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.
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