2013-02-23 03 59 28 Graupel (snow pellets) in Elko, Nevada

Graupel (; ), also called soft hail, hominy snow, or snow pellets, is

precipitation 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 drizzle, rain, sleet, snow, ice pellets, graupel and hai ...

that forms when

supercooled Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid or a gas below its melting point without it becoming a solid. It achieves this in the absence of a seed crystal or nucleus around which a crystal ...

water droplets in air are collected and freeze on falling

snowflake A snowflake is a single ice crystal that has achieved a sufficient size, and may have amalgamated with others, which falls through the Earth's atmosphere as snow.Knight, C.; Knight, N. (1973). Snow crystals. Scientific American, vol. 228, no. ...

s, forming balls of crisp, opaque rime. Graupel is distinct from

hail Hail is a form of solid precipitation. It is distinct from ice pellets (American English "sleet"), though the two are often confused. It consists of balls or irregular lumps of ice, each of which is called a hailstone. Ice pellets generally fal ...

and ice pellets, in regards to their formation and appearance. However, both hail and graupel are common in

thunderstorm A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are somet ...

s with

cumulonimbus Cumulonimbus (from Latin ''cumulus'', "heaped" and ''nimbus'', "rainstorm") is a dense, towering vertical cloud, typically forming from water vapor condensing in the lower troposphere that builds upward carried by powerful buoyant air currents. ...

clouds, though graupel also falls in

winter storm A winter storm is an event in which wind coincides with varieties of precipitation that only occur at freezing temperatures, such as snow, mixed snow and rain, or freezing rain. In temperate continental climates, these storms are not necess ...

s, and at higher elevations as well. The

METAR METAR is a format for reporting weather information. A METAR weather report is predominantly used by aircraft pilots, and by meteorologists, who use aggregated METAR information to assist in weather forecasting. Raw METAR is the most common form ...

code for graupel is GS.

Formation

Under some

atmospheric An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...

conditions,

snow Snow comprises individual ice crystals that grow while suspended in the atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet ...

crystals may encounter

water droplets. These droplets, which have a diameter of about on average, 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), a daily newspaper in Columbia, South Carolina, United States * '' Our ...

at temperatures as low as , far below the normal freezing point as long as above the homogeneous

nucleation In thermodynamics, nucleation is the first step in the formation of either a new thermodynamic phase or structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically defined to be the process that de ...

point of water. 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. 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 and has become ball-like, 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 in some cases, the circumstances in which it falls. Ice from

is formed in hard, relatively uniform layers and usually falls only during

s. Graupel forms fragile, soft, oblong crystals and falls in place of typical

s in wintry mix situations, often in concert with ice pellets. However, Graupel does also occur in thunderstorms. Graupel is also fragile enough that it will typically fall apart when pressed on.

Microscopic structure

The frozen droplets on the surface of rimed crystals are difficult to see even when zoomed in, 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 light and a system of lenses to generate magnified images of small objects. Optical microscopes are the oldest design of microsco ...

because of the limited resolution and

depth of field The depth of field (DOF) is the distance between the nearest and the furthest objects that are in acceptably sharp focus in an image captured with a camera. Factors affecting depth of field For cameras that can only focus on one object di ...

in the instrument. However, observations of snow crystals with a low-temperature scanning electron microscope (LT-SEM) clearly show frozen 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 than ordinary

, due to its rimed exterior and higher density. Macroscopically, graupel resembles small beads of

polystyrene Polystyrene (PS) is a synthetic polymer made from monomers of the aromatic hydrocarbon styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin per unit weight. It is ...

. The combination of density and low viscosity makes fresh layers of graupel unstable on slopes, and layers of or higher present a high risk of dangerous slab

avalanche An avalanche is a rapid flow of snow down a slope, such as a hill or mountain. Avalanches can be set off spontaneously, by such factors as increased precipitation or snowpack weakening, or by external means such as humans, animals, and ear ...

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 or otherwise making surfaces slippery. 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

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, accessed December 21, 2008. Snow or ice weather phenomena Precipitation Hail