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[[Image:Galerie flocons.jpg|thumb|center|700px|Snowflakes]] |
[[Image:Galerie flocons.jpg|thumb|center|700px|Snowflakes]] |
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EAT YELLOW SNOW |
|||
==Snow on the ground== |
==Snow on the ground== |
Part of a series on |
Weather |
---|
![]() |
Snow is a type of precipitation in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. The process of precipitation is called snowfall.
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. The METAR code for snow is SN.
Snow crystals form when tiny supercooled cloud droplets (approx 10μm in diameter) freeze. These droplets are able to remain liquid at temperatures colder than 0°C because in order to freeze, a few molecules in the liquid droplet need to get together by chance to form an arrangement close to that in an ice lattice; then the droplet freezes around this 'nucleus'. Experiments show that this 'homogenous' nucleation of cloud droplets only occurs at temperatures colder than -35°C. [1] In warmer clouds an aerosol particle or 'ice nucleus' must be present in (or in contact with) the droplet to act as a nucleus. Our understanding of what particles make efficient ice nuclei is poor - what we do know is they are very rare compared to that cloud condensation nuclei which liquid droplets form on. Clays, desert dust and biological particles may be effective, [2] although to what extent is unclear. Artificial nuclei include Silver Iodide and dry ice, and these form the basis of cloud seeding.
Once a droplet has frozen, it grows in the supersaturated environment (air saturated with respect to liquid water is always supersaturated with respect to ice) and grows by diffusion of water molecules in the air (vapour) onto the ice crystal surface where they are deposited. Because the droplets are so much more numerous than the ice crystals (because of the relative numbers of ice vs droplet nuclei) the crystals are able to grow to hundreds of micrometres or millimetres in size at the expense of the water droplets (the Wegner-Bergeron-Findeison process). The corresponding depletion of water vapour causes the droplets to evaporate, meaning that the ice crystals effectively grow at the droplets' expense. These large crystals are an efficient source of precipitation, since they fall through the atmosphere due to their weight, and may collide and stick together in clusters (aggregates). These aggregates are snowflakes, and are usually the type of ice particle which falls at the ground. [3] The exact details of the sticking mechanism remains controversial (and probably there are different mechanisms active in different clouds), possibilities include mechanical interlocking, sintering, electrostatic attraction as well as the existence of a 'sticky' liquid-like layer on the crystal surface.
The individual ice crystals often have an hexagonal symmetry. Although the ice is clear scattering of light by the crystal facets and hollows/imperfections mean that the crystals often appear white in colour.
Ice crystals formed in the appropriate conditions can often be thin and flat. These planar crystals may be simple hexagons, or if the supersaturation is high enough, develop branches and dendritic (fern-like) features and have six approximately identical arms, as per the iconic 'snowflake' popularised by Wilson Bentley. The 6-fold symmetry arises from the hexagonal crystal structure of ordinary ice, the branch formation is produced by unstable growth, with deposition occurring preferentially near the tips of branches. [4]
The shape of the snowflake is determined broadly by the temperature and humidity at which it forms. [3] Rarely, at a temperature of around −2 °C (28 °F), snowflakes can form in threefold symmetry — triangular snowflakes. [5] The most common snow particles are visibly irregular, although near-perfect snowflakes may be more common in pictures because they are more visually appealing.
