The circumzenithal arc (CZA) is a vividly coloured arc centred on the zenith and extending for about 90°. Appearing like an ‘upside down rainbow’, this arc is caused by horizontal plate crystals acting as 90° prisms (light passes through a horizontal hexagonal face and one of the vertical side faces).
This coloured arc is caused by horizontally oriented columnar crystals acting as 90° prisms (light passes through a vertical end hexagonal face and one of the long side faces of the column).
Upper Tangent Arc
The gull-wing shaped arc above the 22° halo is formed by refraction through side faces inclined at 60° to each other of horizontally oriented hexagonal column crystals.
A solar halo is a bright circle centred on the sun and is formed by refraction. The 22° halo is the most common and occurs when hexagonal crystals with a range of different orientations act as tiny 60° prisms (light enters through one side of the hexagonal cross section and exits through a next-to-adjacent side).
Also known as a ‘mock sun’ or ‘sundog’, a parhelion is a bright, coloured spot seen on either side of and at the same height as the sun. Parhelia are most commonly seen 22° away from the sun and are formed by refraction through the vertical side faces (inclined at 60° to each other) of horizontal plate crystals.
Caused by reflection off vertical faces of ice crystals (both externally and internally), this faint white circle forms parallel to the horizon at the same height as the sun and can extend for as much as 360°.
This is a sundog appearing 120° away from the sun and occurs when light passing through horizontal plate crystals is reflected internally multiple times.
Warning: when looking for solar halos, never look directly at the sun.
(Featured in Atmospheric Optics Picture of the Day)