The atmosphere can display phenomenal optical features through the bending of light. You may have looked up at the sky on a day with high-level clouds, e.g. cirrus clouds, which are composed of ice crystals. These hexagonal ice crystals act as prisms, and when a photon (a particle of light) passes through the ice crystal, the crystal can scatter the light rays such that it reveals the colors of visible light. The colors of visible light are: red, orange, yellow, green, blue, indigo, and violet; otherwise known as ROY-G-BIV.
The processes of reflection and refraction play a crucial role in the scattering of light particles. In the atmosphere, refraction occurs when a beam of light passes through an ice crystal and/or raindrop, and thus breaks down the particle into the visible colors. This is what causes the rainbow effects we commonly observe after thunderstorm events.
Reflection occurs when a ray of light bounces off a surface, such as sunlight reflecting off of the side mirrors on a car. During the process of reflection, the photons are not broken down and thus do not reveal the colors of visible light. However, reflection does play an important role in atmospheric optics.
Halos
A halo is a ring of light that forms around the sun and/or moon when a layer of cirrus clouds is present. Cirrus clouds are composed of tiny hexagonal ice crystals. These hexagonal crystals, as mentioned above, act as a prism that scatters the light particles into the visible spectrum. Halos can have a white appearance depending on the depth of the ice clouds, and sometimes halos can reveal the full colorful array of visible light.
Halos can also occur at an angle of 22°, otherwise known as the 22° halo. It is also the most common type of halo that occurs in the sky. With some complex mathematics, the effective angle of refraction is calculated to be 60° as light enters the ice crystal through one face and exits through another face of the crystal. The angles of light passage involved with 22° halos can erode the magnitude of the array of colors refracted by the crystal. This often gives the white-like appearance of halo rings, as seen in the photo above.
The composition of the ice crystals is important for the magnitude of colors that are displayed in the sky. The concentration of crystals is also important, but studies have shown that ice crystals that have random orientations (meaning the hexagonal structures are not properly aligned with one another) can often result in a full halo.
Parhelia (sun dogs)
One of the more fascinating, although a commonly observed atmospheric optical phenomena, is the sun dog (otherwise known as the parhelion in singular form). Sun dogs occur at an angular minimum of 22°, and these angles increase or decrease accordingly depending if the sun is setting or rising on the horizon, or when the sun is at its peak solar zenith angle (SZA) during midday.
The formation of sun dogs requires hexagonal ice crystals to be vertically aligned, which is optimal for the refracting of light. When the ice crystals are vertically aligned and the light passes through them, the longer wavelength particles (such as deep red colors) are seen on the side of the parhelion nearest the sun. The higher the SZA, the larger the displacement of the parhelia.
Sun Pillars
Sun pillars are a form of a halo that unveils the illusion that the pillar of light is positioned vertically above or below the sun. However, this is an optical illusion given the orientation of the ice field aloft due to the process of reflection. Sun pillars occur when the ice crystals have their major axis aligned horizontally.
The vertical structure of the pillar requires the azimuthal position of the sun to be at a critical angle along the horizon with respect to the cloud deck, as the light particles from the sun reflect off of the ice crystals. Typically, when a deck of cirrostratus clouds is present, sun pillars can most often be seen during sunrise and sunset.
Another form of the pillar phenomenon is known as light pillars. Similar to sun pillars, light pillars can form from artificial light sources from cities. These light pillars can occur at night when ice crystals are present in the atmosphere.
The light emitted from cities passes through these horizontally oriented, hexagonal ice crystals and reflects off their surface. As a result, a similar effect to that of the sun pillar can occur at night near cities and/or any area that has a light source under the appropriate atmospheric conditions.
Other atmospheric optical phenomena
More forms of atmospheric optical phenomena can be seen on certain occasions. Other forms of rainbow-like displays in the sky include circumhorizontal and circumzenithal arcs, infralateral and supralateral arcs, upper and lower tangent arcs, and parry arcs. These optical phenomena are indeed rare, and require atmospheric conditions that can be difficult to achieve with the solar azimuth.
However, in January, 2015, a very interesting optical effect occurred in the sky in Red River, New Mexico. Nine atmospheric optical phenomena were captured in a single image. The National Weather Service field office in La Crosse, Wisconsin, home to a few atmospheric optic experts, compiled an informative graphic that labeled each optical effect.
Ice clouds play an important role in the formation of halo and other optical effects that occur in the sky. When should you look for these optical phenomena? Typically, cirrus clouds form ahead of an approaching warm front. These cloud optics tend to become more prevalent in the colder seasons as well, so be sure to be on the lookout for these spectacular atmospheric features!