Snow

Snow consists of fine ice crystals and is the most common form of solid precipitate.

• 7.1 Impact on the climate and the environment
• 7.2 Importance to humans

• 8.1 history
• 8.2 snow measurements

Etymology

Snow - Old High German sneo, genitive snēwes ( 8th century ), Middle High German / Middle Low German sne, altsächisch 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', akin to the Greek ( accusative singular) NIPHA ( νίφα ), latin nix ( genitive: nivis ) welsh nyf ' snow'.

All forms are ( ablautende ) Abstract courses for Indo-European word * sneigṵh - ' snowing, ( be ) cluster together, stick together '.

Crystallization

Snow occurs when the clouds of nuclei accumulate fine droplets supercooled water (eg, dust particles ) and freeze there. However, this process is only at temperatures below -12 ° C, where water can remain liquid in the absence of crystallization approaches to down to -48 ° C. The resulting ice crystals less than 0.1 mm in size, fall through increasing weight down and grow by the difference in vapor pressure between ice and supercooled water to grow. Also the water vapor contained in the air resublimated, so goes directly into ice, and thus contributes to the crystal growth. Are formed from the known hexagonal shapes. Because of the special structure of the water molecules only angle of exactly 60 ° or 120 ° are possible.

The different stem-forms of snow crystals depends on the temperature - at lower temperatures, platelets or prisms are formed, at higher temperatures, six-armed dendrites (stars). The humidity affects the crystal growth. Any build-up of snow crystals increases in the cloud and the temperature, because when it freezes, the crystals give off heat as they absorb heat during evaporation.

There is a high thermals, so the crystals move several times vertically through the atmosphere, where they are partially melted and then can re- crystallize. Thus, the regularity of the crystals is broken and formed complex mixed forms of the basic forms of. They have an amazingly large variety of shapes. About 5000 different snow crystals have been photographed by Wilson A. Bentley from 1885. As the first people get to the highest standards Johann Flögel 1879 photographic images of snow crystals. With high probability, there are and there have never been two complex snow crystals which were exactly the same. The reason for this lies in the very large combinatorial possibilities of many individual features. A snowflake contains about 1018 water molecules, including approximately 1014 deuterium atoms. In the visible region of a light microscope can be easily distinguished even hundred features that can be formed at different locations. In combination, very many possible variations, which is why the possible forms of complex crystals are extremely numerous showed far greater than the number of atoms in the universe.

Also amazing how the observed variability is, the pronounced symmetry, which gives a high self-similarity of some snow crystals and let them become a preferred example of fractal geometry ( Koch curve ). The various branches sometimes grow in one copy in a similar way and apparently even if their tips, where they are to continue to grow at a similar rate, often several millimeters apart. One possible explanation, which requires no assumption of an interaction over this distance of time, consists in pointing out that the growth conditions at various comparable nucleation sites at the tips on the same days are sometimes quite similar. Far more often than beautiful, symmetrical snowflakes but are asymmetrical and misshapen. The periodical forms are, however, photographed and mapped more frequently.

The greatest complexity of the snow crystals is reflected in high humidity, as this also allows more intricate structures growing. At very low temperatures, the ice crystals are built not only smaller and easier, but it's snowing even less than at temperatures just below freezing because the air then no longer contains moisture.

Snowflakes

If the air temperature near the freezing point, the individual ice crystals are bonded together by small water droplets and were built into a cotton ball resembling snowflakes. In dry air in colder air layers formed snow with temperatures of about 5 degrees more than snow to reach the Earth, as part of the flake sublimated and the costs borne energy cools the remaining flake. On the other hand, it happens that even at sub-zero rain falls, then as freezing rain. For this effect, the term black ice is used in some media. These components depend on the structure and stratification stability of the upper and lower layers of air of geographical influences and weather elements such as cold air drops. At low temperatures, only very small flakes, the so-called snow grains form.

The white color of the snow lies in the fact that the snow is composed of ice crystals. Each individual crystal is - as the ice as such - transparent; the light of all visible wavelengths is reflected from the interfaces between the ice crystals and the surrounding air and diffused. A sufficiently large collection of ice crystals with random positional relationship to each other a total thus leads to diffuse reflection; Snow therefore appears white. A similar effect can for example be observed in salt when comparing powder and larger crystals.

