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Graupel (; ), also called soft hail, corn snow, hominy snow, or snow pellets, is
precipitation In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw mod ...
that forms when
supercooled Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isoc ...
water
droplet A drop or droplet is a small column of liquid A liquid is a nearly incompressible In fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics Mechanics (Ancient Greek, Greek: ) is the area of ph ...

droplet
s are collected and freeze on falling
snowflake A snowflake is a single ice crystal Ice is water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the mai ...

snowflake
s, forming balls of crisp, opaque
rime Rime may refer to: *Rime ice Rime ice forms when supercooled water liquid droplets freeze onto surfaces. Meteorology, Meteorologists distinguish between three basic types of ice forming on vertical and horizontal surfaces by deposition of superco ...
. Graupel is distinct from
hail Hail is a form of solid precipitation In meteorology Meteorology is a branch of the atmospheric sciences Atmospheric science is the study of the Earth's atmosphere File:Atmosphere gas proportions.svg, Composition of Earth's atmo ...

hail
and
ice pellets Ice pellets are a form of precipitation (meteorology), precipitation consisting of small, translucent balls of ice. Ice pellets are different from graupel ("soft hail") which is made of frosty white hard rime, rime, and from rain and snow mixed, ...

ice pellets
. Hail is common in
thunderstorm A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning Lightning is a naturally occurring electrostatic discharge Electrostatic discharge (ESD) is the sudden fl ...

thunderstorm
s, while graupel typically falls in
winter storm A winter storm is an event in which wind coincides with varieties of precipitation In meteorology Meteorology is a branch of the atmospheric sciences Atmospheric science is the study of the Earth's atmosphere File:Atmosphere g ...
s and in convective showers especially at higher elevations. The
METAR METAR is a format for reporting weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloud cover, cloudy. On Earth, most weather phenomena occur ...
code for graupel is GS.


Formation

Under some
atmospheric An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet A planet is an astronomical body orbi ...

atmospheric
conditions,
snow Snow comprises individual ice Ice is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). ...

snow
crystals may encounter
supercooled Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isoc ...
water
droplets Raindrops in a plant. A drop or droplet is a small column of liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, ...
. These droplets, which have a diameter of about , can exist in the liquid
state State may refer to: Arts, entertainment, and media Literature * ''State Magazine'', a monthly magazine published by the U.S. Department of State * The State (newspaper), ''The State'' (newspaper), a daily newspaper in Columbia, South Carolina, Un ...
at temperatures as low as , far below the normal
freezing Freezing is a phase transition where a liquid turns into a solid when its temperature is lowered below its freezing point. In accordance with the internationally established definition, freezing means the solidification phase change of a liquid o ...

freezing
point. Contact between a snow crystal and the supercooled droplets results in freezing of the liquid droplets onto the surface of the crystal. This process of crystal growth is known as
accretion Accretion may refer to: Science * Accretion (astrophysics), the formation of planets and other bodies by collection of material through gravity * Accretion (meteorology), the process by which water vapor in clouds forms water droplets around nucle ...
. Crystals that exhibit frozen droplets on their surfaces are often referred to as rimed. When this process continues so that the shape of the original snow crystal is no longer identifiable, the resulting crystal is referred to as graupel. Graupel was formerly referred to by meteorologists as "soft hail." However, graupel is easily distinguishable from hail in both the shape and strength of the pellet and the circumstances in which it falls. Ice from
hail Hail is a form of solid precipitation In meteorology Meteorology is a branch of the atmospheric sciences Atmospheric science is the study of the Earth's atmosphere File:Atmosphere gas proportions.svg, Composition of Earth's atmo ...

hail
is formed in hard, relatively uniform layers and usually falls only during
thunderstorm A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning Lightning is a naturally occurring electrostatic discharge Electrostatic discharge (ESD) is the sudden fl ...

thunderstorm
s. Graupel forms fragile, oblong crystals and falls in place of typical
snowflake A snowflake is a single ice crystal Ice is water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the mai ...

snowflake
s in
wintry mix Rain and snow mixed is precipitation (meteorology), 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 transluce ...
situations, often in concert with
ice pellets Ice pellets are a form of precipitation (meteorology), precipitation consisting of small, translucent balls of ice. Ice pellets are different from graupel ("soft hail") which is made of frosty white hard rime, rime, and from rain and snow mixed, ...

ice pellets
. Graupel is also fragile enough that it will typically fall apart when touched.