Planar crystals (thin and flat) grow in air between 0 °C (32 °F) and −3 °C (27 °F). Between −3 °C (27 °F) and −8 °C (18 °F), the crystals will form needles or hollow columns or prisms (long thin pencil-like shapes). From −8 °C (18 °F) to −22 °C (−8 °F) the habit goes back to plate like, often with branched or dendritic features. Note that the maximum difference in vapour pressure between liquid and ice is at approx. −15 °C (5 °F) where crystals grow most rapidly at the expense of the liquid droplets. At temperatures below −22 °C (−8 °F), the crystal habit again becomes column-like again, although many more complex habits also form such as side-planes, bullet-rosettes and also planar types depending on the conditions and ice nuclei. [6]
Interestingly, if a crystal has started forming in a column growth regime, say at around −5 °C (23 °F), and then falls into the warmer plate-like regime, plate or dendritic crystals sprout at the end of the column producing so called 'capped columns'. [3]
There is a widely held belief that no two snowflakes are alike. Strictly speaking, it is extremely unlikely for any two macroscopic objects in the universe to contain an identical molecular structure; but there are, nonetheless, no known scientific laws that prevent it. In a more pragmatic sense, it's more likely—albeit not much more—that two snowflakes are virtually identical if their environments were similar enough, either because they grew very near one another, or simply by chance. The American Meteorological Society has reported that matching snow crystals were discovered in Wisconsin in 1988 by Nancy Knight of the National Center for Atmospheric Research.[ citation needed] The crystals were not flakes in the usual sense but rather hollow hexagonal prisms.
EAT YELLOW SNOW
Snow remains on the ground until it melts or sublimes. In colder climates this results in snow lying on the ground all winter. When the snow does not all melt in the summer it becomes a glacier.
The water equivalent of the snow is the thickness of a layer of water having the same content. For example, if the snow covering a given area has a water equivalent of 50 centimetres (20 in), then it will melt into a pool of water 50 centimetres (20 in) deep covering the same area. This is a much more useful measurement to hydrologists than snow depth, as the density of cool freshly fallen snow widely varies. New snow commonly has a density of between 5% and 15% of water. Snow that falls in maritime climates is usually denser than snow that falls in mid-continent locations because of the higher average clouds over oceans than over land masses. Cloud temperatures and physical processes in the cloud affect the shape of individual snow crystals. Highly branched or dendritic crystals tend to have more space between the arms of ice that form the snow flake and this snow will therefore have a lower density, often referred to as "dry" snow. Conditions that create columnar or platelike crystals will have much less air space within the crystal and will therefore be more dense and feel "wetter".
Once the snow is on the ground, it will settle under its own weight (largely due to differential evaporation) until its density is approximately 30% of water. Increases in density above this initial compression occur primarily by melting and refreezing, caused by temperatures above freezing or by direct solar radiation. By late spring, snow densities typically reach a maximum of 50% of water. [7]
Spring snow melt is a major source of water supply to areas in temperate zones near mountains that catch and hold winter snow, especially those with a prolonged dry summer. In such places, water equivalent is of great interest to water managers wishing to predict spring runoff and the water supply of cities downstream. Measurements are made manually at marked locations known as snow courses, and remotely using special scales called snow pillows.
Many rivers originating in mountainous or high-latitude regions have a significant portion of their flow from snowmelt. This often makes the river's flow highly seasonal resulting in periodic flooding. In contrast, if much of the melt is from glaciated or nearly glaciated areas, the melt continues through the warm season, mitigating that effect.
The energy balance of the snowpack is dictated by several heat exchange processes. The snowpack absorbs solar shortwave radiation that is partially blocked by cloud cover and reflected by snow surface. A longwave heat exchange takes place between the snowpack and its surrounding environment that includes overlaying air mass, tree cover and clouds. Convective (sensible) heat exchange between the snowpack and the overlaying air mass is governed by the temperature gradient and wind speed. Moisture exchange between the snowpack and the overlaying air mass is accompanied with latent heat transfer that is influenced by vapor pressure gradient and air wind. Rain on snow could induce significant heat input to the snowpack. A generally insignificant conductive heat exchange takes place between the snowpack and the underlying ground. That is the reason there is a small temperature rise after or before the snowfall. [8]
Substantial snowfall can disrupt public infrastructure and services, slowing human activity even in regions that are accustomed to such weather. Air and ground transport may be greatly inhibited or shut down entirely. Populations living in snow-prone areas have developed various ways to travel across the snow, such as skis, snowshoes, and sleds pulled by horses, dogs, or other animals and later Snowmobiles. Basic infrastructures such as electricity, telephone lines, and gas supply can also fail. In addition, snow can make roads much harder to travel and cars attempting to traverse them can easily become stuck. The combined effects can lead to a " snow day" on which gatherings such as school, work, or church are officially canceled. In areas that normally have very little or no snow, a snow day may occur when there is only light accumulation or even the threat of snowfall, since those areas are ill-prepared to handle any amount of snow.
