Rain and snow mixed

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A small amount of slush can be produced from a mixture of rain and snow 2014-03-04 06 31 33 Slush produced from a mixture of rain and snow (Commonwealth definition of sleet).JPG
A small amount of slush can be produced from a mixture of rain and snow

Rain and snow mixed 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]

Contents

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

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]

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

"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

Related Research Articles

Sleet is a regionally variant term for two distinct forms of precipitation:

An ice storm is a type of winter storm characterized by freezing rain, also known as a glaze event or, in some parts of the United States, as a silver thaw. The U.S. National Weather Service defines an ice storm as a storm which results in the accumulation of at least 0.25-inch (6.4 mm) of ice on exposed surfaces. From 1982 to 1994, ice storms were more common than blizzards in the U.S., averaging 16 per year. They are generally not violent storms but instead are commonly perceived as gentle rains occurring at temperatures just below freezing.

Freezing rain is the name given to rain maintained at temperatures below freezing by the ambient air mass that causes freezing on contact with surfaces. Unlike a mixture of rain and snow, ice pellets, or hail, freezing rain is made entirely of liquid droplets. The raindrops become supercooled while passing through a sub-freezing layer of air hundreds of meters above the ground, and then freeze upon impact with any surface they encounter, including the ground, trees, electrical wires, aircraft, and automobiles. The resulting ice, called glaze ice, can accumulate to a thickness of several centimeters and cover all exposed surfaces. The METAR code for freezing rain is FZRA.

A winter storm is an event in which varieties of precipitation are formed that only occur at low temperatures, such as snow or sleet, or a rainstorm where ground temperatures are low enough to allow ice to form. In temperate continental climates, these storms are not necessarily restricted to the winter season, but may occur in the late autumn and early spring as well. Very rarely, they may form in summer, though it would have to be an abnormally cold summer, such as the summer of 1816 in the Northeastern United States.

Precipitation product of the condensation of atmospheric water vapour that falls under gravity

In meteorology, precipitation is any product of the condensation of atmospheric water vapour that falls under gravity. The main forms of precipitation include drizzle, rain, sleet, snow, graupel and hail. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor, so that the water condenses and "precipitates". Thus, fog and mist are not precipitation but suspensions, because the water vapor does not condense sufficiently to precipitate. Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapor to the air. Precipitation forms as smaller droplets coalesce via collision with other rain drops or ice crystals within a cloud. Short, intense periods of rain in scattered locations are called "showers."

Black ice Thin coating of glazed ice on a surface

Black ice, sometimes called clear ice, is a thin coating of glaze ice on a surface, especially on roads. The ice itself is not black, but visually transparent, allowing the often black road below to be seen through it. The typically low levels of noticeable ice pellets, snow, or sleet surrounding black ice means that areas of the ice are often practically invisible to drivers or people stepping on it. There is, thus, a risk of slippage and subsequent accident due to the unexpected loss of traction.

A winter storm warning is a statement made by the National Weather Service of the United States which means a winter storm is occurring or is about to occur in the area, usually within 36 hours. Generally, a Winter Storm Warning is issued if the following criteria, at least, are forecast: usually between 4 inches (10 cm) to 7 inches (18 cm) or more of snow or usually 3 inches (7.6 cm) or more of snow with a large accumulation of ice. In the Southern United States, where severe winter weather is much less common and any snow is a more significant event, warning criteria are lower, as low as 1 inch (2.5 cm) in the southernmost areas: as one goes from north to south, the necessary accumulations lessen. A warning can also be issued during high impact events of lesser amounts, usually early or very late in the season when trees have leaves and damage can result. Winter Storm Warnings are issued when winds are less than 35mph; if the storm has winds above this wind speed, it becomes a blizzard warning.

A Winter Weather Advisory is issued by the National Weather Service of the United States when a low pressure system produces a combination of winter weather that presents a hazard, but does not meet warning criteria. A Winter Weather Advisory is similar to significant weather advisory, but a winter weather advisory is an official product. A similar warning is issued by Environment Canada's Meteorological Service of Canada offices.

This article describes severe weather terminology used by the National Weather Service (NWS) in the United States. The NWS, a government agency operating as an arm of the National Oceanic and Atmospheric Administration (NOAA) branch of the United States Department of Commerce (DoC), defines precise meanings for nearly all of its weather terms. This article describes NWS terminology and related weather scales used by the agency. Some terms may be specific to certain cities or regions.

Ice pellets are a form of precipitation consisting of small, translucent balls of ice. Ice pellets are smaller than hailstones which form in thunderstorms rather than in winter, and are different from graupel which is made of frosty white 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 a solid mass unless mixed with freezing rain. The METAR code for ice pellets is PL.