The average diameter is about five millimeters of snow, with a weight of 4 mg. The higher the temperature is, the greater the flakes, since the crystals melt and stick to large flakes. The Guinness Book of Records recorded the largest ever seen snowflake a diameter of 38 centimeters.

If a snowflake on the water, then it produces a shrill high pitched sound with a frequency of 50 to 200 kilohertz, which is inaudible to humans. Not all researchers in this field of research confirmed this effect.

Snowfall

As snowflakes have a large surface area and thus a high air resistance, fall at speeds of about 4 km / h relatively slowly - for comparison, moderate rain falls at approximately 20 km / h and hail can reach even much higher speeds. The rate of fall of snowflakes is largely independent of their size, since the surface of the flakes ( almost) increases proportionally to its size, so the air resistance remains constant.

Snow crystals, as well as all other irregularly shaped objects tend to fall with their flattest side down. This seems illogical because one would think yes, that objects should be oriented so that they move with the least resistance through the air. If the flat side of the snowflake would be exactly parallel to the direction of fall (lowest resistance), they would stay there. However, it is very likely that they while their case due to small disturbances ( turbulence ) leans even to the falling direction. Thus, the snowflake experiences due to the air flowing around it a couple of forces; because of the greater flow velocity at the outer ends. This pair of forces then rotates the snowflake so that its flat side down ( plane of maximum expansion of the flake normal to the direction of fall ). The same mechanism follow a falling leaf from a tree, a dropped piece of paper, Rayleigh disc to measure the speed of sound. Characteristic of two-dimensional snowflakes therefore is this known tumbling event, which is in a suitable slight upward flow about in exhaled air or in a warm house facade to the stationary dance.

Another effect of turbulent flow is that snowflakes and other objects tend to arrange themselves one after the other and then catch another. A snow crystal, which falls into the slipstream behind another, it can fall faster so that it collides with and lumpy, like a cyclist in the slipstream behind another less driving force needed to keep the same pace. However (in V formation following birds use the upward flow of the outer half ( n ) of the edge vortex of the advance aviator to fly horizontally with less energy. ) Are snow particles so small as dust, they fall substantially no turbulence in the flow around, the Reynolds number, a product of length, velocity, and viscosity is then very small, as for Stahlkugerln in honey.

Thaw

A snow lose substance, when power is supplied. This may be due to radiation ( short-wave solar radiation, or long-wave thermal radiation), heat conduction happen ( at air temperatures above 0 ° C ) or by falling in the rain and snow, which is warmer than 0 ° C. How fast is the mass degradation is strongly depends not only on the applied amount of energy, but also of air temperature and humidity. Specifically, the degradation is slower, the drier the air is, as sublimation, ie for the direct transfer of water from the solid to the gaseous phase, a certain amount of energy must be applied, whereby the remaining snow is cooled.

On the basis of wet bulb temperature and dew point temperature are distinguished three stages of the decomposition process. Wet bulb temperature is in this case the temperature is measured by the wet side of a psychrometer, and always less than (at 100 % relative humidity equal to) the air temperature. The dew point temperature is the temperature at which the hot air was saturated with water vapor and is in turn less than the wet bulb temperature. Is the wet bulb temperature is below 0 ° C, sublimation of the snow. This process has the slowest rate of degradation, the snow remains completely dry. It can take place at the air temperature up to 7 ° C, this, however, the relative humidity must be below 20%. If the wet-bulb temperature above 0 ° C, the dew point but not including, the melting snow, that is, it moves both in the gas phase and in the liquid phase. With dew points above the zero point of the snow melts, it goes exclusively into the liquid phase. This process has the fastest degradation rates. At a mean relative humidity of 50 % snow sublimes below 3.5 ° C, it melts at from 3.5 to 10 ° C and melts above 10 ° C.

Melting snow on a large area at a high rate by strong sunlight and warm wind and penetrates the melt water already thin snowpack melting warm and strikes already water- soaked soil, can spring floods occur.

Types of snow

There are several criteria by which one can classify snow. A common misconception says that the Inuit ( Eskimo ) possess very many terms for snow.

Age

Freshly fallen snow is called snowfall. His ice crystals are still finely branched with pointed tips. Changes in the structure of the underlying snow is called snow transformation or metamorphosis. Your type and speed is dependent on external influences such as temperature.

