Snow

Wednesday, December 29, 2010

Snow is a type 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 external pressure. Snowflakes come in a variety of sizes and shapes. Types which fall in the form of a ball due to melting and refreezing, rather than a flake, are known as graupel, with ice pellets and snow grains as examples of graupel. Snowfall amount and its related liquid equivalent precipitation amount are determined using a variety of different rain gauges.
 The process of precipitating snow is called snowfall. Snowfall tends to form within regions of upward motion of air around a type of low-pressure system known as an extratropical cyclone. Snow can fall poleward of these systems' associated warm fronts and within their comma head precipitation patterns (called such due to the comma-like shape of the cloud and precipitation pattern around the poleward and west sides of extratropical cyclones). Where relatively warm water bodies are present, for example due to water evaporation from lakes, lake-effect snowfall becomes a concern downwind of the warm lakes within the cold cyclonic flow around the backside of extratropical cyclones. Lake-effect snowfall can be locally heavy. Thundersnow is possible within a cyclone's comma head and within lake effect precipitation bands. In mountainous areas, heavy snow is possible where upslope flow is maximized within windward sides of the terrain at elevation, if the atmosphere is cold enough.

Most people perceive snow simply as frozen water. Delving in a little deeper, snow is actually a form of precipitation in the form of ice crystals. These ice crystals are hexagonal prisms that form when snow freezes up. Prisms are formed due to the molecular structure of water. As these ice crystals are formed, they come down in one of the following forms:
  • Snow crystals -- Individual, single ice crystals, often with six-fold symmetrical shapes. These grow directly from condensing water vapor in the air, usually around a nucleus of dust or some other foreign material. Typical sizes range from microscopic to at most a few millimeters in diameter.
  • Snowflakes -- Collections of snow crystals, loosely bound together into a puff-ball. These can grow to large sizes (up to about 10 cm across in some cases) when the snow is especially wet and sticky. A snowflake consists of up to 100 snow crystals clumped together.
  • Rime -- Super cooled tiny water droplets (typically in a fog), that quickly freeze onto whatever they hit. An example of this is the small droplets of rime on large snow crystals.
  • Graupel -- Loose collections of frozen water droplets, sometimes called "soft hail."
  • Hail -- Large, solid chunks of ice. 
 If you read ski weather reports, you have heard many different types of snow mentioned. From packed powder to granular, it can be difficult to know what kind of snow is actually being mentioned. However, knowing the types of snow is important if you want to know what you'll actually be skiing on. Below, the different snow types are explained.

Corduroy - Corduroy is the finely ridged surface of the snow after a snowcat has groomed a ski trail.
Corn Snow - Typically seen during spring conditions, corn snow results from cycles of nightly freezing and daily thawing. This snow is wet and granular, and as it melts more in the day it may become sloppy and heavy.
Crud - Basically, crud is powder that has been skied on. Think of crud as powder that's been trampled. It's snow that is uneven, packed down in some places, and piled up in others.
Crust - Crust is soft snow that has a layer of harden, frozen crust (hence the name) on the top. Crust can be from a number of things. Freezing rain, direct sunlight, or the melting and refreezing of the top layer of powder can result in crust.
Powder - Powder is freshly fallen snow that is very light. Formed by tiny snow flakes, it is extremely soft. Many skiers love powder.
Packed Powder- Packed powder is snow that is compressed and flattened either by skier and snowboarder traffic or by grooming equipment.
Slush - Slush is snow that is starting to melt, and it's very heavy and very wet. Some would say that slush doesn't even look like snow, and those who've seen slush during spring conditions know how difficult skiing in slush can be.
Granular Snow
  • Loose Granular Snow - Loose Granular is small, loose pellets of snow that is created by the grooming of wet or icy snow.
  • Wet Granular Snow - Wet Granular is very wet snow, often found in spring conditions. This snow will form a snowball.
  • Frozen Granular Snow - Frozen granular is frozen snow with a consistency like sugar.
Damage
When heavy, wet snow with a snow-water equivalent (SWE) ratio of between 6:1 and 12:1 and a weight in excess of 10 pounds per square foot (~50 kg/m2) piles onto trees or electricity lines – particularly if the trees have full leaves or are not adapted to snow – significant damage may occur on a scale usually associated with hurricanes. An avalanche can occur upon a sudden thermal or mechanical impact upon snow that has accumulated on a mountain, which causes the snow to rush downhill en masse. Preceding an avalanche is a phenomenon known as an avalanche wind caused by the approaching avalanche itself, which adds to its destructive potential. Large amounts of snow which accumulate on top of man-made structures can lead to structural failure.During snowmelt, acidic precipitation which previously fell into the snow pack is released, which harms marine life.

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