The Science of Snowflakes

Beautifully delicate in structure, but what is the science behind the snowflake?

The Science of Snowflakes

What are Snowflakes?

Snowflakes are ice crystals that form when super-cooled cloud droplets freeze high in the Earth's atmosphere before tumbling slowly to the ground in the flurries that we all know and love (or hate if you have somewhere to drive).

Water droplets will more readily freeze around a nucleus which acts as a catalyst for crystallisation. This nucleus can either be an assemblage of water particles that happen to collide with each other or airborne particulates such as dust or fine sand.

Clumping snowflakes.

Clumping snowflakes.

Structure

Atmospheric temperature ultimately affects how the snowflake grows. In the 1930s a scientist devised some lab-based experiments which found that at temperatures around -2 degrees Celsius (C) the snowflake forms plate-like, hexagonal shapes, at -5C you get delicate needles and at -15C you will get the classic six-fold snowflake. If you go colder than this the snowflake will form large, complex, column-like structures. On rare occasions snowflakes can also form a three-fold triangle!

The classic six-fold structure of a snowflake is due to the chemical structure of water. Each water molecule possesses two hydrogen atoms connected to an oxygen atom, and when these water molecules are connected together they naturally align into a six-fold structure. This structure detracts from the common mistake of cutting paper snowflakes in an eight-fold pattern - try cutting your snowflake in the six-fold pattern.

Although most images of snowflakes show them as almost perfectly symmetrical, this is a very rare occurrence. Confirmation bias sees photographers selecting the most aesthetically appealing, and therefore most symmetrical, snowflakes for their photos.

Snow

As the snowflake tumbles towards the surface of the Earth and the air temperature warms towards 0C it will get stickier and forms into larger snowflake clumps - this is why just around freezing is the ideal snowman and snowball temperature! You can even use this to tell the approximate outside temperature from the falling snowflakes - larger ones will mean a relatively warmer temperature.

Diffuse reflection through its clear structure, reflecting every wavelength of light, means that snowflakes appear white to our eyes, even though they are clear. However, you can also get a yellowy-red snow when Saharan sand blown high into the atmosphere acts as the condensation nuclei for the formation of the snowflakes. That's right, yellow snow.