Ice pellets

Ice pellets are a form of precipitation consisting of small, translucent ice balls. This form of precipitation is also known as sleet in the United States and Canada. [cite web|url=http://www.weather.gov/glossary/index.php?word=sleet|title= Sleet (glossary entry)|publisher= National Oceanic and Atmospheric Administration's National Weather Service|accessdate=2007-03-20] Ice pellets are usually (but not always) smaller than hailstones. [cite web|url=http://www.weather.gov/glossary/index.php?word=hail|title= Hail (glossary entry)|publisher= National Oceanic and Atmospheric Administration's National Weather Service|accessdate=2007-03-20] They often bounce when they hit the ground, and generally do not freeze into a solid mass unless mixed with freezing rain. The METAR code for ice pellets is PL.

Formation

Ice pellets form when a layer of above-freezing air is located between 1500 meters (approximately 5,000 feet) and 3000 meters (approximately 10,000 feet) 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. 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, and freezing rain will be the result at the surface. 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. [http://www.weatherquestions.com/What_causes_ice_pellets.htm What causes ice pellets (sleet)?] Retrieved on 2007-12-08.]

Effects

In most parts of the world, ice pellets only occur for brief periods and never accumulate a significant amount. However, across the eastern United States and southeastern Canada, warm air flowing north from the Gulf of Mexico 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 2-5 cm are not unheard of. The effects of a significant accumulation of ice pellets are not unlike an accumulation of snow. One significant difference 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 will take significantly longer to melt compared to an equal volume of fresh snowfall.

See also

*Precipitation (meteorology)
*Black ice

References