Graupel, also called soft hail or snow pellets, is precipitation that forms when supercooled water droplets are collected and freeze on falling snowflakes, forming 2–5 mm (0.08–0.20 in) balls of rime. The term graupel is the German language word for sleet.

December 21–24, 2004 North American winter storm

A historic snowstorm struck the Ohio Valley of the United States, as well as Ontario in Canada, on December 22 and December 23 and is not the same storm that led to snow in Texas on Christmas Eve. It lasted roughly 30 hours, and brought snowfall amounts up to 29 inches (74 cm) to portions of the Midwestern United States. Damages from the storm totaled US$900 million (2004 dollars). A total of 18 died during the storm, one from Canada, mainly due to car accidents.

Snow in Florida snow events in Florida, USA

It is very rare for snow to fall in the U.S. state of Florida, especially in the central and southern portions of the state. With the exception of the far northern areas of the state, most of the major cities in Florida have rarely recorded measurable snowfall, though trace (T) amounts have been widely recorded, or flurries observed a few times each century. According to the National Weather Service, in the Florida Keys and Key West there is no known occurrence of snow flurries since the settlement of the region. In Miami, Ft. Lauderdale, and Palm Beach there is only one known report of snow flurries observed in January 1977.

Cold-air damming

Cold air damming, or CAD, is a meteorological phenomenon that involves a high-pressure system (anticyclone) accelerating equatorward east of a north-south oriented mountain range due to the formation of a barrier jet behind a cold front associated with the poleward portion of a split upper level trough. Initially, a high-pressure system moves poleward of a north-south mountain range. Once it sloshes over poleward and eastward of the range, the flow around the high banks up against the mountains, forming a barrier jet which funnels cool air down a stretch of land east of the mountains. The higher the mountain chain, the deeper the cold air mass becomes lodged to its east, and the greater impediment it is within the flow pattern and the more resistant it becomes to intrusions of milder air.

Precipitation types

In meteorology, the various types of precipitation often include the character or phase of the precipitation which is falling to ground level. There are three distinct ways that precipitation can occur. Convective precipitation is generally more intense, and of shorter duration, than stratiform precipitation. Orographic precipitation occurs when moist air is forced upwards over rising terrain, such as a mountain.

Outline of meteorology Overview of and topical guide to meteorology

The following outline is provided as an overview of and topical guide to meteorology:

Classifications of snow

Classifications of snow describe and categorize the attributes of snow-generating weather events, including the individual crystals both in the air and on the ground, and the deposited snow pack as it changes over time. Snow can be classified by describing the weather event that is producing it, the shape of its ice crystals or flakes, how it collects on the ground, and thereafter how it changes form and composition. Depending on the status of the snow in the air or on the ground, a different classification applies.

December 1969 noreaster

The December 1969 nor'easter was a strong winter storm that mainly affected the Northeastern United States and southern Quebec between December 25 and December 28, 1969. The multi-faceted storm system included a tornado outbreak, record snow accumulations, a damaging ice storm, and flooding rains.

2013–14 North American winter

The 2013–14 North American winter refers to winter in North America as it occurred across the continent from late 2013 through early 2014. The winter of 2013–14 was one of the most significant for the United States, due in part to the breakdown of the polar vortex in November 2013, which allowed very cold air to travel down into the United States, leading to an extended period of very cold temperatures. The pattern continued mostly uninterrupted throughout the winter and numerous significant winter storms affected the Eastern United States, with the most notable one being a powerful winter storm that dumped ice and snow in the Southeast and Northeast in mid-February. Most of the cold weather abated by the end of March, though a few winter storms did affect the western portions of the U.S. towards the end of the winter.

Trace (precipitation)

In meteorology, a trace denotes an amount of precipitation, such as rain or snow, that is greater than zero, but is too small to be measured by standard units or methods of measurement. The designation of a trace rather than zero is used to indicate that precipitation did fall, but not enough to be measured reliably. This is important for both weather forecasting and climatological purposes, because even precipitation amounts too small to be measured can have significant societal impacts.

References

  1. "Rain and snow mixed - AMS Glossary". Glossary.ametsoc.org. 2012-01-26. Retrieved 2019-06-13.
  2. "SLEET | meaning in the Cambridge English Dictionary". Dictionary.cambridge.org. Retrieved 2019-06-13.
  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. "Glossary - NOAA's National Weather Service". W1.weather.gov. 2009-06-25. Retrieved 2019-06-13.
  5. Crh.noaa.gov [ dead link ]
  6. "wintry mix - Definition of wintry mix in English by Lexico Dictionaries". Lexico Dictionaries - English.