Contrails are a result of water vapour, produced as a product of combustion, being ejected from the aircraft engines (→ article)
When a contrail forms near the sun, it’s possible to see a rather beautiful ‘rainbow effect’, as in this example. Such iridescent clouds are a diffraction phenomenon caused by small water droplets or small ice crystals individually scattering light. The aerodynamic contrail formed by the reduction of pressure in the air as it moves over the wing. When the pressure of a gas falls, then its temperature also falls (the same principle as is used by your refrigerator). The reduced temperature causes small drops of water to condense, which then may freeze. The (frozen) drops get larger as more water condenses on them. The iridescent colours are sunlight diffracted by millions of water droplets condensed by the airflow over the wings. The droplets all have similar life histories and therefore similar sizes, ideal conditions for iridescence.
The photograph was taken by Ron Smith at around 1300 local on 18 July 2015 at Henstridge, Somerset, UK. The aircraft was flying from East to West and, when first seen, was only producing an intermittent contrail. The iridescent contrail appeared as the aircraft approached a cloud layer just below its flight altitude.
One of nature’s works of art!
Authors: Ron Smith, Somerset, UK and Claudia Hinz, Germany