Snowfall can be beneficial to agriculture by serving as a thermal insulator, conserving the heat of the Earth and protecting crops from subfreezing weather. Some agricultural areas depend on an accumulation of snow during winter that will melt gradually in spring, providing water for crop growth.
In areas near mountains, people have harvested snow and stored it as layers of ice covered by straw or sawdust in icehouses. This allowed the ice to be used in summer for refrigeration or medical uses.
A mudslide, flash flood, or avalanche can occur when excessive snow has accumulated on a mountain and there is a sudden change of temperature. Large amounts of snow that accumulate on top of man-made structures can lead to structural failure.
The highest seasonal total snowfall measured in the United States was at Mount Baker Ski Area, outside of the town Bellingham, Washington during the 1998–1999 season. Mount Baker received 1,140 inches (29 m) of snow, [9] thus surpassing the previous record holder, Mount Rainier, Washington, which during the 1971–1972 season received 1,122 in. (28.5 m) of snow. [10] Guinness World Records list the world’s largest snowflakes as those of January 1887 at Fort Keogh, Montana;. allegedly one measured 15 inches (38 cm) wide. [11]
{{
cite journal}}
: Unknown parameter |coauthors=
ignored (|author=
suggested) (
help)
{{
cite journal}}
: Unknown parameter |coauthors=
ignored (|author=
suggested) (
help)CS1 maint: extra punctuation (
link) CS1 maint: multiple names: authors list (
link)
m Reverted 1 edit by
75.181.96.21 (
Talk) to last version by Snigbrook. (
TW) |
REDSAUCEONPASTA (
talk |
contribs) |
||
Line 56: | Line 56: | ||
{{-}} |
{{-}} |
||
[[Image:Galerie flocons.jpg|thumb|center|700px|Snowflakes]] |
[[Image:Galerie flocons.jpg|thumb|center|700px|Snowflakes]] |
||
EAT YELLOW SNOW |
|||
==Snow on the ground== |
==Snow on the ground== |
Part of a series on |
Weather |
---|
![]() |
Snow is a type of precipitation in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. The process of precipitation is called snowfall.
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. The METAR code for snow is SN.
Snow crystals form when tiny supercooled cloud droplets (approx 10μm in diameter) freeze. These droplets are able to remain liquid at temperatures colder than 0°C because in order to freeze, a few molecules in the liquid droplet need to get together by chance to form an arrangement close to that in an ice lattice; then the droplet freezes around this 'nucleus'. Experiments show that this 'homogenous' nucleation of cloud droplets only occurs at temperatures colder than -35°C. [1] In warmer clouds an aerosol particle or 'ice nucleus' must be present in (or in contact with) the droplet to act as a nucleus. Our understanding of what particles make efficient ice nuclei is poor - what we do know is they are very rare compared to that cloud condensation nuclei which liquid droplets form on. Clays, desert dust and biological particles may be effective, [2] although to what extent is unclear. Artificial nuclei include Silver Iodide and dry ice, and these form the basis of cloud seeding.
Once a droplet has frozen, it grows in the supersaturated environment (air saturated with respect to liquid water is always supersaturated with respect to ice) and grows by diffusion of water molecules in the air (vapour) onto the ice crystal surface where they are deposited. Because the droplets are so much more numerous than the ice crystals (because of the relative numbers of ice vs droplet nuclei) the crystals are able to grow to hundreds of micrometres or millimetres in size at the expense of the water droplets (the Wegner-Bergeron-Findeison process). The corresponding depletion of water vapour causes the droplets to evaporate, meaning that the ice crystals effectively grow at the droplets' expense. These large crystals are an efficient source of precipitation, since they fall through the atmosphere due to their weight, and may collide and stick together in clusters (aggregates). These aggregates are snowflakes, and are usually the type of ice particle which falls at the ground. [3] The exact details of the sticking mechanism remains controversial (and probably there are different mechanisms active in different clouds), possibilities include mechanical interlocking, sintering, electrostatic attraction as well as the existence of a 'sticky' liquid-like layer on the crystal surface.
