Atmospheric
Optics
Infralateral & Circumhorizon Arcs
Pictured by Alfred Lee between Kunming and Hong Kong, China.
It's hard to tell whether a single bright spectral arc some two 22° halo radii below a high sun is a circumhorizon (CHA) or an infralateral arc. Here both arcs are bright and clear. They overlap directly beneath the sun and then the infralateral curves upwards and away from the CHA.
The eye is better at identification than a camera. The CHA is everywhere at the same altitude above the horizon. The infralateral curves upwards. Camera lens distortions and pointing directions conspire to alter the arcs' shapes in images examined after the event. See how the circumhorizon arc appears to curve upwards in this image. First use the eye - then the lens!
Vertical bands of colour on the clouds? Not so.
All images ©Alfred Lee, shown with permission
The inner halo can be a 22° halo from randomly oriented prisms or a circumscribed halo from column crystals. Both halos may be there.
When the sun is high the circumscribed halo is almost circular and hard to distinguish by shape alone from the 22°. But it is sharper and has more saturated colours.
Look carefully at appearance of the inner halo and whether two are present.
Horizontal plates generate the CHA. Rays enter a near vertical prism side face and leave through the lower large hexagonal face.
Both arcs are generated by refraction at perpendicular ice crystal facets.
Infralateral rays enter a near vertical end face of a column crystal and leave through a prism side face.
