Snow - Old High German Sneo , genitive snēwes (8th century), Middle High German / Middle Low German SNE , altniederdeutsch Sneo , medium Dutch snee , Dutch sneeuw , Old English snaw , English snow , Old Norse snoer , snjōr , Swedish snö , gothic snaiws (Germanic * snaigwa- ) Russian sneg (снег) , Lithuanian sniẽgas 'snow', related to the Greek (accusative singular) nípha (νίφα) , Latin nix (genitive: nivis ), Welsh nyf 'snow'.
All forms are (ablaut) abstract formations to the Indo-European word * sneig ṵ h- 'snow, ball up, stick together'.
Snow is created when the finest droplets of supercooled water attach to crystallization nuclei (e.g. dust particles) in the clouds and freeze there. However, this process only begins at temperatures below −12 ° C, whereby water in the absence of crystallization can remain liquid at up to −48 ° C. The resulting ice crystals, less than 0.1 mm in size, fall down as the mass increases and continue to grow due to the difference in vapor pressure between ice and supercooled water. The water vapor contained in the air also resublimates, i.e. it is converted directly into ice and thus contributes to crystal growth. The well-known hexagonal shapes are formed. Due to the special structure of the water molecules , only angles of exactly 60 ° or 120 ° are possible.
The different trunk shapes of the snow crystals depend on the temperature - at lower temperatures, platelets or prisms form, at higher temperatures six-armed dendrites (ice stars, snow stars). The air humidity also influences crystal growth. When snow crystals form, the temperature in the cloud also rises because the crystals give off heat when they freeze (heat of crystallization ), while they absorb heat when they evaporate.
If the thermals are high , the crystals move vertically through the earth's atmosphere several times , whereby they are partially melted and can recrystallize again. This breaks the regularity of the crystals and creates complex mixed forms of the basic shapes. They have an amazingly wide variety of shapes. Over 5000 different snow crystals were photographed by Wilson A. Bentley from 1885 onwards. According to the latest information, Johann Heinrich Ludwig Flögel was the first person to take photographs of snow crystals in 1879. It is very likely that there are and have never been two complex snow crystals that were exactly the same. The reason for this lies in the very large combinatorial possibilities of many individual features. A snowflake contains about 10 18 water molecules, including about 10 14 deuterium atoms. Even in the visible range of a light microscope, it is easy to distinguish a hundred features that can be formed in different places. In combination, there are many possible variations, which is why the possible shapes of complex crystals are extremely numerous, far greater than the number of atoms in space.
Just as amazing as the variety of shapes observed is the pronounced symmetry , which gives some snow crystals a high degree of self-similarity and made them a preferred example of fractal geometry ( Koch curve ). The various ramifications sometimes grow in a specimen in a similar way and apparently at a similar speed, even if their tips, at which they continue to grow, are often several millimeters apart. One possible attempt at an explanation, which does not require an interaction across this distance, consists in pointing out that the growth conditions at different, comparable germinal sites at the tips at the same point in time are sometimes quite similar. Far more common than beautiful, symmetrical snowflakes, however, are asymmetrical and misshapen. The shapes that appear regularly, however, are more frequently photographed and illustrated.
The greatest complexity of the snow crystals is evident when the air humidity is high, as this allows even more filigree structures to grow. At very low temperatures, the ice crystals are not only smaller and simpler, but it also snows less than at temperatures just below freezing, as the air then hardly contains any moisture.
If the air temperature is close to freezing point , the individual ice crystals are stuck together by small water droplets and snowflakes reminiscent of a cotton ball are created. With dry air, snow formed in colder air layers can still reach the earth as snow even at temperatures around 5 ° C, as part of the flake sublimates and the energy to be applied cools the remaining flake. On the other hand, it also happens that rain falls below 0 ° C , then as freezing rain . In some media, the term lightning ice is used for this effect. These components depend on the structure and stratification stability of the upper and lower air layers , on geographic influences and weather elements such as cold air droplets . At low temperatures only very small flakes, the so-called snow gravel, form .
