep enough (>500 m), the drops refreeze completely into pellets; if too shallow, they arrive as supercooled liquid and freeze on contact as glaze ice.
Sleet is often a transitional phenomenon — it frequently indicates that conditions are near the rain/snow boundary and may shift in either direction as the temperature profile evolves. For drivers, sleet creates hazardous conditions as ice pellets accumulate on roads. In aviation meteorology, sleet and freezing precipitation are reported with specific codes in METAR observations. In Spain, sleet most commonly occurs in the northern meseta and mountain areas during cold-air outbreaks between November and March, especially when warm fronts approach from the Atlantic.