Snowflakes - Why is Snow white?

Snow can be great fun to play in. People enjoy sledging in the snow, making a snowman or having snowball fights! But what exactly is snow? 

The air is full of water vapour which has evaporated from lakes, rivers and the sea. Some of the moisture in the air comes from the leaves of plants and trees. On a cold day, you can see the clouds of water vapour that you breathe out. 

This water vapour rises into the atmosphere and gathers together in clouds. When it falls from the sky, it is called precipitation. Precipitation can fall as rain, snow, hail or sleet depending on the temperature of the air. Sometimes, water vapour freezes into ice crystals, instead of drops of rain. If the temperature is cold enough (it would have to be −35 °C or lower) other water droplets attach to a single first ice crystal, or, in clouds above -35 °C, the freezing droplets can form around a dust or pollen particle. More and more ice crystals form around the first few, creating a snowflake.

 Ice-crystals come in millions and millions of shapes and patterns.  The shapes of snowflakes depend on how high up in the clouds they are formed. Every single tiny variation in the air where the snowflake forms affects its final shape, so no two snowflakes are exactly alike. 

Every snowflake is a hexagon, meaning it has six sides. This is because water molecules each contain two hydrogen atoms and one oxygen atom. (This is why water is called H2O). The most efficient way for the molecules to attach together is in a hexagon shape.  

The patterns that these frozen crystals make depends on the temperature of the clouds that they fall through as well as the amount of water in the air. An ice-crystal needs a temperature of -15°C to keep growing. The colder the temperature the sharper the ice-crystal tips.

Very complicated snowflakes are called dendrite snowflakes. They look as though they have branches, like those on a tree. This type of snowflake forms high up in very cold clouds.  In warmer temperatures, the ice-crystals grow more slowly and are smoother. This means the crystals are plainer to look at. Snowflake which are formed when there is not enough moisture in the air to form ‘branches’ are called thin plate snowflakes. 

 Each snowflake may look as if it is white as it falls from the sky. In fact, the ice crystals are translucent, which means that they allow some light to pass through them. The sides of the snowflakes diffuse (spread out) the light which passes through them, so all the colours in the light reflect off the crystals. This makes the snowflake appear white.

Image: Snowflake by John Britt