SNOW

To have snow, it is necessary to have something more than water!
Indeed, very chemically pure liquid water can stay liquid up to - 40° Celsius. This surfused state is very unstable and a little perturbation will change everything: an impurity like dust will allow crystallisation. A first ice stone is formed and then the first crystal will grow as an hexagonal shape around it. That is the birth of a snowflake that will grow in different shapes depending on the temperature and moisture encountered in the layers of air it will go through its way down. Each snowflake has its own history therefore they are all different.

Here are the 9 main crystals' shapes - the complete classification contains 82 sorts of crystals! :

Once the sky is grey, to have a snowfall, the air temperature should be inferior to -12°C and the soil temperature should not exceed +4°C.

Then, once fallen, snow flakes are broken and form a layer. Each snow fall forms a layer of snow with its own density and all layers are well separated constituting a snow coat. That is why it is very difficult to predict avalanches: in a same area, you can have very different conditions depending on the history of each layers.

Snow field waiting for snowshoers!