On Friday, Joshua Thomas captured this amazing photo of nine atmospheric optical phenomena in Red River, N.M., which was then shared on social media by the National Weather Service.
The Weather Service in La Crosse, Wisc., made it easy by breaking the image down into its individual phenomena, and we will do our best to describe how and why they form, though there’s an entire website devoted to these amazing features that we highly recommend if you’re interested. Various forms of ice crystals need to be present and correctly aligned in the atmosphere for each of these to occur!
Supralateral arc: The largest halo in the image. In contrast with the more common 22-degree halo, this rare arc actually varies in shape depending on how high the sun is in the sky. The supralateral arc is often confused with the 46-degree arc, according to Les Crowley’s Atmospheric Optics (a fantastic wealth of optical phenomenon knowledge). It is formed when sunlight passes through the base of rod-shaped hexagonal ice crystals, and exits through a prism side of the crystal.
Parry arcs: This includes both the sunvex and the upper suncave Parry arcs. These phenomena also change shape depending how high the sun is. Crowley has a nice diagram of the different shapes these arcs can take. The orientation of the ice crystals needs to be just perfect for this feature to appear; the length (axis) of the column crystal must be perfectly horizontal, as well as two of the prism sides. “If the near perfect orientation of column crystals sounds improbable, that of Parry oriented crystals must seem wildly so,” writes Crowley.
Upper tangent arc: This phenomenon appears over the 22-degree halo. It also changes shape depending on the elevation of the sun. This arc appears when hexagonal rod-shaped crystals are oriented horizontally, but are free to rotate around their long axis (as opposed to the Parry arcs).
Helic arcs: These arcs seem to originate from the sun itself, traveling up and away. These arcs are also formed by light passing through crystals in the same position as the Parry arcs.
22 degree halo: Likely the most common optical phenomenon relating to the sun. The 22-degree circular halo is formed when sunlight passes from one prism side of a hexagonal crystal to another. These halos are more common than the other phenomena because the ice crystals are randomly oriented.
Sun pillar: This might appear like a large ray of sun shooting upward, but it’s actually the appearance of light passing through millions of ice crystals. According to Crowley, they are most common 30 to 60 minutes after the sun as set at ground level.
Sun dog: We probably get the largest number of questions about this phenomenon, since they are slightly less common than the 22-degree halo. Sun dogs are also called “mock suns,” or scientifically, “parhelia,” and are often seen in conjunction with a 22-degree halo, since they are formed from the same kinds of ice crystals. However, as the plate-shaped crystals sink through the air, they can become vertically aligned instead of randomly oriented, which allows the light to pass through horizontally, creating the sun dogs. Each bright spot is a single sun dog, or parhelion.
Infralateral arc: This looks most similar to a typical rainbow, except you won’t see it when it’s rainy! It also changes shape based on the elevation of the sun, and seems to originate from the base of the supralateral arc. Infralateral arcs are formed when light passes through the base of hexagonal rod-shaped crystals, and exits through one of the sides of the crystal.