Snow

Snowflakes are an aggregation of ice crystals formed from vapour condensing within clouds at a temperature below 0°C. Contrary to what you might think, water vapour and low temperatures are not the only conditions needed to form ice crystals. Dust particles are crucial for this process, as water vapour molecules form around them aggregating to create crystals. A newly-formed ice crystal arranges into a column-shaped hexagonal structure. Then, aggregating with other water molecules, it changes an infinite number of times gaining an incredible array of different natural shapes. In some cases, ice crystal are higher rather than larger and create needle shapes. Other crystals are larger and create wide hexagonal plates. Arms sprout from the six corners of the initial hexagonal prism and further branches develop on them creating spectacular shapes (dendritic growth).
Each ice crystal has a unique story: from its origin to the moment it falls to the ground, it passes through different atmospheric areas which vary in terms of temperature and humidity, the main factors influencing ice crystal shaping. Moreover, every ice crystal is formed by billions of water molecules aggregating unpredictably. For this reason, it’s impossible for two ice crystals to be alike!
Snowflakes: the air-cleaners
Many scientists have focused their research on ice crystals and snowflake formation. One of the first to study this phenomenon was Descartes who published a treaty on their morphology. To this day the formation mechanisms of ice crystal aren’t completely known. Scientists haven’t found out yet why water vapour aggregates to existing crystals favouring either the prism walls, bases or corners according to temperature and humidity. Scientists’ main goal is understanding why snow is the best “air-cleaner”. Among all pollutant substances depositing on the ground, as much as 90% are incorporated in ice crystals and snowflakes. These substances are snow aggregation nuclei: particles are incorporated in the ice crystals during formation and deposited on the ground when snow falls. Some scientists believe that understanding the mechanisms of their formation could be useful to create more effective anti-pollution filters.

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