What would cause this to happen to a plane’s window, and why would we all need to swap planes as a result?
Answer by Thomas Zerbarini:
What you are looking at is the moisture between the plastic window (inside the pressure vessel) and the aircrafts exterior glass window.
The plastic window is part of the plastic structure that makes up the passenger cabin interior. It is there to keep passengers from scratching the glass window. The plastic has no structural purpose nor is it part of the outer pressure vessel.
It is common for air in between the plastic window and the glass window to have more moisture in it because it is not easily vented. As you can see there is a small vent hole at the bottom of the plastic window. That is to keep the air slightly vented in between the two panes. Here you can see some fogging up of the interior glass of the outer window. The small hole in the plastic is venting warmer air from the cabin into that small space creating a defogging affect in a small circle on the glass window interior.
This is all normal. There is no compromise of the structure. You can also see this fogging frost up as a plane climbs into much colder air aloft and moist air is still trapped in between the plastic interior and the glass window.
Here are some pictures of a cabin with and without the plastic passenger moldings.