Ice_pellets

Sleet is a term used in a variety of ways to describe precipitation that is intermediate between rain and snow but distinct from hail stones.

United Kingdom and Commonwealth country usage

In British English and in many Commonwealth countries, sleet refers to snow that has partially melted on its fall to the ground due to warmer surrounding air, yet has not completely melted into rain.Water in the atmosphere. Retrieved on 2007-03-20. This mix of rain and snow does not usually accumulate on the ground. On rare occasions, however, a very shallow layer of sub-freezing air right at the ground can cause the non-frozen portion of the precipitation to freeze on contact. The result of this mix of freezing rain and snow is a dangerously invisible layer of ice popularly known as "black ice". This "black ice" derives its name from the fact that affected surfaces appear to be wet, but instead coated in an extremely slippery layer of ice.

The METAR code for this usage varies. If surface temperatures are above freezing, SNRA is the proper coding when snowflakes predominate, and RASN is the proper coding when raindrops predominate. If surface temperatures are below freezing, FZRA replaces RA to indicate that the liquid portion is freezing on contact.

Canadian and United States usage

An accumulation of ice pellets

In Canadian usage, sleet is a form of precipitation consisting of frozen raindrops or refrozen partially melted snowflakes, also known as ice pellets. This term has also spread to the United StatesSleet (glossary entry). Retrieved on 2007-03-20. These pellets are considered a form of soft hail in Great Britain, but in Canada and the United States, the term hail is reserved for falling pieces of ice which have cycled through the melting/freezing layer more than once, which results in their growing larger due to the accretion of multiple layers of ice. Comparatively, individual ice pellets are the same size as the raindrops from which they formed, and are usually (but not always) smaller than hailstones.Hail (glossary entry). Retrieved on 2007-03-20. Ice pellets often bounce when they hit the ground, and they generally do not freeze into a solid mass unless mixed with freezing rain. The METAR code for ice pellets is PL.

Snow pellets are another form of precipitation, considered distinct from both hail and sleet in Canada and the United States.

Formation of ice pellets

Ice pellets form when partially or completely melted snowflakes refreeze before reaching the ground. In order for the snowflakes to significantly melt, the temperature aloft needs to rise a few degrees Celsius above the melting point. However, if the subfreezing layer below the warm layer is too narrow, freezing rain will be the result. A temperature profile showing warm air aloft 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.

See also

• Precipitation (meteorology)
• Black ice

References

This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia