Sausage, think of the atmosphere like a sponge cake, so viewed side on there are lots of layers such as the bottom sponge, then jam, then cream and then more sponge.
In the atmosphere if any one of those layers is above 0c then it will melt the snow from higher up and it can't then be turned back in to it. The bottom layer where we are can still be cold, as it is today but of course we just then get the melted snow ie rain.