What is sleet?

Sleet is specifically defined as the phenomenon that occurs when raindrops freeze while falling through a layer of cold air, transforming them into small ice pellets before they reach the ground. This process typically occurs when the atmospheric conditions include a warm layer of air aloft and a colder layer closer to the surface. As the droplets pass through the colder air, they freeze, resulting in sleet, which can then accumulate on surfaces, creating hazardous conditions.

This is distinct from the other options: supercooled drops that freeze on contact with a surface refer to a different phenomenon known as freezing rain, wherein droplets freeze upon hitting the ground or other surfaces rather than while falling through the cold air. Liquid precipitation with a diameter less than 0.5 mm describes drizzle, which is a type of light rain but does not involve freezing or ice. Lastly, heavy snow formed from rimed particles refers to a process related to how snow can gain mass and density due to the freezing of supercooled water on its surface, but this does not relate to the definition or characteristics of sleet.