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
On this image one can see colour in a contrail at both 22 AND 46 degree positions (the latter just above the electrical power pole on the wide-angle image). It’s analysed carefully recently. The angles were measured using calibration images. I cannot recall seeing other reports of this kind of observation. The aircraft was crossing the Rocky Mountains from west to east in the afternoon.
Author: Alan Clark
This phenomenon is well known from the transit of Venus in front of the disc of the sun. This effect appears when the objects are not exactly focused (see: The black drop effect is not an atmospheric phenomenon). On Oct. 13th, 2013 I photographed this effect under unexpected circumstances:
The sun had just set behind the skyline of the Palatinate Hills across the Rhine valley, when a very bright contrail due to forward scattering of sunlight raised from behind the Hills. One of the hills covered the contrail, and the brightness contrast showed the drop phenomenon very nicely: the slope of the hill appears almost vertical where it is intersected by the contrail. Due to the far distance, I used the 13x zoom of the Canon Powershot A510. As the optics of such a small camera is limited, it provided the defocusing needed to show the effect. The sequence show the raising of the contrail during a time lapse of 4 1/2 minutes just after sunset.
If you may ask now: where is the “black drop”?: The “black drop” is somewhat hidden: it is the interface area between the bright contrail and the dark silhouette of the hill, where the “drop effect” raises the skyline showing an almost vertical slope of the hill in front of the contrail. The “drop” is best seen on the second and third frames from the bottom.
Author: Christoph Gerber, Heidelberg
Another article to this topic: The black drop effect is not an atmospheric phenomenon