The appearance of a white ring around the moon is a common atmospheric sight, known as a Lunar Halo or a 22-degree halo. This large, bright, and perfectly circular phenomenon is not physically located near the moon but is an optical effect created within Earth’s atmosphere. Its uniform white or slightly colored appearance and consistent size are determined by the physics of light interacting with specific atmospheric particles.
The Refraction of Light and the 22-Degree Angle
The formation of the lunar halo is a precise demonstration of light refraction through ice crystals suspended high above the ground. Moonlight passes through countless tiny, six-sided ice crystals, typically hexagonal columns or plates. When light enters one face of this hexagonal prism and exits through an alternate face, it is bent, or refracted, twice.
This double refraction process bends the light at a minimum deviation angle consistently near 22 degrees. The ice crystals, regardless of their orientation, all work to deflect the light by this specific amount. This constant angle is why the resulting ring always appears to have a radius of approximately 22 degrees from the center of the moon. The area inside the halo often appears darker because no light is refracted toward the observer at an angle less than 22 degrees. The consistent geometry of the hexagonal ice crystals dictates the optical properties that create this fixed-radius ring.
High-Altitude Ice Crystals and Approaching Weather
The ice crystals required for a lunar halo are found in thin, high-altitude clouds, specifically cirrostratus or cirrus clouds, which float at elevations of 20,000 feet (about 6,000 meters) or higher. These clouds are the earliest visual sign of an advancing low-pressure weather system or a warm front. A warm front causes warmer, moist air to slide up and over colder air, leading moisture to condense and freeze at high altitudes.
The presence of cirrostratus clouds, and thus the halo, often suggests that a change in weather is imminent, typically within the next 12 to 24 hours. As the warm front approaches, these high clouds gradually thicken and descend into lower layers like altostratus and nimbostratus clouds. These lower, moisture-laden clouds ultimately bring precipitation, such as rain or snow, to the ground. This association with the leading edge of a major weather system makes the optical phenomenon a reasonably accurate short-term weather indicator.
Folklore and Traditional Weather Prediction
The sight of a ring around the moon has long been interpreted as a signal of impending weather change, often leading to various proverbs and cultural sayings. Mariners and farmers, whose lives depended on predicting the weather, frequently relied on this atmospheric sign. A common saying reflects this observation: “A ring around the sun or moon, means rain or snow coming soon.”
This traditional wisdom is rooted in the observation that the high, thin clouds producing the halo are often followed by rain-bearing storm systems. The halo was viewed as a celestial warning; some cultures believed the number of stars visible within the ring indicated the number of days until the storm’s arrival. One poetic example of this folklore states, “If the moon show a silver shield, be not afraid to reap your field; but if she rises haloed round, soon we’ll tread on deluged ground.” These historical interpretations align closely with the modern meteorological understanding of warm fronts and cloud progression.