Iridescent Plumes

Mario Stoltz looked through his Hamburg office window at mist plumes from the evaporative coolers of nearby industrial air conditioners.   It was early morning and he was looking almost into the low sun. Surprisingly, the plumes were coloured.

Images ©Mario Stoltz, shown with permission

Freshly condensed mist droplets diffract sunlight to form the iridescent colours.   
Sunlight scatters mainly from the drop surface and the scattered waves interact to form a diffraction pattern.  

The colours showed on several mornings.

Ambient air temperature and humidity matter.

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The cleanest and most colourful pattern forms when all the drops have the same size.   A mixture of sizes gives an overlapping mixture of diffraction patterns that tends to a whitish blur.

Freshly formed drops have similar sizes.  As the mist ages, collisions and vapour exchange between droplets increases their average size and widens the size distribution.
Colours show high up in the sky
One of the earliest sightings.