Iridescent Aircraft Trail

Monika Landy-Gyebnar spotted this iridescent contrail over Hungary.

"I had to go buy some food for the weekend and had my camera with me as usual. [Looking up] I noticed a nice contrail shadow. I switched my camera lens (55-300 Nikkor) to 300mm and pointed it upwards. But before it could find the shadow a very vivid coloured contrail from another airplane swam into the field of view! I almost fell backwards, it was so stunning!

The vivid colours were visible until the contrail itself began to weaken as the plane reached a less humid air mass. The sequence of images [below] shows how the trail changed and developed gaps as the humidity decreased. Very soon after the last pic the contrail formation stopped and the plane flew without a trail.

You can also see some faint shadow cast by the plane across the humid air.  

Back home I checked for the details of the plane. It was a Boeing 747 cargo plane of Singapore Airlines flying east from Dallas and Brussels at 10,058m [33,000ft] high and 883 km/h [550mph]."
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Aircraft contrails have two sources. The major one is water droplets condensed from water vapour generated in the engines. Combustion of aviation fuel gives water vapour, carbon dioxide and traces of NOx and soot. Opaque engine trails are prominent in Monika’s images. The second generator is water vapour already in the air condensed to droplets by airflow over the fuselage and wings. Air passing over the top of wings or convex fuselage sections travels faster, expands and cools. Sometimes, if the air is sufficiently humid, water vapour then condenses out into a fine droplet mist. These airflow droplets appear to be the source of the iridescence in Monika’s trail.

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. Similar sized droplets usually come from a similar life history, the passage of an aircraft, airflow through a lenticular cloud, a banner cloud or freshly formed fog.