Atmospheric
Optics
Sundog & Parhelic Circle
~ Imaged from a bus in Newcastle, England by Su Stewart. At right a colourful sundog or parhelion. A white parhelic circle shining in fibrous cirrus crosses the picture. Not quite white! - Its upper and lower red fringe is real, see below.
Image ©Su Stewart, shown with permission
Entry/exit point
Internal reflection
Parhelic Circle - Same height as sun. Circles the sky when complete. Plate crystals, horizontal columns and Parry crystals make it. Made by more ray routes than any other ice halo. Those above are a sample.
The common ray path feature is their symmetry. The entry and exit angles are equal and opposite and so there is no net dispersion into colours. The halo is white. But sometimes, as here, it is fringed with colour. Rays from the crystals are diffracted by lower level water drops or very small ice particles. The colour fringe is effectively a linear corona or aureole.
Sundog (parhelion) - Always at the same height as the sun and 22° or more from it.
Hexagonal plate crystals in cirrus create it. Sun rays effectively cross a 60° prism and are dispersed into colours.
