Hi alamin. Snow and ice are made of water molecules with a regular crystal structure, meaning all of the water molecules in ice are arranged in the same way. This should let light pass through without it being scattered, making it transparent. This is the case for Ice, as in an ice cube or shard as they have smooth strait edges. However snow flakes are very irregularly shaped and light passing through them will be scattered and difflected. This means if you try to look at an object through snow, the light carrying its image will be scattered in different directions as it goes through the snow so it won’t reach your eye.
That may not be the whole story and hopefully someone with a bit more physics background than myself will be able to fill in or clear up that explanation 🙂
I can’t believe that Will stole my crystallography answer! 🙁
Individual snow crystals still follow a strict crystal structure, but the problem is that each crystal is not layered up directly on top of the other, so their crystal structures don’t line up. They are all piled on top of one another to make a snowflake. One snowflake is made up of loads of tiny snow crystals. Because these don’t line up, like they do in a bigger crystal like a block of ice, the light is reflected in a crazy way, rather than smoothly, and that is why they look white, because the light is reflected and scattered all about the place, rather than passing through. More of it is lost bouncing around the little crystals in each snowflake.
The others are right. You can see almost the same thing if you chew the end of a transparent “biro” – it goes white. That’s not because your spit has done something, it’s because the plastic is toughened and it develops lots of tiny, tiny cracks when you deform it. (This is called crazing”). These absorb energy (which is why it is tough and not as brittle as it could be), and they also scatter light so it looks white!