The refraction of light through ice crystals in the air causes an atmospheric phenomenon called a sun halo. /SANDRA JAMES

By Sandra James

It was a lovely sunny day in McBride when I looked to the sky and noticed a large sun halo.

Sundogs, halos, and light pillars are atmospheric phenomena caused when the light of the sun or moon is reflected or refracted by ice crystals. The size and shape of the light crystals along with their distance from the earth’s surface determines which optical phenomenon people will observe. The angle of the observer also plays a role in the viewing.

Randomly oriented hexagonal ice crystals in the sky are responsible for a sun halo. Light passes through an ice crystal and bends, once a second bend is made the light appears as a halo in the sky.

There are a few requirements for sun halo/dogs to be visible:

  1. Ice crystals become suspended in cirrus clouds.
  2. They move vertically down through the air.
  3. Some of those crystals are flat hexagonal plates, and tend to orient with their flat sides horizontal.
  4. These crystals begin to act as prisms that refract light.
  5. Sun rays are refracted by all these crystals, thereby forming a sun dog or halo.

Cirrus clouds are short, detached clouds found at high altitudes. They look like long whips of hair. In the atmosphere, under certain conditions, water droplets and ice crystals can act as a prism allowing us to see the various colours which make up visible light. Because of these properties we get various atmospheric optical effects.

The halo is a ring of light which forms around the sun or moon and is when the light refracts off ice crystals present in a thin veil of cirrus clouds. Usually the halo is a bright white ring but has been known to have colour. Sundogs are coloured spots of light that develop from refraction of light through the ice crystals. They are located 22 degrees from the sun either left, right, or both, depending on where the ice crystals are present. A sundog is also known as a mock sun.

Light pillars develop as ice crystals slowly fall through the air reflecting the sun and creating a pillar. These usually occur at sunrise or sunset.