When a ring of light appears to encircle the Sun, this atmospheric optical phenomenon is known as a solar halo. The most frequently observed is the 22-degree halo, which is the direct answer to what this “rainbow around the Sun” is called. This display is not a feature of the Sun itself, but an interaction between sunlight and ice crystals high within Earth’s atmosphere.
Defining the Solar Halo
The 22-degree halo presents as a large, luminous ring of light with the Sun precisely at its center. Its name comes from the fact that its radius measures approximately 22 degrees away from the Sun in all directions. The circular light band often displays a faint spectrum of colors, with the inner edge appearing slightly reddish and the outer edge being more bluish or white. The sky area enclosed by the ring is noticeably darker than the rest of the sky, giving the halo a sharp inner boundary.
The Role of Ice Crystals in Formation
The existence of a solar halo depends on the presence of high-altitude clouds, specifically thin cirrus or cirrostratus formations. These clouds exist in the upper troposphere, where temperatures are well below freezing, meaning they are composed of millions of tiny ice crystals rather than water droplets. The light from the Sun must pass through these suspended ice crystals, which act collectively as atmospheric prisms.
This process involves the light being refracted as it enters and exits the crystal structure. This bending disperses the white sunlight into its constituent colors, similar to how a glass prism works. For the 22-degree halo to form, the ice crystals must possess a hexagonal, column-like structure. The light must enter one face of this crystal and exit through an alternate face, creating a pathway that bends the light rays.
Calculating the 22-Degree Angle
The specific measurement of 22 degrees is a direct consequence of the geometry of the hexagonal ice crystal. The six-sided crystal acts as a prism with an apex angle of 60 degrees between the faces the light passes through. Physics dictates that light passing through a prism has a minimum angle by which it can be deviated from its original path. For light traveling through a 60-degree ice prism, this minimum angle is calculated to be approximately 21.84 degrees. This minimum deviation angle is why the halo appears as a sharp ring at 22 degrees, rather than a wash of light across the sky, and explains the distinct darkening inside the halo. The slight color separation is due to dispersion; the refractive index of ice is marginally different for red light (about 21.54 degrees) compared to blue light (about 22.37 degrees), causing the red inner edge and the blue outer edge.
Common Variations and Related Optical Effects
While the 22-degree halo is the most common, it is often accompanied by or confused with other ice-crystal phenomena. Sun dogs, scientifically known as parhelia, appear as bright, colorful spots of light positioned on the 22-degree ring to the left and right of the Sun. They form when hexagonal ice crystals are oriented mostly horizontally, concentrating the refracted light into these two distinct patches. Another related display is the circumzenithal arc, often called an “upside-down rainbow” because of its brilliant colors and high position directly overhead. This arc is created when sunlight enters the top flat face of a hexagonal crystal and exits through a side face.