Ice Crystal Halos

One of the more captivating apparitions of the north is the ice halo. Ice crystals constantly form and fall through the polar atmosphere, creating colored arcs and halos that really have to be seen to be appreciated.

Some of the optical phenomena caused by ice crystals are so complex that they have never been adequately explained. However, the majority can be understood simply by realizing that ice crystallizes in hexagonal geometries, which is why snowflakes have six sides. Unlike snowflakes, ice crystals grow from vapor in subfreezing air layers as variations of thin hexagonal plates or long hexagonal needles. Light refracting through these plates and columns creates the most common displays.

To understand why, picture a ray of light falling on one of the six sides of an ice prism. It is impossible for the ray to emerge from an adjacent face, because it would be reflected back into the prism. It can, however, exit through the opposite face, or through one of the two faces which adjoin the opposite face at angles of 120 degrees. The only other way that the light ray can leave the crystal is through a basal plane of the prism. It is remarkable that simple refraction through surfaces meeting at angles of only 90 and 120 degrees can create so many complicated visual forms.

The most common type of halo is caused by light passing through a shower of very small crystals which float in space with random orientations. These form a ring around the sun (or moon) at a distance of 22 degrees from the light source. (The matter of how one gets from 90 or 120 degrees to 22 degrees is a little beyond the scope of this short article.) This halo is formed from light that has been refracted through the sides (not the base) of the tiny, pencil-shaped ice crystals. Mock suns, or "sun dogs," occur just outside the 22-degree halo when the air contains larger crystals that oscillate with a flat side down, like the fluttering of a falling leaf.

Light refracted through the crystal bases forms a halo 46 degrees from the light source. This halo is sometimes seen through wispy cirrostratus clouds, and has long been associated with coming stormy weather -- with good reason. Cirrus clouds often precede storm fronts.

Many other forms are possible. Some displays include a halo called the parhelic circle which passes through the sun and entirely around the sky at the same elevation. One of the more prominent common features is the Parry arc, named after the British explorer, Sir William Parry, who first reported seeing them while searching for the Northwest Passage in 1819. These arcs form at the top and bottom of the 22-degree halo and change shape with the sun's elevation angle. In this case, the features are caused by light refracted through columnar crystals that are falling with their long dimensions in the horizontal plane.

Sometimes conditions are such that only a particular type of ice crystal is present and only one type of apparition seen. The most common of these are the mock suns to either side of the real thing. When all the apparitions are present, however, it is a rare and spectacular sight.