How is a \(22^{\circ}\) halo formed? Describe a measurement technique you might use to distinguish a genuine \(22^{\circ}\) halo from some other "halolike" phenomenon seen surrounding the Sun or the Moon.

The fascinating display of a 22° halo is one mesmerizing instance of atmospheric optics, the study of how light behaves as it travels through the Earth's atmosphere. Understanding how light interacts with various atmospheric particles provides insight into the formation of stunning visual phenomena, such as halos, rainbows, glories, and auroras.

When discussing 22° halos specifically, we're looking at light that crosses paths with ice crystals suspended in the atmosphere. Different atmospheric conditions produce different phenomena. For example, when light passes through raindrops, it creates a rainbow, but when it interacts with ice crystals, it often results in a halo. The crystal's shape significantly affects the kind of pattern it casts in the sky, which in the case of the 22° halo, leads to a ring exactly 22 degrees from the light source.

The halo itself is actually a circle that forms with a radius that seems to encircle the Sun or Moon. Moreover, the size of the halo—a constant 22° radius regardless of where it's viewed from on Earth—implies a unifying principle in atmospheric optics: the angle at which the light is refracted is the same regardless of the observer's location, creating a consistent visual experience worldwide.

Light refraction is a key concept in understanding the formation of 22° halos. It refers to the bending of light as it passes from one medium to another—such as from the near vacuum of space into the Earth's atmosphere filled with air and various particles. This bending occurs due to a change in the speed of light in different media, based on the principle known as Snell's Law.

In the case of a 22° halo, the medium change happens when light enters hexagonal ice crystals in cirrus clouds. The light bends, or refracts, as it enters the crystal, and then once again as it exits. The special significance of the 22° halos is that this angle is roughly the minimum deviation angle for light passing through a hexagonal ice crystal in a specific orientation.

Understanding the Refraction Process

As sunlight or moonlight enters the upper atmosphere, it encounters these ice crystals, which can be thought of as tiny prisms. The hexagonal symmetry of the ice crystals dictates the specific pathway the light follows, leading it to deviate from its original path by about 22° before it reaches an observer's eyes. This process creates a circle of light at a constant radius around the Sun or Moon, which is recognized from the ground as a halo.

Hexagonal ice crystals are central to creating the spectral ring around the Sun or Moon. These crystals typically form in high-altitude clouds, such as cirrus or cirrostratus clouds, where temperatures are below freezing. The signature hexagonal shape comes from the way water molecules bond as they freeze.

The orientation of these ice crystals is particularly significant for halo formation. Preferably, the crystals are randomly oriented as they float through the air, which allows sunlight or moonlight to enter through one hexagonal face and exit through another, bending at the precise angle required to create a 22° halo.

Ice Crystals as Natural Prisms

The perfection of a halo depends largely on the symmetry and consistency of the ice crystals. When ice crystals are well-formed and have a consistent hexagonal shape, the light refraction is uniform, resulting in a clear and distinct halo. Variations in crystal quality, shape, or orientation can lead to blurred or incomplete halos, or even different phenomena like sundogs or circumzenithal arcs.

If the ice crystals are misshapen or aligned in multiple orientations, however, the light can scatter in various directions, and the halo may become fragmented or exhibit other anomalies. Hence, the symmetry of the hexagonal ice crystals plays a crucial role not only in the formation of 22° halos but also in their appearance and visibility to the observer.