Question Corner

Symmetrical snow

QUESTION: How do snowflakes know how to be symmetrical? Are there any circumstances in which they can be persuaded to be asymmetrical?

ANSWER 1: The snowflakes that you can see falling slowly from the sky are loose collections of tiny snow crystals. These snow crystals are indeed hexagonally symmetrical and can develop the elegant, feathery star shapes that are so beloved of Christmas card designers.

However, snowflakes are not themselves symmetrical. Unfortunately, confusion often arises because snow crystals are also commonly referred to as snowflakes.

Snow crystals are hexagonally symmetrical because their constituent water molecules initially join up in a set of interlocking rings of six molecules as the water freezes.

A snow crystal will contain more than a million trillion water molecules, but the pattern from which the crystal is assembled is hexagonally symmetrical.

Within that hexagonal symmetry, snow crystals can take a huge variety of forms. The starting point is a simple six-sided prism, like that shown. But depending on exact conditions it encounters in the atmosphere, that prism can grow in different ways.

If water molecules from the air attach themselves largely to the six prism faces, then the snow crystal will grow into a hexagonal plate.

But if they attach more to the top and bottom basal faces, then a needle or column-shaped crystal can result.

The temperature and humidity conditions that favour different types of crystal growth are well known, although we still don't fully understand the physical processes involved.

At certain temperatures and when the air is supersaturated with water, the growth of the crystal is determined by how fast water vapour can diffuse through the air and stick to the crystal surface.

The growth pattern then becomes unstable. Parts of the crystal that stick out further into the surrounding air will be able to mop up more water vapour and grow faster.

The corners of a hexagonal prism will begin to grow the six characteristic dendritic arms of a Christmas card "snowflake".

Protuberances on these arms will themselves grow faster, and so on. The most complex snow crystals are formed when a developing crystal experiences different conditions as it passes through the atmosphere, each favouring a different type of crystal growth.

Because of the sensitivity to growth conditions in the atmosphere, it is generally true that no two snowflakes are exactly the same.

Exceptions to this occur when atmospheric conditions are stable and favour only a simple type of snow crystal. In Antarctica most snow falls as simple hexagonal prisms.

ANSWER 2: Although snow crystals are approximately symmetrical, it has long been known that they are never perfectly symmetrical down to the very last detail.

Studies of the pictures, taken by Wilson Alwyn Bentley, show that absolute, perfect symmetry is hard to find.

-- New Scientist

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