Originally Posted by
911slf
But I was under the impression that the amount of water vapour in the atmosphere was very low at high altitude.
Not necessarily. If that was the case, clouds wouldn't exist.
What you are probably thinking of is how conventional air conditioning packs/air cycle machines extract most of the humidity from the ambient air, which the 787 doesn't.
So if the 787 has more humidity in the cabin, where does it come from?
See above.