A basic distinction between depleting and uplifting metamorphosis, as well as the melting metamorphosis. When degrading metamorphosis the crystals such as wind take by temperature fluctuations, the pressure of the snow cover and environmental influences on less branching and more rounded shapes. They are thus solid and dense and are then referred to as felted or about grained snow. When building metamorphosis form in deeper layers new, larger crystal forms, which only have low strength due to large air pockets. Both degradative and constructive snow transformation taking place at temperatures below freezing over a period of several weeks. The melting metamorphosis can be at temperatures above 0 ° C round crystal shapes. In the interplay with refreezing of the water at the surface ( Auffirnen ) is crud otherwise compact Harsch, and the influence of wind can make even wind pressed snow, which also contributes to the cornices, but also snow board education. Under strong sunlight caused by sublimation of the penitents and other special shapes which are typical of the high mountains.

Old snow of the previous winter, after at least one year called firn and has a high density ( about 0.6 g / cm ³). Over longer periods of time may eventually arise from Firn glacier.

Moisture

• Powder is dry snow that do not stick together, even under pressure. Its density is less than 60 kg / m³
• If already the snow with particular drought, caused little to no verzahnende flakes that build deep baseless snow layers, the American Rocky Mountains with its special location between the Pacific Ocean and the dry continent it is called Champagne powder
• Wet snow sticks together under pressure and is therefore particularly suitable for snowballs and snowmen, it can be squeezed out but no water. It is also called sticky snow, because he zusammenpappt.
• Wet snow or corn snow ( adj slushy ) is very heavy and wet, he also sticks together and you can squeeze out water.
• Faul snow is a mixture of water and larger chunks of snow that no longer hold together well ( slush ).
• At the temperature limit ( elevation change ), or when the weather drops rain and snow, that is, a mixture of snow and rain.

Color

• Blood Snow is dyed reddish snow. It is usually caused by a build up mass development of green algae ( Chlamydomonas nivalis, for example ), the red carotenoids. Less common cause is the descent of reddish dust masses that are transported by winds from the desert regions.
• A also caused by kryophile snow algae green staining was detected in glaciers and Arctic snow surfaces.

Density

Thermal Properties

The thermal conductivity of the snow depends on its structure and texture and decreases its density. The thermal conductivity lies between that of air [ 0.0247 W / ( m · K) ] and the ice [ 2.2 W / (m · K) ]. The insulating effect of snow can learn to use, even though the air temperatures are below the melting point depending on substrate temperature melting processes at the bottom of the snow layer. The heat capacity of snow roughly corresponds to the ice, 2,106 J / (kg K ∙ ) ( at 0 ° C). It decreases with decreasing temperature. The latent heat of melting of snow is 335 kJ / kg ( at 0 ° C and atmospheric pressure ).

The from excessive hypothermia protective effect of snow cover is particularly advantageous in agriculture. Is used Technically, the insulating effect of snow in the construction of igloos.

Occurrence and origin

• Drifting snow is very fine snow, which penetrates through the action of the wind in houses.
• A snowdrift is a caused by wind transport of snow accumulation, the amount of which may be located well above the actual rainfall.
• Artificial snow is artificially produced snow.

Importance

Impact on the climate and the environment

In areas with a well-trained snow cover, more sunlight is reflected back into the atmosphere by the high albedo of snow, so that the soil heats up less. The long-wave thermal radiation of the atmosphere, however, is particularly well absorbed by snow. In particular, it is used during the melting process as a so-called heat of fusion to overcome the bonding energy of the molecules of water, without heating the snow and the water formed. Freshly fallen snow is up to 95% of entrapped air and thus also forms a good thermal insulator, the plants under the snow cover protects against sharp frost wind and Kahl frosts.

Relevance to humans

Where snow is usually only in the winter, the landscape associated change also has an aesthetic significance. As a metaphor of snow for the winter is quite general. Habits, sensations and leisure differed considerably from periods without snow. For tourism snow plays an important role (see also Winter sports). Popular with children is the building of snowmen and the discharge of snowball fights.

A great danger is particularly exposed locations of snow avalanches, entire villages may buried under. Heavy snowfalls (snow disaster) can also lead to severe loss events ( congested building or buildings, tree falls, cut off towns, etc.).

