Aircraft Iridescence
Bernardo Malfitano pictured this F-22 silhouetted at the top of its loop over Miramar, California.
©Bernardo Malfitano, shown with permission.

The iridescent colours are sunlight diffracted by millions of very small water droplets condensed by the airflow over the wings and fuselage. The droplets all have similar life histories and therefore similar sizes, ideal conditions for iridescence.

Why does the condensation occur?   

The air flow over parts of the airframe (the top of aircraft wings in ordinary flight) is faster than elsewhere. The fast flowing air is at a lower pressure and expands. In doing so it cools. If the humidity is high enough and the air temperature also fairly high (warm air can hold more water as vapour than cold air) then conditions might be reached for heterogeneous or even homogeneous nucleation followed by rapid droplet growth. In some cases the air might already be supersaturated before the aircraft passes. The result? We see trails of condensed droplets and sometimes - as here - they diffract light and iridesce spectacularly.


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