Contrary to a fairly widespread opinion, the snow does not form simply when the temperature drops below freezing, the formation of snow is actually a phenomenon more complex and surprising. Water is an element that has this important property: it has the power to maintain its temperature longer than the material that surrounds . This ability is unaffected even when the comparison is made between the droplets of steam into the air and airborne dust (ie ' aerosol).
It so happens that at high altitudes, in temperatures several degrees below zero, while the droplets of water continue to remain in the vapor state, the dust becomes airborne cold. 
When a droplet steam comes into contact with the speck of dust or a cold glass of ice, cool istanteneamente sublimating : being born a "crystal" of snow. Around "core aggregation (powder or crystal ice cold) water freezes according to the key characteristic of the ice crystals (hexagonal): that is why the" crystals "of snow, while varying in shape, always keep a star schema with six arms.
coming into contact with other crystals of ice, "crystal" of snow becomes increasingly large, endless repetition of the initial design. In colder areas of the planet falls into the snow crystals separated, while in warmer and humid crystals tend to join each other giving rise to "snowflakes" .
A snowflake made up of thousands of crystals, can be increased to 15 mm in diameter varying in shape according to the manner in which it falls: there are so perfectly hexagonal snowflakes (if they fall vertically with a rocking motion), or bows to "foot" to "flat" to "column", etc.. 
A property of snow, is well known to skiers that, precisely because of the characteristic shape of the crystals (hexagonal), it holds a large amount of air. For example, a cubic meter of "powder snow" (the best one for skiing!) Can weigh less than 200 kilograms (1,000 Kilograms and would weigh if it were a single cubic meter of water!). (Informations All of these have been taken and adapted from "MisericordiaOnLine", run by the Brotherhood of Mercy of Andria)
Source: Physics easy
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