The white color of the snow is due to the fact that the snow consists of ice crystals. Like ice as such, every single crystal is transparent ; the light of all visible wavelengths is reflected and scattered at the interfaces between the ice crystals and the surrounding air. A sufficiently large collection of ice crystals with a random positional relationship to one another thus leads to diffuse reflection overall ; Snow therefore appears white. A similar effect can be observed , for example, with salt when comparing powder and larger crystals.
The mean diameter of snowflakes is about five millimeters, with a weight of 4 milligrams. The higher the temperature, the larger the flakes, as the crystals thaw and then stick together to form large flakes. The Guinness Book of Records lists the largest snowflake ever documented as 38 cm in diameter.
If a snowflake falls on water, it produces a high- pitched sound with a frequency of 50 to 200 kilohertz that is inaudible to humans ( ultrasound ). Not all researchers in this research area confirmed this effect.
Since snowflakes have a large surface and therefore a high level of air resistance, they fall relatively slowly at speeds of around 4 km / h - for comparison: moderate rain falls at around 20 km / h, hail can reach much higher speeds . The falling speed of snowflakes is largely independent of their size, since the surface of the flakes increases (almost) proportionally to their size, which means that the air resistance remains roughly constant. The microplastic contained in the air can be pulled to the ground by snowfall .
Like all irregularly shaped objects, snow crystals also tend to fall with their flattest side down. At first glance, this may seem illogical, as objects actually move with the least amount of air resistance. This would in fact be the case if the flat side of the snowflake were always oriented exactly parallel to the direction of fall. However, it is very likely that during its fall it will lean towards the direction of fall due to small disturbances (turbulence). Due to the air flowing around it, the snowflake is subject to a couple of forces (higher flow velocities at its edges), which turns it so that its flat side points downwards (plane of greatest extent of the flake normal to the direction of fall). A leaf falling from a tree, a dropped sheet of paper and Rayleigh's disk for measuring the speed of sound are also subject to the same mechanism . Flat snowflakes are therefore characterized by the well-known tumbling fall, which becomes a stationary dance in a suitable, slight upward flow, for example in exhaled air or on a warm house facade.
Another effect of turbulent flow is that snowflakes and other objects tend to line up and then catch up with each other. A snow crystal that gets into the vortex zone behind another can fall faster in it, so that it collides and clumps with it, similar to the way a cyclist in the slipstream behind another needs less driving force to keep the same pace. Birds flying in a V-formation use the upward current on the outside of the tip vortex of the person flying ahead to fly horizontally with less energy. However, if snow particles are as small as dust, they essentially fall without turbulence in the flow; the Reynolds number , a product of length, speed and viscosity, is then very small, as for steel balls in honey.
Intensity of snowfall
The intensity of the snowfall can be specified in two ways. Either in the form of the horizontal visibility during snowfall or as the amount of precipitation. The latter can be specified as the amount of precipitation , i.e. the height of the water column of the melted snow or in the form of the average snow height growth in centimeters per hour (cm / h). The term heavy snow , which appears more frequently in the media, can be equated with heavy snowfall , but is rarely used by meteorologists.
The German Weather Service uses the following definition for the precipitation intensity of snow:
- easy: amount of precipitation in 60 minutes <1.0 mm, in 10 minutes <0.2 mm
- Moderate: amount of precipitation in 60 minutes ≥ 1.0 mm to <5.0 mm, in 10 minutes ≥ 0.2 mm to <0.9 mm
- heavy: amount of precipitation in 60 minutes ≥ 5.0 mm, in 10 minutes ≥ 0.9 mm
In addition, as part of its warning criteria, the German Weather Service has defined snowfall levels for weather warnings (level 1), warnings of striking weather (level 2), severe weather warnings (level 3) and warnings of extreme storms (level 4). The following applies to a storm, for example: snowfall of more than 10 cm in 6 hours or more than 15 cm in 12 hours in the flatlands.
A precipitation height of 1.0 mm corresponds to the specification of 1 liter / m². When it snows, 1 mm of precipitation corresponds to approximately 1 to 2 cm of fresh snow.
