Snows

Physical aspects

History

Kepler was one of the first scientists to be been interested in the formation of the flake S. It writes in 1611 a treaty, the New Year's gift or snow sexangulaire . Towards 1930, Japanese Ukichiro Nakaya forms his own flakes under experimental conditions, fixing the temperature and saturation out of water. He realizes whereas the shape of the crystals depends on these two parameters. In 1935, Tor Bergeron develops the theory of growth of the flakes starting from the cannibalization of the superfused drops of water called the Effet Bergeron.

Diversity

In a very cold Cloud, the steam condenses directly out of crystals of ice on suspended particles (dust, smoked…). If they do not meet that layers of air of temperature lower than 0°C during their fall, the crystals are bound and combine to form increasingly broad flakes. The assembly of these crystals depends primarily on the temperatures. Only the characteristic commune with all the crystals is the hexagonal structure related to the angle of 120° of the water molecule. It comes from a minimization of the chemical potential energy of the crystal.

The shape of the crystals varies according to the temperature, but also percentage of moisture:

* of 0 with -4°C: thin hexagonal plates

* of -4 with -6°C: needles

* of -6 and -10°C: hollow columns

* of -10 with -12°C: crystals with six long points

* of -12 with -16°C: thread-like dendrites.

The Densité of coldly fallen snow is very variable. The statistics give an average of 110 kg per cubic meter, with a standard deviation of 40 kg which confirm the dispersed character of this criterion.

Life cycle of a crystal

The formation and the evolution of the crystals integrate:

multiple degrees of chemical freedom of association of the Molecule S of Water; the expression of these possibilities is supported by the relative slowness of crystallization: ten minutes to a few hours. This is at the base of the extreme diversity of the forms created.
various the weather conditions met between the formation and disappearance:

conditions of the education level, before Précipitation
conditions of the atmospheric layers crossed
conditions on the level of the ground, if it is reached.

The weakness of the bonds between water molecules makes these crystals very sensitive to any modification of their environment. One can regard the crystal of snow as unstable and that it must be in phase of crystallization to preserve its Forme, so that recombinations occur as soon as this one stops. This sharp sensitivity makes difficult the microscopic observation of the crystals without special precautions.

Conditions of the education level

The parameters of the ascending movements of air condition the duration of crystallization particularly and the possibilities of penetration in different layers by their Hygrométrie, Température, Pression,… On this level, of the crystals can melt, to sublimate, to combine, but also to be covered with water in Surfusion; the crystals cover initially invisible nodules but which can in certain cases give them an aspect of “flower of mimosa”.

Even if the air is not ascending, the resistance which he opposes request the agglomeration of several crystals before the Précipitation does not start.

Conditions of precipitation

The turbulescence, the hygroscopy in particular will govern the disappearance (cast iron or sublimation) of the crystals and flakes or on the contrary their progressive agglomeration. Partially liquefied flakes can also undergo a brutal crystallization with the meeting of a colder atmosphere; if the phenomenon is massive one speaks about Grésil.

The variation of the weather parameters with the Altitude is characterized in particular by the determination of famous the Limite rain/snow.

Conditions of crystallization on the ground

Under the moderate Latitude S (“hot” ground), the strong capacity isolating from snow still associated with the Albédo makes possible the fast creation of a heat gradient between the hot and insulated ground and cold reflective surface; it can reach 20 degrees. However it is noted that the crystals of a layer of snow, in a Gradient of temperature, return in a process of recrystallization resulting in an increase in the intermediate size of the crystals. From this point of view, one considers that a fifteen centimetres thickness is enough with the establishment to a gradient.

The conditions of crystallizations being quite different from those of the upper atmosphere, crystallization on the ground produces new but less elaborate forms.

Other conditions on the ground

Those cannot be schematized; it is enough to think of the diversity as well of the quantities of snow as of the weather variations to imagine the innumerable possibilities of dissipation of the crystals. Each human community, according to its requirements, has a culture suitable to describe the typical forms of these possibilities.

Evolution of the snowy coat

According to the profile of temperature which the flake between its formation and its arrival on the ground must traverse, one will have types of favoured crystals. When the profile is rather hot and wet, there will be formation of large flakes which imprison little air and gives very dense snow. The relationship between the number of centimetres accumulated in this case and the water which they contain is very weak, about 4 to 8 cm of snow for 1 mm of water. By cold temperature, the reverse occurs and one can easily obtain a coefficient of 25:1 for powder snow. The ologic average Climat is 10:1, that is to say 1 cm of snow for 1 millimetre of water contained.

Coldly fallen snow is prone to the action of the Vent, especially if it is very light. This gives the powder mill Inhabitants of Quebec, and in an extreme case the blizzard. It can concentrate in named dunes snowdrifts (Canada) or Congère S (Europe). It is not the case of the spring snow , compact and rich in water, supply to be melted on the spot. In mountain, the wind is at the origin of cornices which can trap the hikers.

Snow is not an inert material. It is on the contrary in constant evolution and does not cease changing, subjected to the action of its own weight which packs it, like to the differences in temperatures between the day and the night. If the slope is stiff, the coat can become unstable and generate Avalanche S.

The metamorphosis of isothermy

It is held when the thermal Gradient within the layer is weak, lower than 5°C per meter. Because of imbalances of saturated vapor, the dendrite S are destroyed with the profit of the center of the crystal. The crystals swell and their size is gauged. They are called fine grains . The contacts thus created between them correspond to the formation of bridges of ice which weld the crystals the ones with the others. It is the phenomenon of Frittage. The layer of snow gains in cohesion and of density.

The metamorphosis of average gradient

It appears when the heat gradient within the layer lies between 5 and 20 °C per meter. One also observes a transfer of matter by sublimation/Congélation but the privileged direction is the vertical, of bottom upwards. The crystals transform into plane grains with face .

The metamorphosis of strong gradient

When the heat gradient is higher than 20 °C per meter, the vapor flow within the layer of snow becomes very strong. After ten days, it there has appearance of goblets , or frosts of depth, which can reach several millimetres in diameter. The coat becomes very unstable then, being on a true roll of the dice.

The cast iron metamorphosis

It results in the appearance of water liquid in the middle of the snowy coat. It is caused by a fall of rain or a prolonged rise in temperature. It is formed agglomerates known as round grains (“deicing salt”) which make the snowy coat very unstable.

The fact of hydrating snow does not cause necessarily its immediate cast iron. The photographs below of immersed snow were taken in summer.

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