Airplane Window with Circular Ice Pattern around a Breathing Hole

October 15, 2019


Photographer: Mario Freitas
Summary Author: Mario Freitas

During a flight over Southeast Brazil, the ice formation shown on the cabin window in the above picture reminded me of a former EPOD concerning the same phenomenon. What made me curious in this case was the nearly circular symmetry of the geometrical pattern. Ice crystals and water droplets frequently are observed to grow on airplane windows, which normally consists of three parallel acrylic panes. In the picture, my pattern is centered on a small hole in the middle pane. This hole is required by aircraft engineering.

According to flight data at the time the photo was taken, cabin air was at a comfortable temperature of +70 F (21 C), whereas outside it was -72 F (-58 C), at an altitude of 38,000 ft (11,582 m). Furthermore, atmospheric pressure in such conditions reaches nearly 3.0 psi. Breathing a low rate of oxygen, as happens to mountain climbers, can provoke relevant lesions to occur in the brain. So, pressurization systems are designed to keep the cabin pressure between 11.0 and 12.0 psi, not so different from 14.7 psi that we typically experience at sea level.

This circular symmetry pattern is likely formed on the inner face of the outer pane, where very low temperature combined with normal air pressure enables moistened air to undergo the thermodynamical processes of condensation and crystallization. The circular gap close to the pattern center, where no ice formation has occurred, is due to the small airflow through the breath hole. One of the purposes of this hole is to equilibrate air pressure between the cabin and the air between the panes during flight so that any mechanical stresses are mostly applied on the outer pane. Additionally, the hole prevents the buildup of moisture between the window panels, controlling fog or frost that would otherwise form on the window’s surface.

Thanks to this design, passengers with window seats can observe natural wonders, including geological formations, clouds, atmospheric optical phenomena, etc., from high above the Earth’s surface.