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Freezing drizzle is drizzle that freezes on contact with the ground or an object at or near the surface. Its METAR code is FZDZ. 
Although freezing drizzle and freezing rain are similar in that they both involve liquid precipitation above the surface in subfreezing temperatures and freeze on the surface, the mechanisms leading to their development are entirely different. Where freezing rain forms when frozen precipitation falls through a melting layer and turns liquid, 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, but in subfreezing conditions.  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), via supercooling.  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 low-level stratus-type clouds where air saturation occurs entirely below the layer in which ice crystals can develop and grow. 
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 unlike it's rain counterpart. However, even thin layers of slick ice deposited on roads as black ice can be very slippery and cause extremely hazardous conditions resulting in vehicle crashes.
Freezing drizzle is extremely dangerous to aircraft in icing conditions, as the supercooled water droplets will freeze onto the airframe, degrading aircraft performance considerably. The loss and accident of American Eagle Flight 4184 on October 31, 1994, has been attributed to ice buildup due to freezing drizzle aloft. 
Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or 32 degrees Fahrenheit. Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaque bluish-white color.
Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water throughout its life cycle, starting when, under suitable conditions, the ice crystals form in the atmosphere, increase to millimeter size, precipitate and accumulate on surfaces, then metamorphose in place, and ultimately melt, slide or sublimate away.
Cumulus clouds are clouds which have flat bases and are often described as "puffy", "cotton-like" or "fluffy" in appearance. Their name derives from the Latin cumulo-, meaning heap or pile. Cumulus clouds are low-level clouds, generally less than 2,000 m (6,600 ft) in altitude unless they are the more vertical cumulus congestus form. Cumulus clouds may appear by themselves, in lines, or in clusters.
Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air at or near the Earth's surface. Fog can be considered a type of low-lying cloud usually resembling stratus, and is heavily influenced by nearby bodies of water, topography, and wind conditions. In turn, fog affects many human activities, such as shipping, travel, and warfare.
Diamond dust is a ground-level cloud composed of tiny ice crystals. This meteorological phenomenon is also referred to simply as ice crystals and is reported in the METAR code as IC. Diamond dust generally forms under otherwise clear or nearly clear skies, so it is sometimes referred to as clear-sky precipitation. Diamond dust is most commonly observed in Antarctica and the Arctic, but can occur anywhere with a temperature well below freezing. In the polar regions of Earth, diamond dust may persist for several days without interruption.
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. They are generally not violent storms but instead are commonly perceived as gentle rains occurring at temperatures just below freezing.
Freezing rain is 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 or ice pellets, 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.
Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid or a gas below its freezing point without it becoming a solid. It achieves this in the absence of a seed crystal or nucleus around which a crystal structure can form. The supercooling of water can be achieved without any special techniques other than chemical demineralization, down to −48.3 °C (−55 °F). Droplets of supercooled water often exist in stratus and cumulus clouds. An aircraft flying through such a cloud sees an abrupt crystallization of these droplets, which can result in the formation of ice on the aircraft's wings or blockage of its instruments and probes.
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzling, rain, sleet, snow, ice pellets, graupel and hail. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor, so that the water condenses and "precipitates" or falls. Thus, fog and mist are not precipitation but colloids, 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.
In physics and chemistry, flash freezing is the process whereby objects are frozen in just a few hours by subjecting them to cryogenic temperatures, or through direct contact with liquid nitrogen at −196 °C (−320.8 °F). It is commonly used in the food industry.
Cloud physics is the study of the physical processes that lead to the formation, growth and precipitation of atmospheric clouds. These aerosols are found in the troposphere, stratosphere, and mesosphere, which collectively make up the greatest part of the homosphere. Clouds consist of microscopic droplets of liquid water, tiny crystals of ice, or both. Cloud droplets initially form by the condensation of water vapor onto condensation nuclei when the supersaturation of air exceeds a critical value according to Köhler theory. Cloud condensation nuclei are necessary for cloud droplets formation because of the Kelvin effect, which describes the change in saturation vapor pressure due to a curved surface. At small radii, the amount of supersaturation needed for condensation to occur is so large, that it does not happen naturally. Raoult's law describes how the vapor pressure is dependent on the amount of solute in a solution. At high concentrations, when the cloud droplets are small, the supersaturation required is smaller than without the presence of a nucleus.
Rime ice forms when supercooled water liquid droplets freeze onto surfaces. Meteorologists distinguish between three basic types of ice forming on vertical and horizontal surfaces by deposition of supercooled water droplets. There are also intermediate formations.
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 (0.04 in) 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.
Nucleation is the first step in the formation of either a new thermodynamic phase or a new structure via self-assembly or self-organization. Nucleation is typically defined to be the process that determines how long an observer has to wait before the new phase or self-organized structure appears. For example, if a volume of water is cooled below 0 °C, it will tend to freeze into ice, but volumes of water cooled only a few degrees below 0 °C often stay completely free of ice for long periods. At these conditions, nucleation of ice is either slow or does not occur at all. However, at lower temperatures nucleation is fast, and ice crystals appear after little or no delay. Nucleation is a common mechanism which generates first-order phase transitions, and it is the start of the process of forming a new thermodynamic phase. In contrast, new phases at continuous phase transitions start to form immediately.
The Wegener–Bergeron–Findeisen process, is a process of ice crystal growth that occurs in mixed phase clouds in regions where the ambient vapor pressure falls between the saturation vapor pressure over water and the lower saturation vapor pressure over ice. This is a subsaturated environment for liquid water but a supersaturated environment for ice resulting in rapid evaporation of liquid water and rapid ice crystal growth through vapor deposition. If the number density of ice is small compared to liquid water, the ice crystals can grow large enough to fall out of the cloud, melting into rain drops if lower level temperatures are warm enough.
Graupel, also called soft hail, hominy snow, 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 crisp, opaque rime.
Glaze or glaze ice, also called glazed frost, is a smooth, transparent and homogeneous ice coating occurring when freezing rain or drizzle hits a surface. It is similar in appearance to clear ice, which forms from supercooled water droplets. It is a relatively common occurrence in temperate climates in the winter when precipitation forms in warm air aloft and falls into below-freezing temperature at the surface.
In meteorology, the different types of precipitation often include the character, formation, 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 and condenses on the slope, such as a mountain.
This glossary of meteorology is a list of terms and concepts relevant to meteorology and atmospheric science, their sub-disciplines, and related fields.
A shower is a mode of precipitation characterized by an abrupt start and end and by rapid variations in intensity. Often strong and short-lived, it comes from convective clouds, like cumulus congestus. A shower will produce rain if the temperature is above the freezing point in the cloud, or snow / ice pellets / snow pellets / hail if the temperature is below it at some point. In a meteorological observation, such as the METAR, they are noted SH giving respectively SHRA, SHSN, SHPL, SHGS and SHGR.