Atmospheric
Optics
Antarctica Lunar Halo & Snow
~ LI Hang imaged this scene on 30th July at Zhongshan Station (Chinese Permanent Station in Antarctica), Larsemann Hills. “I was kind of upset when the rarely seen "blue moon" hid behind the clouds and it started to snow. Some minutes later I walked out the door and was astonished to see such a fantastic Moon Halo in the snow above my head.”
Images ©LI Hang, shown with permission
22° halo rays only
A flash lit the snowflakes in the top image. Here is the scene sans flash.
Far out from the sun/moon the 46° halo shines.
Rays parallel to face 'A' form the halo's inner edge.
Ones like that shown here light the halo's outer regions.
All rays. The light inside the 22° halo is external reflections from crystal faces.
The snowflakes were not responsible for the 22° halo – they are too complicated and optically imperfect. The halo comes from simpler objects, probably tumbling clusters of hexagonal prisms larger than 50 micron. Rays cross two prism side faces inclined 60°.
The halo is often wrongly described as a ‘22° ring’. This image clearly shows halo light well outside the inner 22° radius edge. In theory it stretches out to 50°. The bright inner edge is from rays refracted through a minimum deviation angle but other rays deflect more as this animation shows.
At right a HaloSim ray tracing shows individual coloured rays. The red inner edge shows up. Beyond the inner rim are all colours which our eyes blend together as white.
The rays inside the halo are from direct external reflections off crystal faces. At far right a filtered simulation shows only rays passing through two side faces. That is the ‘pure’ 22° halo.
