Physics at the Airplane Window
Above the clouds, the temperatures and pressures outside the aircraft are dramatically different from those in the cabin. Impressive optical and thermodynamic phenomena occur on the disks that separate the two kingdoms.
Aircraft designers would probably prefer not to use windows at all, as these are weak points in the fuselage. This is due to the difference in pressure: At today's usual cruising altitude of around ten kilometers, the air pressure is only around a quarter of the normal value on the earth's surface of around 1000 hectopascals (hPa). That would be life-threatening for the passengers. Most of us are used to lower regions and can at best endure pressure such as that prevailing on the Zugspitze for a long time. Accordingly, about three quarters (750 hPa) of normal atmospheric pressure is maintained in the cabin. Because of the higher pressure inside, the hull is subject to great loads, similar to an inflated balloon. Corners on windows would distribute the forces unevenly and would be the starting points for cracks. That's why the familiar, rounded shape is less a matter of design and more of sheer physical necessity…
