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Exquisite geometric perfection is characterized by many forms of atmospheric ice - snow, hoarfrost, ice dust, cereals, hail.
Snow forms in the clouds under certain temperature conditions: droplets of supercooled water are deposited on the icy tiny crystals contained in the clouds. In the troposphere, the bulk of clouds stay at a temperature below 0 ° C, but the water vapor that gets there during evaporation from the surface of the planet does not immediately turn into ice. In appreciable amounts, ice crystals appear there only in the temperature range from -12 to -16 ° C, intensive crystallization occurs at -22 ° C, but even at -41 ° C, individual drops of supercooled water are found in the clouds.
The clouds receive moisture from the ascending air currents and air masses circulating in the atmosphere. These masses contain the bulk (90%) of atmospheric moisture. The complex regime of the ascending air currents, which feed the clouds with moisture, introduces a variety in the formation and growth of ice crystals in the cloud. Gradually they acquire such dimensions and weight that they overcome the lifting force of the ascending air currents and fall to the ground in the form of snow.
It is necessary to ice the pressure of saturated steam is always less than over supercooled water at the same temperature. When a drop of water near the formation of a snowflake approaches the ice crystal, molecules of water rush out from the surrounding shell of saturated water vapor to the ice surface, from greater pressure to smaller. Settling on a crystal, they increase its size. And the drops gradually evaporate: due to the water molecules composing them, they create all the new vapor shells and immediately lose them.
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The structure of the snow crystal depends on the temperature, the amount of water vapor, due to which it grows, and the intensity of their arrival. All this creates an amazing variety of its forms. Specialists studying the shape of snowflakes to determine their connection with the course of atmospheric processes, number thousands of their varieties.
But for all the variety, snowflakes are predominantly six- and twelve-pointed stars - dendrites, as well as hexagonal plates and hexagonal prisms. In the temperature range from -8 to -12 ° C, mainly dendrites form in the cloud. Under these conditions, the greatest difference between the saturated vapor pressure above the water and above the ice surface is noted.
At -15 ° C there are interesting adjacent forms - "cufflinks". These are crystalline needles of ice, terminating on both sides with ice stars or plates that have grown up in the form of a base. A figurine, in which both bases are plates, is called tsuzumi for its resemblance to a traditional Japanese drum. There is an assumption that almost all flat adjacent crystals are tsuzumi with an extremely shortened column. Only with comparatively weak frosts do snowflakes fly from the sky.
In high latitudes - in the Arctic and Antarctica - severe colds (-30 ... -50 ° C and below) are most often encountered, and very "unflattering" snow falls on the ground: each snowflake is a sharp pointed rod at one end. Such crystals are usually formed in cirrus clouds at an altitude of 7 ... 10 kilometers above the Earth's surface, that is, almost in the stratosphere.