Atmospheric Optical Phenomena

The passage of sunlight through cirrus clouds can produce any one of the optical phenomena known as haloes, sundogs, or sun pillars. One explanation for phenomena of this kind is that cirrus clouds consist of tiny ice crystals that refract light through very specific angles, namely 22° and 46°. When sunlight shines through a cirrus cloud, each tiny ice crystal acts like a glass prism, refracting light at an angle of 22° (more commonly) or 46° (less commonly).

A halo is one example of this phenomenon. Sunlight shining through a cirrus cloud is refracted in such a way that a circle of light—the halo—forms around the sun. The halo may occur at 22° or 46°.

Sundogs are formed by a similar process, and occur during sunrise or sunset. When relatively large (about 30 microns) crystals of ice orient themselves horizontally in a cirrus cloud, the refraction pattern they form is not a circle (a halo), but a reflected image of the sun. This reflected image is located at a distance of 22° from the actual sun, often at or just above the horizon. Sundogs are also known as mock suns or parhelia.

Sun pillars are, as their name suggests, narrow columns of light that seem to grow out of the top or (less commonly) from the bottom of the sun. This phenomena is a result not of refraction, but of reflection. Sunlight reflects off the bottom of flat ice crystals as they settle slowly toward Earth's surface. The exact shape and orientation of the sun pillar depends on the position of the sun above the horizon and the exact orientation of the ice crystals to the ground.