A solar halo is an atmospheric phenomenon appearing as luminous rings or arcs around the sun. These optical displays arise from interactions between sunlight and ice crystals suspended in the atmosphere.
What a Solar Halo Looks Like
A solar halo typically manifests as a circular ring of light centered on the sun. The most commonly observed form is the 22-degree halo, appearing at an angular radius of approximately 22 degrees from the sun. This means if you extend your arm and spread your fingers, the distance from your thumb to your little finger tip roughly covers this angle. Halos often appear white or faintly colored, but can display a subtle spectrum of colors, with red on the inner edge and blue on the outer edge, similar to a faint rainbow. The inner boundary of the halo can appear sharply defined, while the outer edge might be more diffuse.
The sky within the 22-degree halo often appears noticeably darker than the sky outside it. This effect occurs because no light is refracted at angles smaller than 22 degrees, leading to a comparative absence of light within the ring. Halos can be seen globally at any time of year, given the right atmospheric conditions. The appearance of a full, unbroken ring depends on the consistent presence of ice crystals across the observer’s line of sight.
How Ice Crystals Create Halos
Solar halos form when sunlight interacts with millions of tiny ice crystals suspended high in the atmosphere. These crystals are typically found in thin, wispy cirrus or cirrostratus clouds, which exist at altitudes of 6,000 meters (20,000 feet) or higher where temperatures are consistently below freezing. The water vapor at these heights freezes directly into ice, rather than forming liquid water droplets. The hexagonal (six-sided) shape of these ice crystals is fundamental to halo formation.
Sunlight passing through these hexagonal ice crystals undergoes both refraction and reflection. Refraction causes the light to bend as it enters and exits the crystal, similar to how a prism disperses light. The hexagonal shape, particularly the 60-degree angle between alternating faces, bends light at a minimum deviation angle of approximately 22 degrees. As light bends, it separates into its component colors, with red light bending least and blue light bending most, leading to the faint coloration observed in some halos. The collective effect of countless randomly oriented ice crystals refracting sunlight at this consistent 22-degree angle creates the circular halo around the sun.
Different Types of Solar Halos
Beyond the common 22-degree halo, other forms exist, produced by specific ice crystal shapes and orientations. Sun dogs, or parhelia, are bright spots of light appearing on either side of the sun, typically at the same 22-degree angular distance as the main halo. These mock suns are often colored, with red closest to the sun, and result from sunlight passing through horizontally oriented, plate-shaped hexagonal ice crystals. Sun dogs frequently accompany the 22-degree halo.
The 46-degree halo is a less common, larger, fainter ring around the sun, roughly twice the radius of the 22-degree halo. It forms when sunlight enters randomly oriented hexagonal ice crystals through a prism face and exits through a hexagonal base, resulting in a 90-degree inclination between the faces. The circumzenithal arc is a brightly colored, rainbow-like arc forming high above the sun, appearing like an upside-down rainbow. It is caused by sunlight entering the top flat face of horizontally oriented plate-like ice crystals and exiting through a side prism face.
Halos Versus Other Atmospheric Optics
Solar halos are distinct from other atmospheric optical phenomena like rainbows and coronas, though sometimes confused due to their circular appearance. The primary difference lies in their formation: solar halos result from refraction and reflection of sunlight through high-altitude ice crystals, while rainbows form from refraction and internal reflection of sunlight within spherical water droplets. Rainbows always appear opposite the sun, with the observer’s shadow at the center of the arc, and typically show more vibrant and distinct colors than halos.
Coronas, another atmospheric optical display, also appear as rings around the sun or moon but are smaller and closer to the light source than halos. Unlike halos, coronas are created by light diffraction around small water droplets or ice crystals, not refraction. Coronas exhibit concentric colored rings, with blue inside and red outside, the reverse color order of a halo. When observing any atmospheric phenomenon involving the sun, it is important to exercise caution. Never look directly at the sun, even when a halo is present, as this can cause severe eye damage. Safe viewing methods include shielding the sun with a hand, a building, or a tree, or by using certified solar viewing glasses.