Iridescence over Boulder, Colorado Flatirons

A time-lapse video by Will Schaleben

“Imaginary cloud creatures prance across the sky above the Flatirons of Boulder, Colorado.

High winds, probably over 100 mph, drive baby clouds along as if wild horses. Beautiful iridescent clouds form at the leading edge of a standing wave lenticular formation.

Shot January 18, 2012 with HTC EVO View smartphone using Time lapse app at one frame every 4 seconds at 15 frames per second play speed."

©2012 Will Schaleben, shown with permission

Play this several times. Watch the clouds’ wave motions. Watch iridescence flash in the new fragments then fade leaving them white light before they fade and evaporate. Watch the cloud bank at upper left with banded iridescence where it is thin but thickening at its boundary.

Wave motion induced by passage over high ground forces moist air upwards and downwards. As an air pocket climbs it expands and cools. If it is moist enough some water vapour condenses into small droplets which diffract sunlight to give the complex colours. Age takes its toll, the droplet sizes spread through collisions, more condensation and evaporation – their collective diffraction effects blur and the colours fade. Then the pocket is forced downwards where it warms and the droplets are gone. All in a second or so before the pocket is swept upwards yet again...

The colours are produced by individual droplets. Light is scattered mainly at their surface to form outgoing spherical waves. The waves overlap and in some directions - and for some wavelengths - their wave crests are in phase and give bright colour.

Iridescent Cloud Beings Over Flatirons of Boulder Colorado .

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