How is cabin humidity managed?
The 787 claims to have higher humidity in the cabin. It is said that the composite construction enables this to be achieved without creating corrosion problems. Also the cabin altitude is around 6000 feet compared with a more typical 8000 feet in other aircraft, so the cabin pressure is higher and the partial pressure of water vapour is correspondingly higher.
But I was under the impression that the amount of water vapour in the atmosphere was very low at high altitude. This could be because the partial pressure of water vapour must be lower unless there are high altitude clouds, and I think that with extreme low temperatures, say minus 40 or lower, the air must necessarily be dryer. So even if you heat and compress this air, I would expect it to contain little water vapour.
So if the 787 has more humidity in the cabin, where does it come from?