|Hole in the sky?
A near perfect 22° halo over Silverdale, WA, USA. Imaged by Dale Ireland on May 27, 2011. ©Dale Ireland, shown with permission.
The sky inside the halo appears darker. It’s often called 'a hole in the sky' – But is it?
The halo has an inner edge at 22° from the sun because that is the minimum angle that light rays passing between ice crystal faces inclined 60 degrees to one another can be deflected. Rays refracted through larger angles form a sky brightness outside the inner circle that theoretically extends out to ~50° from the sun.
2) Halo light refracted by large ice crystals starts to light the sky at ~22 degrees from the sun (and not before). The measurements clearly show up the halo's red inner edge. The halo light is evident out to ~35° as an excess brightness over the grey line background. This nicely demonstrates that the 22° halo is not a ring but a disk extending outwards with decreasing brightness from 22 degrees.
3) The sky out at 50° is more blue than that within the halo. This could be due to Rayleigh scattering by air molecules which is much stronger for blue light than red and gives us the blue sky. More measurements on other sky images are needed to test the generality or otherwise of the effect.