The individual ice crystals often have an hexagonal symmetry. Although the ice is clear scattering of light by the crystal facets and hollows/imperfections mean that the crystals often appear white in colour.
Ice crystals formed in the appropriate conditions can often be thin and flat. These planar crystals may be simple hexagons, or if the supersaturation is high enough, develop branches and dendritic (fern-like) features and have six approximately identical arms, as per the iconic 'snowflake' popularised by Wilson Bentley. The 6-fold symmetry arises from the hexagonal crystal structure of ordinary ice, the branch formation is produced by unstable growth, with deposition occurring preferentially near the tips of branches. [4]
The shape of the snowflake is determined broadly by the temperature and humidity at which it forms. [3] Rarely, at a temperature of around −2 °C (28 °F), snowflakes can form in threefold symmetry — triangular snowflakes. [5] The most common snow particles are visibly irregular, although near-perfect snowflakes may be more common in pictures because they are more visually appealing.
Planar crystals (thin and flat) grow in air between 0 °C (32 °F) and −3 °C (27 °F). Between −3 °C (27 °F) and −8 °C (18 °F), the crystals will form needles or hollow columns or prisms (long thin pencil-like shapes). From −8 °C (18 °F) to −22 °C (−8 °F) the habit goes back to plate like, often with branched or dendritic features. Note that the maximum difference in vapour pressure between liquid and ice is at approx. −15 °C (5 °F) where crystals grow most rapidly at the expense of the liquid droplets. At temperatures below −22 °C (−8 °F), the crystal habit again becomes column-like again, although many more complex habits also form such as side-planes, bullet-rosettes and also planar types depending on the conditions and ice nuclei. [6]
Interestingly, if a crystal has started forming in a column growth regime, say at around −5 °C (23 °F), and then falls into the warmer plate-like regime, plate or dendritic crystals sprout at the end of the column producing so called 'capped columns'. [3]
There is a widely held belief that no two snowflakes are alike. Strictly speaking, it is extremely unlikely for any two macroscopic objects in the universe to contain an identical molecular structure; but there are, nonetheless, no known scientific laws that prevent it. In a more pragmatic sense, it's more likely—albeit not much more—that two snowflakes are virtually identical if their environments were similar enough, either because they grew very near one another, or simply by chance. The American Meteorological Society has reported that matching snow crystals were discovered in Wisconsin in 1988 by Nancy Knight of the National Center for Atmospheric Research.[ citation needed] The crystals were not flakes in the usual sense but rather hollow hexagonal prisms.
EAT YELLOW SNOW
Snow remains on the ground until it melts or sublimes. In colder climates this results in snow lying on the ground all winter. When the snow does not all melt in the summer it becomes a glacier.
The water equivalent of the snow is the thickness of a layer of water having the same content. For example, if the snow covering a given area has a water equivalent of 50 centimetres (20 in), then it will melt into a pool of water 50 centimetres (20 in) deep covering the same area. This is a much more useful measurement to hydrologists than snow depth, as the density of cool freshly fallen snow widely varies. New snow commonly has a density of between 5% and 15% of water. Snow that falls in maritime climates is usually denser than snow that falls in mid-continent locations because of the higher average clouds over oceans than over land masses. Cloud temperatures and physical processes in the cloud affect the shape of individual snow crystals. Highly branched or dendritic crystals tend to have more space between the arms of ice that form the snow flake and this snow will therefore have a lower density, often referred to as "dry" snow. Conditions that create columnar or platelike crystals will have much less air space within the crystal and will therefore be more dense and feel "wetter".
