Thank you. I have heard of, and mentioned, clouds. :-) And Wikipedia, which tells me
The percentage water vapor in surface air varies from 0.01% at -42 °C (-44 °F)[16] to 4.24% when the dew point is 30 °C (86 °F).[17] Approximately 99.13% of it is contained in the troposphere.
I had supposed the low amount of water in the atmosphere at altitude was mainly a function of temperature. The Wikipedia entry does not make it quite clear if pressure plays a part. Obviously in the extreme case water boils above 100C down a mine, and at near 0C in a vacuum.
I also found this:
http://www.boeing.com/commercial/aer...ly/m01txt.html , which contains this:
Air-conditioning system design The amount of outside air per occupant supplied to the airplane affects the in-flight humidity level. Increasing the outside air per occupant decreases the cabin humidity, which decreases the condensation rates. and this:
Seating density More people produce more moisture, causing higher cabin humidity levels and increased condensation rates.
Unless I am told different I will assume that much of the water vapour in an aircraft comes from the respiration of the passengers themselves, and that air conditioning removes much of it, but less so in the 787. I also assume that while the cabin pressure might be a bit higher in the 787 than other airliners, the rate of change of air is not necessarily higher, indeed it might even be lower unless there is a means of capturing and recirculating water vapour separately.
I should have asked when I was at Everett.