Freezing drizzle

Freezing drizzle is drizzle that freezes on contact with the ground or an object at or near the surface. Its METAR code is FZDZ.[1]


Although freezing drizzle and freezing rain are similar in that they both involve liquid precipitation at the surface during subfreezing temperatures, the mechanisms leading to their development are entirely different. Where freezing rain forms when frozen precipitation falls through a melting layer, freezing drizzle forms via the supercooled warm rain process in which cloud droplets coalesce until they become heavy enough to fall out of the cloud.[2] Despite this process taking place in a subfreezing environment, the liquid water will not freeze if the environmental temperature is above 18 °F (−8 °C).[3] If ice crystals are already present in this environment, the liquid droplets will freeze onto these crystals and be effectively removed before they can grow large enough to fall out of the cloud. As a result, freezing drizzle develops in shallow stratus-type clouds where saturation occurs entirely below the layer in which ice crystals can develop and grow.[2]


When freezing drizzle accumulates on land it creates an icy layer of glaze. Freezing drizzle alone does not generally result in significant ice accumulations due to its light, low-intensity nature. However, even thin layers of slick ice deposited on roads as black ice can cause extremely hazardous conditions resulting in vehicle crashes.

Freezing drizzle is extremely dangerous to aircraft as the supercooled water droplets will freeze onto the airframe, degrading aircraft performance considerably. The loss of American Eagle Flight 4184 on October 31, 1994 has been attributed to ice buildup due to freezing drizzle aloft.[2]

See also


  1. ^ Spence, Charles F. (2006). Aim/Far. McGraw Hill Professional. p. 294. ISBN 9780071479240. Retrieved 13 October 2017.
  2. ^ a b c Rauber, Robert M; Walsh, John E; Charlevoix, Donna Jean (2012). Severe & Hazardous Weather. ISBN 9780757597725.
  3. ^ "Freezing Drizzle on January 26th; A Look Into Why". National Weather Service. Retrieved 10 December 2017.
Anticyclonic tornado

An anticyclonic tornado is a tornado which rotates in a clockwise direction in the Northern Hemisphere and a counterclockwise direction in the Southern Hemisphere. The term is a naming convention denoting the anomaly from normal rotation which is cyclonic in upwards of 98 percent of tornadoes. Many anticyclonic tornadoes are smaller and weaker than cyclonic tornadoes, forming from a different process.


Drizzle is a light liquid precipitation consisting of liquid water drops smaller than those of rain – generally smaller than 0.5 mm (0.02 in) in diameter. Drizzle is normally produced by low stratiform clouds and stratocumulus clouds. Precipitation rates from drizzle are on the order of a millimetre per day or less at the ground. Owing to the small size of drizzle drops, under many circumstances drizzle largely evaporates before reaching the surface and so may be undetected by observers on the ground. The METAR code for drizzle is DZ and for freezing drizzle is FZDZ.

Fog season

The fog season is a season of fog that occurs in some places, because of special meteorological and topographical characteristics, after a rainy period. The fog season is usually based in the cooler months (late autumn, winter and early spring).

An example is found in the San Joaquin Valley and Sacramento Valley areas of California's Great Central Valley, where a thick ground fog, known as Tule fog, may form, in particular in the months from November through March. In the Tampa Bay area of Florida, the fog season is from December to February. Sydney's fog season is longer, starting from April through to October. Though it's more frequent in June due to more rain.

It is not generally true that fog season in a given area is during Fall or Winter (the cooler months); for example, the Japanese coast of the Pacific Ocean has a dense fog season from May to August. The June Gloom, a cloudy and foggy phenomena, experienced in the southern coast of California occurs in late spring and early summer (May and June).

This also occurs in St. Louis, MO during the fall and spring season. Sometimes during the winter as well. Its the Midwest.

Freezing rain

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 storm that produces a significant thickness of glaze ice from freezing rain is often referred to as an ice storm. Although these storms are not particularly violent, freezing rain is notorious for causing travel problems on roadways, breaking tree limbs, and downing power lines from the weight of accumulating ice. Downed power lines cause power outages in affected areas while accumulated ice can also pose significant overhead hazards. It is also known for being extremely dangerous to aircraft since the ice can effectively 'remould' the shape of the airfoil and flight control surfaces. (See atmospheric icing.)

