Cap Cloud iridescence ~ Florent Pin made this magnificent image over the Alps while flying from Paris to Milan. Two iridescent cap clouds shroud an Alpine peak.
All images ©Florent Pin, shown with permission
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Cap clouds are a variety of orographic cloud.   They hover almost stationary over mountain peaks but inside them is all transience and change.

Cap clouds form when wind passing over the mountain forces layers of moist air upwards.   Air made to rise has its pressure reduced.   The air expands and in doing so it adiabatically cools.   If its water vapour content is high enough, there is sudden condensation into myriad small droplets – a cap cloud.

The apparently quiescent cloud actually had wind howling through it. Droplets condense on the upwind side. They re-evaporate on the other side as the air layer starts to descend, compress and warm.

The sudden condensation favours iridescence because the droplets all have similar histories and therefore sizes.   The tiny droplets diffract sunlight to form the characteristic pastel hues.

Thanks to Ian Loxley of the Cloud Appreciation Society for help with cloud identification.