Drilled holes in the lower part of aircraft windows
 

Something I've always wondered and never yet worked out.

Normally I sit on an aisle seat so tend to forget about it but today I managed a window seat so, be an nosey thing that I am, decided to watch the world pass by. I then noticed the small "holes" (ok, they are only shallow) drilled into the bottom of each window. I remembered that this is the best place for answers to state what their purpose is.

Many thanks

little holes in windows
 

the little hole is only in the inner of the two plastic window pressure panes at passenger positions.
the outter pane does not have a hole and takes the pressure difference of inside to outside pressure of 8 psi.
if this pane fails or blows out then the inner pane takes the pressure plus a small leak of cabin air out of the little hole.
between these plastic panes and passengers is a third pane of plastic which is there to stop the pressure panes being scratched and damaged by acid sweat and ball point pens.

I thought the small hole was to allow pressure equalisation between the cabin and the space between the passenger-facing window and the exterior-facing pressure window. I imagine it would help reduce condensation too.