Once the snow is on the ground, it will settle under its own weight (largely due to differential evaporation) until its density is approximately 30% of water. Increases in density above this initial compression occur primarily by melting and refreezing, caused by temperatures above freezing or by direct solar radiation. By late spring, snow densities typically reach a maximum of 50% of water. [7]
Spring snow melt is a major source of water supply to areas in temperate zones near mountains that catch and hold winter snow, especially those with a prolonged dry summer. In such places, water equivalent is of great interest to water managers wishing to predict spring runoff and the water supply of cities downstream. Measurements are made manually at marked locations known as snow courses, and remotely using special scales called snow pillows.
Many rivers originating in mountainous or high-latitude regions have a significant portion of their flow from snowmelt. This often makes the river's flow highly seasonal resulting in periodic flooding. In contrast, if much of the melt is from glaciated or nearly glaciated areas, the melt continues through the warm season, mitigating that effect.
The energy balance of the snowpack is dictated by several heat exchange processes. The snowpack absorbs solar shortwave radiation that is partially blocked by cloud cover and reflected by snow surface. A longwave heat exchange takes place between the snowpack and its surrounding environment that includes overlaying air mass, tree cover and clouds. Convective (sensible) heat exchange between the snowpack and the overlaying air mass is governed by the temperature gradient and wind speed. Moisture exchange between the snowpack and the overlaying air mass is accompanied with latent heat transfer that is influenced by vapor pressure gradient and air wind. Rain on snow could induce significant heat input to the snowpack. A generally insignificant conductive heat exchange takes place between the snowpack and the underlying ground. That is the reason there is a small temperature rise after or before the snowfall. [8]
Substantial snowfall can disrupt public infrastructure and services, slowing human activity even in regions that are accustomed to such weather. Air and ground transport may be greatly inhibited or shut down entirely. Populations living in snow-prone areas have developed various ways to travel across the snow, such as skis, snowshoes, and sleds pulled by horses, dogs, or other animals and later Snowmobiles. Basic infrastructures such as electricity, telephone lines, and gas supply can also fail. In addition, snow can make roads much harder to travel and cars attempting to traverse them can easily become stuck. The combined effects can lead to a " snow day" on which gatherings such as school, work, or church are officially canceled. In areas that normally have very little or no snow, a snow day may occur when there is only light accumulation or even the threat of snowfall, since those areas are ill-prepared to handle any amount of snow.
Snowfall can be beneficial to agriculture by serving as a thermal insulator, conserving the heat of the Earth and protecting crops from subfreezing weather. Some agricultural areas depend on an accumulation of snow during winter that will melt gradually in spring, providing water for crop growth.
In areas near mountains, people have harvested snow and stored it as layers of ice covered by straw or sawdust in icehouses. This allowed the ice to be used in summer for refrigeration or medical uses.
A mudslide, flash flood, or avalanche can occur when excessive snow has accumulated on a mountain and there is a sudden change of temperature. Large amounts of snow that accumulate on top of man-made structures can lead to structural failure.
The highest seasonal total snowfall measured in the United States was at Mount Baker Ski Area, outside of the town Bellingham, Washington during the 1998–1999 season. Mount Baker received 1,140 inches (29 m) of snow, [9] thus surpassing the previous record holder, Mount Rainier, Washington, which during the 1971–1972 season received 1,122 in. (28.5 m) of snow. [10] Guinness World Records list the world’s largest snowflakes as those of January 1887 at Fort Keogh, Montana;. allegedly one measured 15 inches (38 cm) wide. [11]
{{
cite journal}}
: Unknown parameter |coauthors=
ignored (|author=
suggested) (
help)
{{
cite journal}}
: Unknown parameter |coauthors=
ignored (|author=
suggested) (
help)CS1 maint: extra punctuation (
link) CS1 maint: multiple names: authors list (
link)