|Cloud Contrast Bow seen by Mario Freitas (site) of Universidade Tecnológica Federal do Paraná
Curitiba, Brazil. ©Mario Freitas, shown with permission.
Curving across the clouds below the aircraft is a barely perceivable feature. There are patches (arrowed) where the cloud detail is strangely more sharply defined than elsewhere. This is a rarely imaged "cloud contrast bow".
The bow is a subtle form of cloudbow or fogbow - look for it as an arc of ~40° radius centered on the antisolar point. Contrast bows are hard to spot and are most easily seen by eye as the aircraft movement drifts cloud features across the zones of higher contrast.
Cloudbows/fogbows arise from single scattering by small cloud droplets. Here single scattering in the sunlit outer and uppermost skin of the cloud is forming a broad and essentially colourless bow and at the bow angles there is therefore a very slight brightening of the cloud surface. Droplets deeper in the cloud also scatter sunlight but rays interact with several droplets and are scattered at all angles leading externally to no brightening at any particular angle occurs.
Why is contrast increased at the fogbow positions? When the upper cloud envelope is structured, parts are in partial shadow. These parts are not brightened in the bow positions because they are in less direct sunlight and their light has undergone more multiple scattering thus losing the bow directionality. The net effect is that the brightness difference between the top surface and the hollows is increased making the cloud detail more easily visible.