Why Is There a Tiny Hole in Airplane Windows?

Why do airplane windows have a tiny hole? A passenger window is really three panes. The inner one is a non-structural scratch pane. The tiny "breather" or "bleed" hole sits in the middle pane and lets cabin air into the gap, so the outer pane carries the cabin pressure while the middle pane stays in reserve as a fail-safe backup. The hole also lets moisture out of the gap, which helps keep the window from fogging or frosting up. So it's not a flaw: it's a backup plan, and a defogger.

Sources:
– GKN Aerospace (Marlowe Moncur) on window design, via Gizmodo:
– Pilot Mark Vanhoenacker's explanation, via TIME:
– Boeing engineer Bret Jensen on moisture and fog, via Migflug: