Author: Liya Johnson
The first things that come to our minds when we speak about winters are Christmas and snowflakes. We all have seen snowflakes or have at least made those paper cuttings of snowflakes in our childhood. Flakey, cold white stuff which is so delicate and intricate, each having a pattern that is never identical to another. It's nothing but crystalline water yet so unique and marvellous.
But have you ever found yourself wondering -Why snowflakes are so intricate if it's frozen water? why are they symmetrical?. How are they formed? Why is each snowflake so unique? What is the shape of a real snowflake? And what exactly is the science behind snowflakes?
The study of snowflakes was firstly undertaken by Wilson Bentley. He was an American meteorologist and a photographer. He was the first to take a detailed microscopic picture of snowflakes. The study took a scientific turn in 1930 as Japanese physicist Ukichiro Nakaya began his study on the formation of snowflakes.
To understand the physics behind them, it is essential to understand how they are formed. They are formed due to the crystallisation of water droplets around impurities in supersaturated air. The water droplets don't freeze once it’s below the freezing point as water can be supercooled even below -40 degrees celsius. But once a droplet freezes the rest of the water droplets crystallise to form a seed crystal.
The hexagonal shape of snowflakes has a lot to do with the arrangement of hydrogen and oxygen atoms in the water molecules. The molecules of water are formed by means of covalent bonding between the constituent atoms. As oxygen has more affinity towards electrons than hydrogen, it acquires a partial negative charge and the latter acquires a partial positive charge. As like charges repel each other the 4 electron pairs repel each other making the angle between 3 constituent atoms 104.5 degrees. The molecules align themselves to maximise attractive forces and minimise repulsive ones(refer to the diagram below). The positive and the negative charges give rise to the formation of a hydrogen bond between the hydrogen and oxygen atoms of different molecules. When water freezes this bonding occurs on repeat ultimately forming a hexagonal crystal lattice and the snowflakes retain this hexagonal structure as it grows.
Bond formed due to sharing of electrons
But how do these forces operating at a molecular scale lead to the hexagonal shape of a relatively larger snow crystal? The answer to this has to do with how crystals form facets. The seed crystal may not be perfectly hexagonal but the molecules tend to attach themselves to irregular surfaces rather than smooth surfaces(Faceting - SnowCrystals.com). This eventually creates a hexagonal crystal. Once this structure is formed the water molecules tend to attach themselves to the corners and the crystal grows further.
Top and bottom flat surfaces of a hexagonal prism
Snowflakes can be classified into plates, stellar crystals, columns, needles, spatial dendrites, capped columns, and irregular forms on the basis of their shapes. But they all have hexagonal faces. The initial seed crystal has 2 basal facets and 6 prism facets. If the basal facets grow faster you get a column and if the prism facets grow faster you get a plate.
The side surfaces of a hexagonal prism (3D hexagon)
The shape that a snowflake might take on depends on the temperature and humidity that the crystal is subjected to; minute differences in these conditions will alter the shape of each flake. And this is why it is said that no two snowflakes are identical as each flake takes different paths in the atmosphere hence the conditions will be slightly different.
The snowflakes have six-fold symmetry. The first scientist to theorize about the six-fold symmetry of snow crystals was German scientist Johannes Kepler. They are symmetrical because the changes in environmental conditions take place over a large area compared with the size of a single snowflake, so all regions of the same flake are similarly affected.
Isn't it amusing that such a simple thing has so many complicated explanations behind it? This exactly is what makes science interesting. Even simple things lead to most groundbreaking discoveries and can give rise to many mysterious questions that are yet to be answered.
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