When classifying according to visibility, three levels are also defined, whereby the visibility is over 4 km, between 1 to 4 km or less than 1 km.
A snow cover loses substance when energy is supplied. This can be done by radiation (short-wave solar radiation or long-wave thermal radiation), heat conduction (at air temperatures above 0 ° C) or by rain falling into the snow that is warmer than 0 ° C. How fast the mass reduction takes place depends not only on the amount of energy introduced, but also on the air temperature and humidity. Specifically, the breakdown proceeds more slowly, the drier the air, because for sublimation , i.e. for the direct transition of the water from the solid to the gaseous phase, a certain amount of energy has to be applied, whereby the remaining snow is cooled.
A distinction is made between three stages of the degradation process on the basis of wet temperature and dew point temperature . The wet temperature is the temperature that is measured by the humid side of a psychrometer and is always lower (equal to 100% humidity) than the air temperature. The dew point temperature is the temperature at which the moist air would be saturated with water vapor and is in turn lower than the wet temperature. If the wet temperature is below 0 ° C, the snow sublimates . This process has the slowest rate of degradation, the snow remains completely dry. It can take place at an air temperature of up to 7 ° C, but the relative humidity must be below 20%. If the wetness temperature is above 0 ° C, but the dew point temperature is still below, the snow melts , i.e. it changes into both the gas phase and the liquid phase. At dew point temperatures above zero , the snow thaws, it only changes into the liquid phase. This process has the fastest degradation rates. At an average relative humidity of 50%, snow sublimes below +3.5 ° C, it melts at 3.5–10 ° C and thaws above 10 ° C.
If snow melts over a large area at a high rate due to strong sunlight and warm wind and if the meltwater penetrates the already thin, melt-warm snow cover and hits soil that is already water-soaked, spring floods can occur.
Types of snow
There are several criteria that can be used to classify snow :
Freshly fallen snow is called fresh snow . Its ice crystals are finely branched with sharp points. Changes in the structure of the lying snow are called snow transformation or metamorphosis . Their type and speed depend on external influences such as temperature.
A basic distinction is made between decomposing and constructive metamorphosis, as well as melting metamorphosis. During the decomposing metamorphosis, the crystals take on less branched and more rounded shapes due to temperature fluctuations, the pressure of the snow cover and environmental influences such as wind. As a result, they become firmer and denser and are then referred to as tomentose or round-grained snow. During the metamorphosis that builds up, new, larger crystal forms are formed in the deeper layers, which have little strength due to large air pockets. Both decomposing and constructive snow changes take place at temperatures below freezing point over a period of several weeks. The melt metamorphosis creates round crystal shapes at temperatures above 0 ° C. Can in interplay with re-freezing of water on the surface (Auffirnen) crud form, otherwise compact HarschUnder the influence of wind and wind-pressed snow , who is also the cornices - but also slab formation contributes. Under strong sunlight caused by sublimation of the penitents and other special forms for the subtropical and tropical mountains are typical.
- Powder snow is dry snow that does not stick together even under pressure. Its density is less than 60 kg / m³.
- If the snow is already falling under particularly dry conditions, little or hardly any interlocking flakes are created that build up deeply unstable layers of snow; in the American Rocky Mountains with their special location between the Pacific and the dry continent, it is known as champagne powder .
- Damp snow sticks together under pressure and is therefore particularly suitable for snowballs and snowmen , but no water can be pressed out. It is also called cardboard snow because it tucks together.
- Wet snow is very heavy and wet, it also sticks together and you can squeeze out water. He is called in Upper German Schneebatz ( batzy , sticky-soft ').
- Lazy snow is a mixture of water and larger chunks of snow that no longer hold together well ( slush in which Switzerland also Pflotsch ).
In relation to precipitation:
- At the temperature limit (transition in height) or when the weather changes, sleet falls, i.e. a mixture of snow and rain (Upper German snow lumps )
- Blood snow is reddish colored snow. It is usually caused by a mass development of green algae (e.g. Chlamydomonas nivalis ), whichaccumulatered carotenoids . Due to the fall of reddish colored dust masses and fine sands, which are transported by winds from desert regions, the snow usually only takes on a very light color, which appears reddish-beige.
