Ice pellets are a form of

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hai ...

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,

windshield The windshield (North American English) or windscreen (Commonwealth English) of an aircraft, car, bus, motorbike, truck, train, boat or streetcar is the front window, which provides visibility while protecting occupants from the elements. ...

s, 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 agency of the United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weather-related products to organizations and the public for the ...

. 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 ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tota ...

. Because of this, Environment Canada 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

snowflake A snowflake is a single ice crystal that has achieved a sufficient size, and may have amalgamated with others, which falls through the Earth's atmosphere as snow.Knight, C.; Knight, N. (1973). Snow crystals. Scientific American, vol. 228, no. ...

s 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 A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass, and is typically located on the equator-facing edge of an isotherm gradient. Warm fronts lie within broader troughs of low pressure than cold fr ...

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 A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface trough of low pressure. It often forms behind an extratropical cyclone (to the west in the Northern ...

, and often with a stationary front. Sleet on the ground

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 eastern

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., federal district, five ma ...

and southeastern

, warm air flowing north from the

Gulf of Mexico The Gulf of Mexico ( es, Golfo de México) is an ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North American continent. It is bounded on the northeast, north and northwest by the Gulf Coast of the United S ...

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.

References

External links

{{Authority control Precipitation Snow or ice weather phenomena

Terminology

Formation

Effects

See also

References

External links