Airframe Induced Iridescence

Lillian Kwok captured this aircraft low over North London.   It sails through sunlit moist air its wings, engines and tail plane casting long shadows.   Its wings rather than any engine exhaust leave intense iridescent trails.   Airflow over the airframe causes water vapour to condense into tiny droplets and then sunlight and diffraction do their colourful work.

All images ©Lillian Kwok, shown with permission

Droplets condense

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Air travels faster over convex surfaces. It expands and cools.
Air passing over the top of wings or convex fuselage sections travels faster, expands and adiabatically cools. If the air is sufficiently humid, as here, water vapour then condenses out into a cloud of small droplets. We see the same when pileus cloud forms atop thunder heads or banner clouds wave from mountain peaks..

The iridescent colours are produced by individual water droplets diffracting sunlight. When the droplets are locally of similar size they all diffract their coloured light into the same direction and so colours become evident to us.