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 hail. ...

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 crysta ...

water

droplet A drop or droplet is a small column of liquid, bounded completely or almost completely by free surfaces. A drop may form when liquid accumulates at the lower end of a tube or other surface boundary, producing a hanging drop called a pendant ...

s in air are collected and freeze on falling snowflakes, forming balls of crisp, opaque rime. Graupel is distinct from hail 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 some ...

s with cumulonimbus clouds, though graupel also falls in winter storms, 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 fo ...

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 ...

conditions,

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 further changes. It consists of frozen crystalline water throughou ...

crystals may encounter

water droplets. These droplets, which have a diameter of about on average, can exist in the liquid state 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 ...

point as long as above the homogeneous nucleation 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 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 nucl ...

. 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 hail 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 snowflakes 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 because of the limited resolution and depth of field 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, aromatic hydrocarbon styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin pe ...

. 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 ea ...

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