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Aircraft Condensation Iridescence. Roberto Figueredo Simonetti imaged this Iberia A340 over Montevideo (Uruguay) on its approach to Carrasco International Airport. ©Roberto Figueredo Simonetti, shown with permission.

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.

Why does condensation occur? Once the pressure of water vapour in the air exceeds the saturation vapour pressure it is no longer stable. Provided there are dust particles present to act as nuclei, the excess vapour over the saturation pressure condenses into droplets - heterogeneous nucleation. Without any nuclei, condensation is more difficult. Instead the air becomes supersaturated. However, when the supersaturation exceeds a certain critical value water condenses out very rapidly as a mist of fine droplets even though nuclei are absent. This is homogeneous nucleation.

The flow over the top of aircraft wings is faster than elsewhere. The fast flowing air is at a lower pressure and expands. In doing so it cools. If the humidity is high enough and the air temperature also fairly high (warm air can hold more water as vapour than cold air) then conditions might be reached for heterogeneous or even homogeneous nucleation followed by rapid droplet growth. In some cases the air might already be supersaturated before the aircraft passes. The result? We see trails of condensed droplets from the wings and sometimes these iridesce spectacularly.

In the above case the droplets evaporated quickly. Sometimes they persist and, with the water generated by combustion in the engines, form a contrail.

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