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Contrail Iridescence ~ Seen from New Jersey, USA by Steve Mattan (recycled photons).     "I was sitting on the deck, binoculars in hand, doing a bit of birding when I looked up at an airplane. The contrail was glowing with color. I quickly ran in to get my camera but I got back too late. The color was gone. I waited as a few more planes flew by, the contrails, if any, boringly white. Then this one. Got it. Taken in the evening, in the light of the setting sun."

All images ©Steve Mattan, shown with permission
Air forced to travel faster over convex surfaces expands and cools

Airflow over the wings and fuselage generates the broad contrail.   The engine exhaust trails are narrower and thicker.

Air is cooled by expansion as it flows over the airframe.   Take a wing section at right, the flow over the wing is faster than elsewhere. The fast flowing air is at a lower pressure and expands (adiabatically) cooling as it does so.  Cool air holds less water as vapour and condensation can occur into tiny droplets that then grow in size.  The result – a condensation trail.

Newly formed droplets can iridesce intensely because in any one part of the trail the particles have similar histories and sizes.   They diffract light with little overlap or mixing of colour.