Snow scene in unity

Making a good 'snow' scene can be done using the particle emitter component that follows your main camera around. Use the smooth follow script prefab with unity helps, and you can tweak that around to get it right. In scripting you can use a boolean varible (i.e. "isItSnowing" or something like that) and attach it to a timer if you want it to snow/on off at particular variables of time. You can make your own snow flakes in photo shop or gimp and place them on an alpha channel so they appear transparent. Import into unity, slap that material onto the particle emitter, and test it out. You'll most likely need to mess around with the variables to get the proper effect your looking for.

That would be the basic "snow falling effect", it'd be far too much for a computer to handle a particle system the size of the terrain. For the terrain itself, I would attach a layer of snow texture, to give the illusion of snow that is piled up onto the ground.

That's generally how I'd go about it. I do this trick with rain too. You can attach sounds if you like in the scripting if you want the sound of wind as the snow comes down. There is plenty of documentation on particle systems on the unity site as well as on how to texture the terrain and attaching the sound.

Hope that general guideline helps!