"Superpilot" says
Once had the privelage of being in the jumpseat of a 747-200 with a certain airline from the indian subcontinent. Holding at LAM at FL120 in rain...we could not find enough tissues, paper towels and hankeys to hold in front of the windshield.
The aircraft is pressurised therefore it is absolutely impossible for this water to be coming from outside. (Any "holes" that rain is trying to get into are filled with cabin air trying to escape.) It is simply that the airframe (inside the aircraft) in the cruise being cooled to approx -55C has had moisture from the air in the cabin freeze onto it and in descent this melts causing "rain" inside the 'plane. It is a common problem with a vast number of aircraft both old and new.