Snow and ice on roads is a significant risk and often leads to a complete breakdown of traffic flow. Roads are often impassable after heavy snow only with the help of tire chains. Specially equipped winter clearing services may be entrusted with the snow removal.

Tourism places that are economically dependent on snow sports, using snow cannons to produce at little or no natural snow artificial snow, with artificial snow is characterized by different properties than natural snow.

Snow also has acoustic effects: Is it easy, there is a lot of trapped air between the individual flakes, thus he has a sound- absorbing. The proverbial winter silence is therefore beside the symbol to understand real ones for a little activity associated with time of suspension and Erholens.

Snow research

History

The strictly hexagonal structure of snowflakes was known in the Empire of China since at least the 2nd century BC. In the West, this property of the English mathematician Thomas Harriot first noticed in 1591, which, however, did not publish his observations. Work on the variety of forms of snow crystals are also known by Johannes Kepler and René Descartes, but first systematic investigations took only Ukichiro Nakaya, the 1936 could produce the first synthetic snowflakes and this 1954 categorized in more than 200 different types.

Snow measurements

Measurements of the amount of snow to be carried out by conventional rain gauge in which, for protection against snow drifts crosses are attached. The thickness of the snow surface is determined with snow levels or snow probes. The increase can also be measured with ultrasound. At the German Weather Service, the snow cover thickness and snow depth are measured daily at 7:30 clock statutory time. Although the amount of new snow is measured over a 24 -hour period, it is sometimes given as a so-called new snow sum over several days summed ( eg 3 -day snow total).

The water content (water equivalent of snow cover) and the snow density have significance for the climatology and hydrology. The snow line is an important climatological characteristic. The snow line separates snow-covered and snow- free areas from each other.

Snow safety

In winter sports area is considered a Snow sure if it has at least 100 days sufficient for skiing snow cover of 30 cm ( Alpine skiing ) and 15 cm ( nordic skiing ). Currently, areas which are higher than about 1200 meters are as guaranteed snow. Snow is one of the strongest motives in choosing a ski resort. This makes the snow safety is not to be underestimated economic factor, in -dependent of winter tourism mountain regions. It could be in the wake of global warming, some low-lying ski areas have problems to have the necessary snow reports. According to OECD estimates, could lose their snow safety in Austria, around 70 % of the ski resorts. To counteract the lack of snow, is investing heavily in artificial snow-making equipment for many years. So EUR 21.8 million in 2007 were invested in artificial snow in the Austrian state of Carinthia in the year. However, the operation of snowmaking equipment is very expensive. So taste one kilometers snow ski slope up to 650,000 euros.

Trivia

• As an industrial snow is called local snowfall caused by power plants and other large facilities.
• Lake effect snow is a weather phenomenon in the Great Lakes in North America, which is leading to locally increased snowfall.
• Great Snow in the forest lead to snow damage in buildings, snow loads in the structural analysis are considered.
• Snow blindness is an injury to the eye, which is caused inter alia by snow and sun.
• With Yukitsuri ( the high binding in japan. Horticulture) branches can be prevented from breaking under the weight of snow.

Snow disasters

• Big snow storm (1888 ) in the United States
• Snow disaster in northern Germany in 1978
• Munsterlander snow chaos in 2005

Snow representation in art history

In art history, the representation of snow is a theme often used, which was given to epoch a different meaning from era: In the Middle Ages the winter brought the care of the dependent nature of the people and their health at risk. Social and technical progress, it is thanks to the winter of threat more and more lost. After the Renaissance gone out of fashion, the winter landscape reached its artistic revival in the late 18th century. First, it is romanticized. Later the artist 's gaze is on the external appearance of the wintry shades.

Pieter Bruegel the Elder: The Hunters in the Snow, 1565

Pieter Bruegel the Elder: Winter landscape with bird trap, 1601

Caspar David Friedrich: The Arctic Ocean, 1823-1824

Joseph Ferdinand de Boissard Boisdenier: Episode on the retreat from Moscow, 1835

Kasimir Malevich Sewerinowitsch: landscape ( Winter ), 1909 (dated 1930 )

Matthias Zimmermann: The redox reaction on the surface of a faded winter presentation, 2012