"Moisture in the air is very important for snow, since without it snowflakes cannot form. When temperatures are below zero degrees celsius, there needs to be enough moisture in the atmosphere for water vapour to condense in clouds, forming small ice crystals. As these ice crystals cling together, they begin to create a six-sided snowflake, and when the snowflake gets heavy enough, it falls out of the clouds. Assuming that the ground level is also below freezing, snowflakes will reach the ground and the warmth and pressure of the ground causes the snow to lose its crystalline shape and become round ice pellets."