Microscopic structure

The frozen droplets on the surface of rimed crystals are difficult to see, and the topography of a graupel particle is not easy to record with a
light microscope The optical microscope, also referred to as a light microscope, is a type of microscope that commonly uses visible spectrum, visible light and a system of lens (optics), lenses to generate magnified images of small objects. Optical microscopes ...
because of the limited
resolution Resolution(s) may refer to: Common meanings * Resolution (debate), the statement which is debated in policy debate * Resolution (law), a written motion adopted by a deliberative body * New Year's resolution, a commitment that an individual make ...
and
depth of field For many cameras, depth of field (DOF) is the distance between the nearest and the farthest objects that are in acceptably sharp focus in an image. The depth of field can be calculated based on focal length The focal length of an optical ...

depth of field
in the instrument. However, observations of snow crystals with a low-temperature
scanning electron microscope A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that ...
(LT-SEM) clearly show cloud droplets measuring up to on the surface of the crystals. The rime has been observed on all four basic forms of snow crystals, including plates, dendrites, columns and needles. As the riming process continues, the mass of frozen, accumulated cloud droplets eventually obscures the form of the original snow crystal, thereby giving rise to graupel. File:Graupel encasing a snow crystal.jpg, Graupel encasing and hiding a snow crystal from view File:Snowflake 300um LTSEM, 13368.jpg, Rime on both ends of a columnar snow crystal


Graupel and avalanches

Graupel commonly forms in high-altitude climates and is both denser and more
granular Granularity (also called graininess), the condition of existing in granules or grains A grain is a small, hard, dry seed A seed is an embryonic plant enclosed in a protective outer covering. The formation of the seed is part of the pro ...
than ordinary
snow Snow comprises individual ice Ice is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). ...

snow
, due to its rimed exterior. Macroscopically, graupel resembles small beads of
polystyrene Polystyrene (PS) is a synthetic Aromaticity, aromatic hydrocarbon polymer made from the monomer known as styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin per un ...

polystyrene
. The combination of density and low viscosity makes fresh layers of graupel unstable on slopes, and layers of present a high risk of dangerous slab
avalanche An avalanche (also called a snowslide) is a rapid flow of snow 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 fur ...

avalanche
s. In addition, thinner layers of graupel falling at low temperatures can act as ball bearings below subsequent falls of more naturally stable snow, rendering them also liable to avalanche. Graupel tends to compact and stabilise ("weld") approximately one or two days after falling, depending on the temperature and the properties of the graupel.


Gallery

File:Schneeflocke wird zu Graupel.jpg, Snowflakes can turn into graupel File:Schneeflocke wird zu Graupel 2.jpg, Almost graupel File:Schneeflocke3.jpg, Graupel in shape of snowflake


See also

* Sleet - term variously used for frozen precipitation


References


External links

{{commons category


Dictionaries


3 results for:graupel
Dictionary.com, accessed September 12, 2006.
Graupel
''Merriam-Webster Online Dictionary'', accessed September 12, 2006.


Weather glossaries



The Weather Channel, accessed September 12, 2006.

National Snow and Ice Data Center (NSIDC), accessed September 12, 2006.

National Oceanic and Atmospheric Administration (NOAA), accessed September 12, 2006.

Weather at
About.com Dotdash (formerly About.com) is an American digital media Digital media means any media (communication), communication media that operate with the use of any of various encoded machine-readable data formats. Digital media can be created, vi ...
, accessed December 21, 2008. Snow or ice weather phenomena Precipitation