Originally Posted by 777fly
I remember from my distant 'A' level studies ( when it was a struggle to get even a 'C' grade) that there was thing called Boyles' Law. Something about P1V1T1=P2V2T2?? What that means , if I remember correctly, is that if a given volume of air (in a cabin) expands rapidly to a new and much larger volume, its pressure will drop and its temperature will fall rapidly.( Please correct me if I am wrong, I did chemistry in 1964).
I think Boyle´s law does not concern adiabatic expansion... as you see, there are 3 variables on both sides. How much the temperature actually falls depends on the things like Cp/Cv.
Originally Posted by 777fly
So, to answer the last question, as the cabin depressurises, the temperature will quickly fall below the cabin dewpoint and a 'mist' will appear throughout the cabin.
The dewpoint is low. I think the relative humidity is under 5%. For a good reason: the moisture must not condense on the inside of the skin and cause corrosion!
Originally Posted by 777fly
Following a decompression, if the aircraft is not immediately descended below about 15000ft max, everyone on board without an oxygen supply would be unconcious within minutes and exposed to ambient temperatures well below - 35C . Yes, the cabin would get VERY cold!
I think that if you have sudden decompression from 8000 feet to 15 000 feet and the initial temperature is, let´s say, +20C, you get about 0C at 15 000 feet. -35C would require decompression to 27 000 feet or so.
But as soon as the pressures equalize, the adiabatic cooling stops, and the body heat as well as heat given off by various systems will start warming the air!