Sleet forms when snows travels through a layer of warm air, melting into raindrops, before re-freezing into tiny ice pellets upon hitting a colder layer of air.
Sleet can only form under very specific weather conditions, which is why it is not as common as other forms of precipitation. It usually occurs during the winter when the proper conditions are most common. Sleet is different than freezing rain. Freezing rain also falls through a layer of cold air towards the ground. However, it freezes upon contact with the object and not in the air. Freezing rain lands as rain, but sleet lands as ice. Sleet is also different than hail. Similarly to sleet, hail is made of ice. Hail, however, freezes while in the clouds. Sleet freezes while it falls.
Additionally, hail freezes in an outward direction, but sleet freezes in an inward direction. Sleet is most common in the eastern United States and Canada, where warm air from the Gulf of Mexico flows north and can create the warm layer of air needed to form sleet. Unlike other forms of precipitation, sleet is very light and does not collect easily. Because of this, sleet cannot harm crops or vehicles.