system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: On a cold day when someone walks into a room from outdoors and you can "smell the cold" what is it that you're smelling and what causes this?

I ask this because some people just walked into the room I'm in from outside and I feel like I can smell the cold on them. Also, there is a certain smell that cold has that seems different than when it's hot out, so what is that and how/why does it happen?
gpt
The predominant factor is the change in the _amount of smell_.  When it is very cold there is almost no moisture in the air.  Moisture in the air allows more particulate matter to be airborne.  When it's really, really cold it simply doesn't smell as much as when it's warm, and when its humid.