Atmospheric
Optics
Parhelic Circle - 3 perspectives
New York City photographer Josh Maready watched this parhelic circle while on location at Flagstaff, Arizona.
At top
is a stitched 360° landscape panorama. Here we see the parhelic circle as often described. White. Stretching all around the sky. Everywhere at the same height above the horizon, that of the sun. Circumhorizontal.
Another fish-eye view, this time centred on the nadir. A 'Little Planet' view.
Here the parhelic circle
is seen as also circumnadiral.
The parhelic circle (PC) is formed by sunlight refracted and reflected by ice crystals in high, cold cirrus cloud.
The crystals must be large enough (greater than about 0.05mm across) to be aerodynamically oriented. Hexagonal plate crystals form the PC, so do horizontal hexagonal columns and also Parry oriented crystals.
The PC can be formed
by a great many different ray paths. Only a few are at right.
'A' is the simplest, an external reflection. It gives a rather weak PC stronger near the sun.
'B' produces the strongest PC. Light internally reflects once. It splits into its component colours at the entrance and exit refractions but the colours overlap completely along the circle to produce white (or the colour of the sunlight). The rays are deflected in azimuth but not in altitude producing a circle at the same height as the sun - or distance from the zenith.. ..or nadir.
'C' - There can be three or more internal reflections. They tend to brighten the PC far from the sun.
This fish-eye view has the zenith at centre.
The parhelic circle is seen as circumzenithal.
This aspect is strongest when the sun is high and the circle shrinks in size.
All images ©Josh Maready, shown with permission
