Rain and snow mixed

A small amount of slush can be produced from a mixture of rain and snow

Rain and snow mixed or sleet is precipitation composed of rain and partially melted snow. Unlike ice pellets, which are hard, and freezing rain, which is fluid until striking an object, this precipitation is soft and translucent, but it contains some traces of ice crystals, from partially fused snowflakes. In any one location, it usually occurs briefly as a transition phase from rain to snow or vice versa. Its METAR code is RASN.[1]

Terminology

This precipitation type is commonly known as sleet in most Commonwealth countries,[2] including Canada.[3] However, the United States National Weather Service uses the term sleet to refer to ice pellets.[4]

Formation

Rain snow mix soundings - Left diagram shows typical skew-T appearance, while right diagram shows variations which result in mixtures of rain and snow

This precipitation occurs when the temperature in the lowest part of the atmosphere is slightly above the freezing point of water (0 °C or 32 °F). The depth of low-level warm air (below the freezing level) needed to melt snow falling from above to rain varies from about 230–460 m (750–1,500 ft) and depends on the mass of the flakes and the lapse rate of the melting layer. Rain and snow typically mix when the melting layer depth falls between these values.[5]

"Wintry showers" or "wintry mixes"

Wintry showers is a somewhat informal meteorological term, used primarily in the United Kingdom, to refer to various mixtures of rain, graupel and snow. Though no "official" criteria exist for the term, in the United Kingdom the term is not used when any significant accumulation of snow on the ground takes place. It is often used when the temperature of the ground surface is above 0 °C (32 °F), preventing accumulation from occurring even if the air temperature is marginally below 0 °C (32 °F); but even then, the falling precipitation must generally be something other than exclusively snow.

In the United States, wintry mix generally refers to a mixture of freezing rain, ice pellets, and snow.[6] In contrast to the usage in the United Kingdom, in the United States it is usually used when air and ground temperatures are below 0 °C (32 °F). Additionally, it is generally used when some accumulation of ice and snow is expected to occur. During winter, a wide area can be affected by the multiple precipitation types typical of a wintry mix during a single winter storm, as counter-clockwise winds around a storm system bring warm air northwards ahead of the system, and then bring cold air back southwards behind it. Most often, it is the region ahead of the approaching storm system which sees the wintry mix, as warm air moves northward and above retreating cold air, causing snow to change to ice pellets, freezing rain and finally rain. The reverse transition can occur behind the departing low pressure system, though it is more common for precipitation to change directly from rain to snow, or for it to stop before a transition back.

See also

References

  1. ↑ http://glossary.ametsoc.org/wiki/Rain_and_snow_mixed
  2. ↑ http://dictionary.cambridge.org/dictionary/british/sleet
  3. ↑ "Weather Glossary". Environment Canada. Retrieved 2015-03-30. 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.
  4. ↑ http://w1.weather.gov/glossary/index.php?word=sleet
  5. ↑ http://www.crh.noaa.gov/lmk/?n=winter-precip
  6. ↑ http://oxforddictionaries.com/us/definition/american_english/wintry-mix
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