system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does eating snow kill you eventually

I've heard many times that if you're trapped somewhere in the mountains (for whatever reason) you must not "drink" the snow-water because it will kill you. Why is that? We can drink rain water, so why not snow-water?

Thanks!
gpt
You ARE supposed to drink snow-water *(edit:) when there isn't any other water available*, it's  a great source of water in an emergency.  But you're never supposed to eat snow unless it has melted first.

Eating snow makes it much easier to die of hypothermia.

It doesn't matter if you're in a good shelter, because the snow freezes you from the inside.  When you are stuck in any place cold enough that you risk freezing to death, it is dangerous to eat or drink very cold things.

It is safe to let the snow melt first, and then drink it.

Edit: You should only eat snow as a last resort.  Snow has more bacteria in it than some other kinds of water.  It also has a bit of whatever was in the air when the snow fell, which could include pollution.   Snow that has been on the ground a long time can be considered comparable to an especially clean pool of  stagnant water.