- A green color, also caused by cryophilic snow algae , was discovered in glaciers and arctic snow surfaces.
According to density
|30 ... 50 kg m −3||dry, loose fresh snow|
|50 ... 100 kg m −3||bound fresh snow|
|100… 200 kg m −3||heavily bound fresh snow|
|200 ... 400 kg m −3||dry old snow|
|300 ... 500 kg m −3||damp old snow|
|150 ... 300 kg m −3||Swimming snow|
|500 ... 800 kg m −3||multi-year firn|
|800 ... 900 kg m −3||ice|
According to occurrence and origin
- Flying snow is very fine snow that penetrates houses through the action of the wind.
- A snowdrift is an accumulation of snow caused by wind transport, the height of which can be well above the actual amount of precipitation . Snow drifts, especially in the mountains, can lead to overhanging structures, the cornices .
- Technical snow (colloquially: artificial snow ) is technically generated snow.
- A snow flurry is a local accumulation of whirling snowflakes in the air caused by strong wind, air suction or strong vibrations.
- The thermal conductivity of snow depends on its structure and texture and increases with its density. The thermal conductivity is between that of air [0.0247 W / (m · K)] and that of ice [2.2 W / (m · K)]. Due to the insulating effect of the snow, depending on the surface temperature, melting processes can already start on the underside of the snow layer, although the air temperatures are below the melting point. The protective effect of a snow cover against excessive cooling is particularly advantageous in agriculture. If the ground is still covered with snow remnants, the first signs of spring, such as the snowdrop named after it, germinate under the snow cover. Technically, the insulating effect of snow is also used against wind and heat radiation when building igloos . Also when digging in, ideally with a bivouac sack , in an emergency bivouac in the snow or by filling the outer walls of simple wooden huts that are not windproof.
- The heat capacity of snow, at 2.106 J / (kg ∙ K) (at 0 ° C), roughly corresponds to that of ice. It decreases as the temperature drops.
- The latent heat of fusion of snow is 335 kJ / kg (at 0 ° C and normal pressure). 1 kg of water at 80 ° C can melt just 1 kg of snow, the end product is 2 kg of water at 0 ° C.
Effects on the climate and the environment
In areas with a well-developed snowpack, the high albedo value of the snow reflects more sunlight back into the earth's atmosphere , so that the ground does not heat up as much. The long-wave heat radiation from the atmosphere, on the other hand, is particularly well absorbed by snow . In particular, during the melting process , it serves as so-called heat of fusion to overcome the binding energy of the water molecules without heating the snow or the water that is produced. Freshly fallen snow consists of up to 95% trapped air and thus also forms a good thermal insulator that protects plants under the snow cover from sharp freezing winds and frosts .
Importance to humans
Where there is normally only snow in winter , the associated change in the landscape also has an aesthetic significance . As a metaphor , the snow stands for the winter in general. Habits, sensory impressions and leisure activities differ considerably from times without snow. For the tourism Snow plays an important role (see also winter sports ). Building snowmen and having snowball fights are popular with children .
Snow avalanches pose a great danger at exposed locations , and entire villages can be buried under them. Heavy snowfalls (snow disasters) can also lead to serious damaging events (overloaded buildings or structures, tree falls, cut-off towns, etc.).
Slippery snow and ice on traffic routes represent a considerable danger and often lead to a complete collapse of the flow of traffic. After heavy snowfall, roads are often only passable with the help of snow chains . Specially equipped winter clearance services can be commissioned to clear snow.
Tourist resorts that are economically dependent on snow sports use snow cannons to produce artificial snow when there is little or no natural snowfall, with artificial snow having different properties than natural snow.
Snow also has acoustic effects: if it is loose, there is a lot of trapped air between the individual flakes, which makes it sound-absorbing . The proverbial winter silence is therefore to be understood as a symbol for a time of rest and relaxation associated with little activity.
