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Graupel (snow)
Encyclopedia
Graupel refers to precipitation
that forms when supercooled droplets of water are collected and freeze on a falling snow
flake, forming a 2–5 mm ball of rime. The term graupel is the German word for this meteorological phenomenon.
Graupel is sometimes referred to as small hail, although the World Meteorological Organization
defines small hail as snow pellets encapsulated by ice, a precipitation halfway between graupel and hail
.
conditions, snow
crystals may encounter supercooled water
droplets. These droplets, which have a diameter of about 10 µm, can exist in the liquid state
at temperatures as low as -40°C (-40°F), far below the normal 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
. Crystals that exhibit frozen droplets on their surfaces are 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
is formed in hard, relatively uniform layers and usually falls only during thunderstorm
s. Graupel forms fragile, oblong shapes and falls in place of typical snowflakes
in wintry mix situations, often in concert with ice pellets
. Graupel is also fragile enough that it will typically fall apart when touched.
The frozen droplets on the surface of rimed crystals are hard to resolve 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 cloud droplets measuring up to 50 µm 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 obscures the identity of the original snow crystal, thereby giving rise to a graupel particle.
Graupel commonly forms in high altitude climates and is both denser and more granular
than ordinary snow
, due to its rimed exterior. Macroscopically, graupel resembles small beads of polystyrene
. The combination of density and low viscosity makes fresh layers of graupel unstable on slopes, and layers of 20-30 cm present a high risk of dangerous slab 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.
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...
that forms when supercooled droplets of water are collected and freeze on a falling snow
Snow
Snow is a form of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by...
flake, forming a 2–5 mm ball of rime. The term graupel is the German word for this meteorological phenomenon.
Graupel is sometimes referred to as small hail, although the World Meteorological Organization
World Meteorological Organization
The World Meteorological Organization is an intergovernmental organization with a membership of 189 Member States and Territories. It originated from the International Meteorological Organization , which was founded in 1873...
defines small hail as snow pellets encapsulated by ice, a precipitation halfway between graupel and hail
Hail
Hail is a form of solid precipitation. It consists of balls or irregular lumps of ice, each of which is referred to as a hail stone. Hail stones on Earth consist mostly of water ice and measure between and in diameter, with the larger stones coming from severe thunderstorms...
.
Formation
Under some atmosphericAtmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
conditions, snow
Snow
Snow is a form of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by...
crystals may encounter supercooled water
Supercooling
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....
droplets. These droplets, which have a diameter of about 10 µm, can exist in the liquid state
State of matter
States of matter are the distinct forms that different phases of matter take on. Solid, liquid and gas are the most common states of matter on Earth. However, much of the baryonic matter of the universe is in the form of hot plasma, both as rarefied interstellar medium and as dense...
at temperatures as low as -40°C (-40°F), far below the normal freezing
Freeze
Freeze may refer to:In liquids turning to solids:*Freezing, the physical process of a liquid turning into a solid*Freeze drying, a method of rapidly removing moisture from food productsIn cessation of movement or change:...
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 (atmosphere)
Accretion is an atmospheric science term for when an ice crystal or snowflake hits a supercooled liquid droplet, which then freeze together. This increases the size of the water particle. A common example of this that is visible to people is graupel....
. Crystals that exhibit frozen droplets on their surfaces are 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
Hail is a form of solid precipitation. It consists of balls or irregular lumps of ice, each of which is referred to as a hail stone. Hail stones on Earth consist mostly of water ice and measure between and in diameter, with the larger stones coming from severe thunderstorms...
is formed in hard, relatively uniform layers and usually falls only during thunderstorm
Thunderstorm
A thunderstorm, also known as an electrical storm, a lightning storm, thundershower or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder. The meteorologically assigned cloud type associated with the...
s. Graupel forms fragile, oblong shapes and falls in place of typical snowflakes
Snowflakes
Snowflakes is the first Christmas album by American R&B singer–songwriter Toni Braxton, released in the United States on October 23, 2001 by Arista Records...
in wintry mix situations, often in concert with ice pellets
Ice pellets
Ice pellets are a form of precipitation consisting of small, translucent balls of ice. Ice pellets usually are smaller than hailstones. They often bounce when they hit the ground, and generally do not freeze into a solid mass unless mixed with freezing rain...
. Graupel is also fragile enough that it will typically fall apart when touched.
Microscopic structure
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...
because of the limited resolution and depth of field in the instrument. However, observations of snow crystals with a low-temperature scanning electron microscope
Scanning electron microscope
A scanning electron microscope is a type of electron microscope that images a sample by scanning it with a high-energy beam of electrons in a raster scan pattern...
(LT-SEM) clearly show cloud droplets measuring up to 50 µm 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 obscures the identity of the original snow crystal, thereby giving rise to a graupel particle.
Graupel and avalanches
Granularity
Granularity is the extent to which a system is broken down into small parts, either the system itself or its description or observation. It is the "extent to which a larger entity is subdivided...
than ordinary snow
Snow
Snow is a form of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by...
, due to its rimed exterior. Macroscopically, graupel resembles small beads of polystyrene
Polystyrene
Polystyrene ) also known as Thermocole, abbreviated following ISO Standard PS, is an aromatic polymer made from the monomer styrene, a liquid hydrocarbon that is manufactured from petroleum by the chemical industry...
. The combination of density and low viscosity makes fresh layers of graupel unstable on slopes, and layers of 20-30 cm present a high risk of dangerous slab avalanche
Avalanche
An avalanche is a sudden rapid flow of snow down a slope, occurring when either natural triggers or human activity causes a critical escalating transition from the slow equilibrium evolution of the snow pack. Typically occurring in mountainous terrain, an avalanche can mix air and water with the...
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.
Dictionaries
- 3 results for:graupel. Dictionary.com, accessed September 12, 2006.
- Graupel. Merriam-Webster Online Dictionary, accessed September 12, 2006.
Weather glossaries
- Weather Glossary, G. The Weather Channel, accessed September 12, 2006.
- All About Snow. National Snow and Ice Data Center (NSIDC), accessed September 12, 2006.
- Terms used by meteorologists, forecasters, weather observers, and in weather forecasts. National Oceanic and Atmospheric Administration (NOAA), accessed September 12, 2006.
- About.com Weather Glossary. Weather at About.coml, accessed December 21, 2008.