“Rainbow”-contrail

Iridescence contrail

Iridescence contrail. Photo: Ron Smith

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

Posted on July 28, 2015, in coronae and iridescence and tagged , , . Bookmark the permalink. 1 Comment.

  1. A contrail is a white line that you see in a clear blue sky (without clouds) during the day from a flying jet airliner, which is between 10,000 feet and 20,000 feet up in the sky. From that distance in the sky, you could only hear that flying jet airliner for only a little bit, (e.g., like a very low softer noise), or it doesn’t make any noise at all. A contrail from a flying jet airliner in a clear blue sky looks like as if someone is drawing a white perfectly straight line in the sky as seen from ground level.
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