When a bright circle appears around the Moon, you are witnessing a lunar halo, sometimes called a “moon ring.” This event is not a physical object near the Moon but a visual effect created by the interaction of moonlight with Earth’s atmosphere. The halo typically appears as a large, whitish ring, though faint colors can sometimes be discerned, especially on the inner edge. This sight is a form of light refraction, transforming the upper atmosphere into a temporary lens that encircles the lunar disk.
How Ice Crystals Create the Halo
The formation of the lunar halo depends on the presence of ice crystals suspended high in the atmosphere. These crystals are found within thin, wispy clouds known as cirrus or cirrostratus, which float at altitudes exceeding 20,000 feet where temperatures are far below freezing. Even in summer, these clouds are composed entirely of frozen water particles. The light we see is reflected sunlight from the Moon, which travels through these clouds toward an observer on the ground.
Each tiny ice crystal acts like a miniature prism, bending the path of the light rays that pass through it. As moonlight enters one face of a crystal and exits another, its direction is altered in a process called refraction. The collective effect of millions of these randomly oriented particles redirects the light toward the viewer, creating the visible ring. The halo is formed by light being scattered by this intervening layer of ice-filled air.
Understanding the 22-Degree Angle
The lunar halo always appears at a radius of approximately 22 degrees from the Moon, a direct consequence of the specific geometry of the ice crystals. These high-altitude crystals are most often shaped as hexagonal prisms. Light enters one side of this hexagonal structure and exits through an alternate side, with the two faces making a 60-degree angle.
When light passes through this 60-degree prism, the laws of physics dictate a minimum angle by which the light ray can be deviated from its original path. For ice, this minimum deviation angle is consistently around 21.84 degrees, which is rounded to 22 degrees. Because no light is refracted toward the observer at an angle smaller than this minimum deviation, the area of the sky inside the halo often appears noticeably darker.
Predicting Weather with a Lunar Halo
The presence of a lunar halo is associated with weather lore suggesting that rain or snow is approaching. This saying has a valid basis in modern meteorology because of the conditions necessary to form the halo. The high cirrostratus clouds containing the ice crystals are often the leading edge of an approaching weather system, specifically a warm front.
A warm front occurs when a mass of warmer air gradually rises over a retreating wedge of cooler air near the surface. As the warm, moist air is lifted high into the atmosphere, it cools and condenses, forming high-altitude cirrus clouds. These clouds, which create the halo, act as an atmospheric signal, arriving hours before the main part of the front. The main frontal system is associated with lower pressure and widespread precipitation. Observing a lunar halo suggests that a significant shift in weather, likely bringing rain or snow, could arrive within the next 12 to 48 hours.