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.


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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.

No ice crystals within 22° of the sun can glint their halo light towards the observer's eye and the sky there is said to be darker.

However,  Dale Ireland measured his image and pointed out that the sky inside the halo is actually brighter than outside – the hole is an illusion.

At right are more red, green and blue brightness measurements taken along a radius from the sun to the halo and beyond. The sky brightness falls off sharply from the sun (overexposed in the image) and there is then a sharp bump where the halo starts at ~22°. The brightness then decreases again.

The sky's brightness has several components:

1) Sunlight is scattered by air molecules, aerosol and dust particles and possibly some very small ice crystals in the cirrus haze. The scattered light decreases smoothly with distance from the sun and is represented in the graph at right by the grey line.

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.

Finally, the background image is a deliberately course HaloSim ray tracing on a black background to show the pure halo contribution to the sky brightness and the dots representing individual glints. That one really does have a hole.

Inside real halos the sky looks darker but Dale has shown that it can actually be brighter there than outside the halo.   Maybe we should rename it the halo hole illusion!