|Lhasa Halos ~ Images by Xiaoshan Huang. A bright circumscribed halo, 22° halo and parhelic circle hang high like a huge eye in the Tibetan sky. From the horizon a colourful infralateral arc curves upwards and leftwards over the Potala Palace. All images ©Xiaoshan Huang, shown with permission|
|A mix of randomly oriented crystals (22° halo) and horizontal column crystals were probably entirely responsible for the display.
Plate crystals contribute to parhelic circles but there is little evidence of a sundog that plates would also have generated.
The circumscribed halo is made by rays passing through horizontal column side faces inclined 60° to each other.
The more widely dispersed colours of the infralateral arcs are from rays passing between a near vertical end face and a side face. In effect, passage through a 90° prism.
|The circumscribed halo/tangent arcs change shape dramatically with sun elevation.|