Freezing rain advisory

A Freezing Rain Advisory was an advisory issued by the National Weather Service in the United States when freezing rain or freezing drizzle is expected to cause significant inconveniences, but does not meet warning criteria (typically greater than 1⁄4 inch or 6.4 millimetres of ice accumulation).On October 2, 2017, issuance of the Freezing Rain Advisory has been discontinued. A Winter Weather Advisory will be issued when the above criteria are forecast to be met. Ice Storm Warnings continue to be issued however.


A gale is a strong wind, typically used as a descriptor in nautical contexts. The U.S. National Weather Service defines a gale as 34–47 knots (63–87 km/h, 17.5–24.2 m/s or 39–54 miles/hour) of sustained surface winds. Forecasters typically issue gale warnings when winds of this strength are expected. In the United States, a gale warning is specifically a maritime warning; the land-based equivalent in National Weather Service warning products is a wind advisory.

Other sources use minima as low as 28 knots (52 km/h; 14 m/s; 32 mph), and maxima as high as 90 knots (170 km/h; 46 m/s; 100 mph). Through 1986, the National Hurricane Center used the term gale to refer to winds of tropical force for coastal areas, between 33 knots (61 km/h; 17 m/s; 38 mph) and 63 knots (117 km/h; 72 mph; 32 m/s). The 90 knots (170 km/h; 46 m/s; 100 mph) definition is very non-standard. A common alternative definition of the maximum is 55 knots (102 km/h; 63 mph; 28 m/s).The most common way of measuring wind force is with the Beaufort scale which defines a gale as wind from 50 kilometres per hour (14 m/s) to 102 kilometres per hour (28 m/s). It is an empirical measure for describing wind speed based mainly on observed sea conditions. On the original 1810 Beaufort wind force scale, there were four different "gale" designations whereas generally today there are two gale forces, 8 and 9, and a near gale 7:

The word gale is derived from the older gail, but its origin is uncertain.


Graupel (German pronunciation: [ˈɡʁaʊpəl]; Enɡlish: [ˈgɹaʊpəl]), 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 comes from the German language.

Graupel is distinct from hail, small hail and ice pellets: the World Meteorological Organization defines small hail as snow pellets encapsulated by ice, a precipitation halfway between snow pellets and hail. Small hail is common in thunderstorms, while graupel typically falls in winter storms. The METAR code for graupel is GS.

Ground blizzard

Ground blizzard refers to a weather condition where loose snow or ice on the ground is lifted and blown by strong winds. This can occur in the absence of precipitation, and can even occur when the sky is clear. This is in contrast to "ordinary" blizzards, which are accompanied by heavy falling snow. They can be especially dangerous as they occur after a winter storm has passed, when it is assumed that all forms of severe winter weather has ended.

Ice accretion indicator

The ice accretion indicator is an L-shaped piece of aluminium 38 cm (15 in) long by 4 to 5 cm (1.6 to 2.0 in) wide. It is used to indicate the formation of ice, frost or the presence of freezing rain or freezing drizzle.

It is normally attached to a Stevenson screen, about 1 m (3 ft 3 in) above ground, but may be mounted in other areas away from any artificial heat sources. The weather station would have two on site and they would be exchanged after every weather observation. The spare indicator should always be at the outside air temperature to ensure that it is ready for use and would normally be stored inside the screen.If the observer notes the presence of ice or frost on the indicator then a remark to that effect should be sent in the next weather observation. Examples of these are 'rime icing on indicator' and 'FROIN' (frost on indicator). As the indicator is at air temperature and is kept horizontal it provides an excellent surface on which to observe freezing precipitation.

Ice storm warning

An Ice Storm Warning is issued by the National Weather Service of the United States when freezing rain produces a significant and possibly damaging accumulation of ice. The criteria for this warning vary from state to state, but typically an ice storm warning will be issued any time more than 1⁄4 inch (6.4 mm) of ice is expected to accumulate in an area; in some areas, the criterion is 1⁄2 inch (13 mm).A freezing rain advisory or freezing drizzle advisory will be issued when a small amount of icing is possible.