The strictly hexagonal structure of snowflakes had been around in the Chinese Empire since at least the 2nd century BC. Chr. Known. The English mathematician Thomas Harriot first noticed this property in the West in 1591 , but did not publish his observation. Work on the variety of shapes in snow crystals is also known from Johannes Kepler and René Descartes , but Ukichiro Nakaya was the first to undertake systematic investigations , who was the first to produce synthetic snowflakes in 1936 and who in 1954 categorized them into over 200 different types.
Measurements of the amount of snow are carried out with the help of conventional rain gauges , where snow crosses are attached to protect against drifts . The thickness of the snow surface is determined with snow gauges or snow probes. The growth can also be measured with ultrasound . Although the amount of new snow is measured over a 24-hour period, it is sometimes given as a so-called new snow total over several days (e.g. 3-day new snow total).
The water content ( water equivalent of a snow cover) and the snow density are important for climatology and hydrology . The snow line is also an important climatological parameter. The snow line separates snow-covered and snow-free areas.
In winter sports, an area is considered to be snow-reliable if it has a snow cover of 30 cm ( Alpine skiing ) or 15 cm ( Nordic skiing ) sufficient for skiing for at least 100 days . Areas that are higher than about 1200 meters are currently considered to be snow reliable. Snow security is one of the strongest motivations when choosing a ski area. That is why guaranteed snow is an important economic factor in the mountain regions that are dependent on winter tourism. In the wake of global warming, some lower-lying winter sports areas could have problems getting the required snow depths. According to estimates by the OECD , around 70% of the ski areas in Austria could lose their snow reliability. In order to counteract the lack of snow, intensive investments have been made in artificial snowmaking systems for years .
Particularly strong and long-lasting, productive snowfalls and blizzards can assume catastrophic proportions. In some regions of the world, snowstorms occur more frequently and have special names there: Blizzard ( North America , Antarctica and Lapland ), Purga ( Central Asia ) and Yalca ( Andes in northern Peru ). The German Weather Service defines a snowfall of more than 10 cm in 6 hours or more than 15 cm in 12 hours as severe weather.
Significant snow disasters were:
- Great snow storm (1888) in the USA
- Snow disaster in Northern Germany in 1978
- Muensterland snow chaos 2005
Representation of snow in art history
In the history of art , the depiction of snow is a theme that is often used, and from epoch to epoch a different meaning was given: In the Middle Ages , winter put the care of people dependent on nature and their health in danger. It is thanks to social and technical progress that the winter has lost more and more of its threat. Out of fashion after the Renaissance , the winter landscape experienced its artistic revival in the late 18th century. At first it is romanticized. Later, the artists focus on the external appearance of the winter color nuances.
Pieter Bruegel the Elder : The Hunters in the Snow , 1565
David Teniers the Younger : Winter Scene with a Man Slaughtering a Pig , 1650
Caspar David Friedrich : The Sea of Ice , 1823-1824
Ernst Oppler : Early spring
Joseph Ferdinand Boissard de Boisdenier : Episode on the Retreat from Moscow , 1835
Claude Monet : Lavacourt: Sunshine and Snow , around 1878–1881
Kazimir Severinovich Malevich : Landscape (The Winter) , 1909 (dated around 1930)
Matthias AK Zimmermann : The redox reaction on the surface of a faded winter image , 2012
- A common misconception is that there are particularly many Eskimo words for snow .
- Indistinguishable from the snow is by Resublimation forming Polar snow or diamond dust.
- As Industrieschnee is called local snowfall, caused by power plants and other large facilities.
- Lake effect snow is a weather phenomenon on the Great Lakes in North America, with increased local snowfall.
- The fall of a lot of snow without wind leads to a snow break in the forest ; in the case of buildings, the snow loads on the roof are taken into account in the statics.
- Snow blindness is damage to the eye caused, among other things, by the reflection of sunlight on the snow.
- With yukitsuri (the high binding Japanese horticulture) to branches can be prevented from breaking under the weight of snow.
- The balling of sufficiently warm snow with two hands into a solid snowball is a sintering process .
- Some fairy tales have the theme of snow, for example The Snow Queen , "her skin as white as snow" in Snow White , "The Snow" - how the snow came to its color through a snowdrop .
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