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Ice pellets ( Commonwealth English) or sleet (
Ice pellets form when a layer of above-freezing air is located between above the ground, with sub-freezing air both above and below it. This causes the partial or complete melting of any snowflakes falling through the warm layer (the French term for sleet, ''neige fondue'', literally means "melted snow" because of this). 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 become freezing 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 a warm front during the cold season,Weatherquestions.com
What causes ice pellets (sleet)?
Retrieved on 2007-12-08. but can occasionally be found behind a passing cold front, and often with a stationary front.
American English
American English, sometimes called United States English or U.S. English, is the set of variety (linguistics), varieties of the English language native to the United States. English is the Languages of the United States, most widely spoken lang ...
) is a form of precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...
consisting of small, hard, translucent balls of ice. Ice pellets are different from graupel ("soft hail"), which is made of frosty white opaque rime, and from a mixture of rain and snow, which is a slushy 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 like jackets, windshields, and dried leaves, compared to the dull splat of liquid raindrops. Pellets generally do not freeze into other solid masses unless mixed with freezing rain. The METAR code for ice pellets is PL (PE before November 1998).
Terminology
Ice pellets are known as sleet in the United States, the official term used by the U.S.National Weather Service
The National Weather Service (NWS) is an Government agency, agency of the Federal government of the United States, United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weathe ...
. However, the term ''sleet'' refers to a mixture of rain and snow in most Commonwealth countries instead, including Canada
Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...
. Because of this, Environment Canada
Environment and Climate Change Canada (ECCC; )Environment and Climate Change Canada is the applied title under the Federal Identity Program; the legal title is Department of the Environment (). is the Ministry (government department), department ...
never uses the term ''sleet'', and uses the terms "ice pellets" or "wet snow" instead.
Formation
Ice pellets form when a layer of above-freezing air is located between above the ground, with sub-freezing air both above and below it. This causes the partial or complete melting of any snowflakes falling through the warm layer (the French term for sleet, ''neige fondue'', literally means "melted snow" because of this). 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 become freezing 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 a warm front during the cold season,Weatherquestions.comWhat causes ice pellets (sleet)?
Retrieved on 2007-12-08. but can occasionally be found behind a passing cold front, and often with a stationary front.
Effects
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
The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
and southeastern Canada
Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...
, warm air flowing north from the Gulf of Mexico
The Gulf of Mexico () is an oceanic basin and a marginal sea of the Atlantic Ocean, mostly surrounded by the North American continent. It is bounded on the northeast, north, and northwest by the Gulf Coast of the United States; on the southw ...
ahead of a strong synoptic-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 of 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.
See also
* Freezing rain * Graupel * Hail * Sleet (disambiguation)References
External links
{{Authority control Precipitation Snow or ice weather phenomena