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Ice pellets (Commonwealth English) orsleet (American English) is a form ofprecipitation consisting of small, hard,translucent balls of ice. Ice pellets are different fromgraupel ("soft hail"), which is made of frosty white opaquerime, and froma mixture of rain and snow, which is aslushy liquid or semisolid. Ice pellets often bounce when they hit the ground or other solid objects, and make a higher-pitched "tap" when striking objects likejackets,windshields, anddried leaves, compared to the dull splat of liquid raindrops. Pellets generally do not freeze into other solid masses unless mixed withfreezing rain. TheMETAR code for ice pellets isPL (PE before November 1998[1]).
Ice pellets are known assleet in the United States, the official term used by the U.S.National Weather Service.[2] However, the termsleet refers to amixture of rain and snow in most Commonwealth countries instead,[3] includingCanada.[4] Because of this,Environment Canada never uses the termsleet, and uses the terms "ice pellets" or "wet snow" instead of sleet.[5]
Ice pellets form when a layer of above-freezing air is located between 1,500 and 3,000 meters (5,000 and 10,000 ft) above the ground, with sub-freezing air both above and below it. This causes the partial or complete melting of anysnowflakes falling through the warm layer (the French term for sleet,neige fondue, literally means "melted snow" because of this)[citation needed]. As they fall back into the sub-freezing layer closer to the surface, they re-freeze into ice pellets. However, if the sub-freezing layer beneath the warm layer is too small, the precipitation will not have time to re-freeze before hitting the surface, so it will becomefreezing rain and freeze on the surface instead. A temperature profile showing a warm layer above the ground is most likely to be found in advance of awarm front during the cold season,[6] but can occasionally be found behind a passingcold front, and often with astationary front.
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In most parts of the world, ice pellets only occur for brief periods and do not accumulate a significant and troublesome amount. However, across the easternUnited States and southeasternCanada, warm air flowing north from theGulf of Mexico ahead of a strongsynoptic-scale storm system can overrun cold, dense air at the surface for many hundreds of miles for an extended period of time. In these areas, ice pellet accumulations of2–5 cm(0.8–2.0 in) are not unheard of. The effects of a significant accumulation of ice pellets are not unlike an accumulation of snow. One significant difference however is that for the same volume of snow, an equal volume of ice pellets is significantly heavier and thus more difficult to clear away. Additionally, a volume of ice pellets takes significantly longer to melt compared to an equal volume of fresh snowfall due to less surface area.
Ice pellets: This is the term Canadians use to describe frozen rain drops which are five millimetres or less in diameter and bounce when they hit a hard surface. Americans call this sleet.
