Why do Snowflakes differ in Size and Shape?

Dave - Snow is made up in the atmosphere and it's basically when water vapour goes directly from a vapour to a solid - straight to ice. It's sublimed into ice and there are lots of different ways it can do this. There are lots of different modes of growth. Some modes which produce long, straight crystals and make the crystals get longer and thinner. Some cause them to branch in different ways. Sometimes you can get plates forming and sometimes you get long straight needles as the temperature and humidity changes. So, if a snowflake moves around in a cloud both the temperature and the humidity will change while it's growing. And so, depending exactly how long it stays at one temperature, at one humidity, then another temperature then another humidity, you'll get different forms of growth. So, the snowflakes which you'll produce will be slightly, slightly different. They don't necessarily all have to be completely different, but there'll be lots and lots of different types.