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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


Colours in a contrail

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

Aircraft Iridescence

While taking wildlife pictures in a nature reserve close to the Vienna, Austria, airport, Franz Kerschbaum noticed this 747 jet aircraft in a landing approach with a huge condensation cloud behind its wings. As it moved closer to the runway, he was evidently at just the right position for this shocking but beautiful artificial iridescent cloud to come into view. Iridescence and coronas are diffraction phenomena. The pastel or metallic colors result from deflection of sunlight about minute water droplets.

Photo details: Canon EOS 30D camera, 100-400 mm/4.5-5.6L lens at 400 mm and f/8.0

Shock wave refraction and iridescence over airplane wing

The observation of Monika with the beautiful colored contrail pictures made me remember to post the following photos. On a flight in a 747-400 from Beijing to San Francisco on August 18th I was seated right over the wing on port side of the aircraft. Initially I was disappointed with this window seat but I did get to see some interesting things.

The first of these was a condensation cloud over the wing. The airplane was still ascending around 20,000 ft, when we flew through (super)saturated air. The faster airflow over the top surface of the wing and associated drop in pressure caused condensation. The cloud showed beautiful iridescent colors. The water droplets in the cloud were so small at this stage and all so uniform in size that they diffracted the light with constructive interference over large angles.

On the second photo this condensation cloud extends behind the wing. This is because it takes energy for a water droplet to form in supersaturated air, but once the droplet forms it quickly grows (or it quickly evaporates if the air is not supersaturated, as in the first photo). In other words, when the air is supersaturated, all that is needed is a trigger to initiate droplet formation, after which growth occurs because the air is supersaturated.

This droplet growth is responsible for the colored trail between the exhaust contrails that can sometimes be seen, as in the photos posted earlier. But most often, the air is either not supersaturated, or already condensed into a cloud. Then, no long trail forms (only the engine contrails perhaps), or it is not visible (within cloud). Therefore it is not common to see this effect, especially from ground.

At times a huge and bright corona formed around the sun. In this photo the airplane was banking right and the sun appeared low over the wing. My apologies for the low quality photo – it is very hard to photograph through an airplane window with the sun in view.

The other interesting thing, which I had observed before, but only now could photograph well, is the vertical standing shock wave that sometimes can be seen dancing above the wing of a commercial jet. Here, because of the condensation cloud, this shock wave actually became visible. Usually it can only be seen due to the feeble refraction and miraging of structures on the wing (you won””t notice this unless you look very carefully and at the right moment when the shock is positioned so that you are in its plane).

The normal shock occurs because air flowing over the wing has to flow faster than below it, and this flow can briefly reach the speed of sound even if the jet airplane is flying slower than the speed of sound. Because the Mach number is about 1, the shock wave is almost perfectly vertically oriented, normal to the airflow.

The shock plane in the photo can be seen as the vertical plane of enhanced condensation. I believe this is the rarefaction shock (as opposed to the compression shock) because condensation is enhanced behind the plane, indicating lower air pressure. Presumably the compression shock was somewhere ahead of this, invisible to me since I was not positioned in its plane.

The sharp density gradient at the shock wave causes a lateral mirage, but any miraging can only be seen if you are looking near grazing incidence along the shock plane (which is usually not perfectly planar but a little curved). The last two photos better show the miraging along the shock plane. Note the effect on the pylon of engine nr. 1.

Author: Harald Edens

Contrail Colours

These two photos were taken by “Controll” and Mónika Landy-Gyebnár (Noli) in Hungary. The pictures show wonderful colour stripes in the contrails. The most fascinating thing is that the stripes have the airplane as their centre instead of the Sun, and as the plane was flying, the colours did not change depending on the angle to or the distance from the Sun

The explanation of this phenomenon may lie in the varying consistency of the exhaust. The water vapour condenses, freezes, and the ice crystals become larger and larger. During this process, the refractive characteristics of the particles constantly change, and if the circumstances are ideal and the trail is not too thick, we might see such beautiful colour patterns. For the explanation, we owe thanks to “Uncinus”.

There are more photos available in this topic. See Controlls collection of colourful contrails and another picture taken by Mónika Gyebnár-Landy.

Posted by Ágnes Kiricsi

Condensation Trail Iridescence

Jurgen de Boer (site) imaged this aircraft and contrail near to sunset on 2nd May. The aircraft was 10-20° from the sun. The structured iridescence is interesting. Seven or more colour bands are visible. The water droplets or ice crystals of the condensation trail will have had similar formation histories. Perhaps this produced rather uniform sizes and mean size variation along the trail thus accounting for the uniform colour bands?

Posted by Les Cowley