|Cloud Bow – Seen by Chris Brown on June 16 between the Shetland Islands and Aberdeen, Scotland. ©Chris Brown, shown with permission.|
Cloudbows are low contrast features with at most pastel colours, sometimes with almost no colour at all. And they huge like the rainbow.
They are more easily seen with the unaided eye as they appear to drift with the aircraft across a lower cloud deck. To find one, look down opposite the sun. Sometimes a glory is at the bow’s centre.
Cloudbows form in a similar way to rainbows. Sunlight enters cloud water drops and some is internally reflected once before leaving. The refractions at entrance and departure produce the colours.
Cloudbows are broader than rainbows and they have more diffuse colours because an additional effect – diffraction – plays a role. The cloud droplets are sufficiently small compared with raindrops that light waves no longer follow the clean straight paths of geometric optics but are instead spread to produce a more diffuse bow.