Megan- That's a very timely question. It's a very cold winter night, so I just set up a vaporizer to try to keep my wife's dry eye from getting worse.
Air contains some water molecules, bouncing around as individual gas molecules. When the water molecule density is high, they bump into any little drop-like clumps more often than molecules fly off the clumps. The clumps grow into drops and fall out, sticking to surfaces. That's what dew is. The colder the molecules, the less often they fly out of the drops, so cold air can hold a lower density of water molecules than hot air.
Of course our body temperature doesn't change much in cold air. So lots of water molecules fly off body surfaces (like the eye), and there aren't many new water molecules coming in from the dry cold air. When you warm the air in your house, it still won't have many water molecules in it, because it comes from the cold outdoor air, even though it could hold more. You can, of course, put water back into it with vaporizers. Which reminds me, I better check if there's enough salt in that vaporizer for it to work- but that's another story, about ionic conduction and electrical heating.
(published on 01/26/09)