In Canada, a Freezing Rain Warning has the same meaning.

List of severe weather phenomena

Severe weather phenomena are weather conditions that are hazardous to human life and property.

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.

Precipitation can fall in either liquid or solid phases, or transition between them at the freezing level. Liquid forms of precipitation include rain and drizzle. Rain or drizzle which freezes on contact within a subfreezing air mass gains the preceding adjective "freezing", becoming known as freezing rain or freezing drizzle. Frozen forms of precipitation include snow, ice needles, sleet, hail, and graupel. Their respective intensities are classified either by rate of fall, or by visibility restriction.

Siberian Express

Siberian Express is a meteorological term in the United States describing the arrival of an extremely cold air mass of Siberian origins. It specifically refers to an origin in Siberia. The term is most commonly employed by the news media when such a frigid air mass moves into areas where it is relatively unusual, such as the Deep South or Eastern Seaboard of the United States. Meteorologically, it is typically associated with a phenomenon called the Pacific-North American teleconnection pattern (PNA).

"Siberian Express" was the nickname coined by a meteorologist to describe the January 17, 1982 cold wave event hitting much of the United States. Also called "Cold Sunday", the event broke many all-time record lows.

Paleoclimatologist Jack A. Wolfe published in 1992 about the geographic origin. The frozen Arctic Ocean produced the frigid air for the "Siberian Express", the high-pressure system in Siberia which was southward blocked by the Himalayas and Tibetan Plateau, that transported the air down into North America.


Sirocco (), scirocco, jugo or, rarely, siroc (Catalan: Xaloc; Sicilian: Sciroccu; Greek: Σορόκος; Italian: Scirocco; Spanish: Siroco; Occitan: Siròc, Eisseròc; Croatian: Jugo, literally southerly; Libyan Arabic: Ghibli; Egypt: khamsin; Tunisia: ch'hilli) is a Mediterranean wind that comes from the Sahara and can reach hurricane speeds in North Africa and Southern Europe, especially during the summer season.


Slush, also called slush ice, is a slurry mixture of small ice crystals (e.g., snow) and liquid water.In the natural environment, slush forms when ice or snow melts. This often mixes with dirt and other materials, resulting in a gray or muddy brown color. Often, solid ice or snow can block the drainage of fluid water from slushy areas, so slush often goes through multiple freeze/thaw cycles before completely disappearing.

In areas where road salt is used to clear roadways, slush forms at lower temperatures than it would ordinarily in salted areas. This can produce a number of different consistencies over the same geographical area.

Snow flurry

A snow flurry is a term for a light snowfall that results in little or no snow accumulation. The US National Weather Service specifically defines snow flurries as intermittent light snow that produces no measurable precipitation (trace amounts). In contrast, bursts of snowfall which do result in measurable snow accumulation are called snow showers.

Snow grains

Snow grains are a form of precipitation. Snow grains are characterized as very small (<1 mm), white, opaque grains of ice that are fairly flat or elongated.

Unlike snow pellets, snow grains do not bounce or break up on impact. Usually, very small amounts fall, mostly from stratus clouds or fog, and never fall in the form of a shower.

The METAR code for snow grains is SG.

Tule fog

Tule fog is a thick ground fog that settles in the San Joaquin Valley and Sacramento Valley areas of California's Great Central Valley. Tule fog forms from late fall through early spring (California's rainy season) after the first significant rainfall. The official time frame for tule fog to form is from November 1 to March 31. This phenomenon is named after the tule grass wetlands (tulares) of the Central Valley. Tule fog is the leading cause of weather-related accidents in California.


In meteorology, visibility is a measure of the distance at which an object or light can be clearly discerned. It is reported within surface weather observations and METAR code either in meters or statute miles, depending upon the country. Visibility affects all forms of traffic: roads, sailing and aviation. Meteorological visibility refers to transparency of air: in dark, meteorological visibility is still the same as in daylight for the same air.

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