Cloud Contrast Bow pictured by Fabiano Diniz on a flight in Argentina from Buenos Aires to Ushuaia. ©Fabiano Diniz, shown with permission.
Atmospheric
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A white arc drifts with the aircraft across the clouds. It is rendered more visible by the enhanced contrast between the cloud tops and hollows.

The arc, a ‘cloud contrast bow’, is a form of cloudbow or fogbow. They are the cloud and mist droplet counterparts of the raindrop generated rainbow. Cloud droplets are far smaller, 1/1000 – 1/10 mm across. They appreciably diffract light waves to form a broad and diffuse cloudbow rather than a sharply defined rainbow.

The contrast enhancement arises because light from upper parts of an uneven cloud deck has a greater probability of being scattered once only. Single scattering produces a bright cloudbow.  Sunlight from cloud hollows is scattered several times and those rays do not make a cloudbow. The hollows therefore appear darker at the cloudbow angles. Contrast between hollows and cloud tops is apparently enhanced.

A second cloudbow by Fabiano Diniz seen this time on a journey over Brazil from Curitiba to Rio de Janeiro.

Here we know that the cloud droplets are larger because the bow has traces of a red outer edge.

The clouds again show enhanced contrast in the region of the bow.