A ring around the moon is an atmospheric event that has captured human attention for centuries. This sight, appearing as a large, faint circle of light, is a common occurrence with a straightforward scientific explanation rooted in the physics of light and ice. The ring is an optical illusion created high above the ground by the interaction between Earth’s atmosphere and the light from our natural satellite.
Defining the Lunar Halo
The luminous circle seen around the moon is scientifically known as a 22-degree halo. This name comes from the fixed angular radius of the ring, which is always 22 degrees outward from the moon. The halo appears circular because the tiny ice crystals that create it are randomly oriented in the atmosphere, bending the light equally in all directions. The ring is typically white because the moonlight is not bright enough for the human eye to perceive distinct color separation.
The Science Behind the Phenomenon
The halo is caused by the refraction of moonlight through thin, high-altitude cirrus or cirrostratus clouds. These clouds form in the upper troposphere, where temperatures are well below freezing, and are composed of microscopic, hexagonal ice crystals that act like tiny prisms.
The hexagonal geometry of these crystals is responsible for the constant 22-degree angle. As moonlight enters one face of the crystal and exits a non-adjacent face, the light is refracted twice. This double refraction results in a minimum deviation angle of 22 degrees. Light cannot be refracted toward the observer at any smaller angle, which explains why the sky inside the ring appears darker than the halo itself.
Connecting Halos to Weather Patterns
The presence of a lunar halo is associated with the folklore saying, “ring around the moon means rain soon.” This historical observation has a scientific basis, as the conditions that create the halo often precede a change in weather.
The high, thin cirrus clouds containing the ice crystals frequently form at the leading edge of an approaching low-pressure system. Low-pressure systems are associated with an incoming warm front, which brings moist air that rises and condenses. These weather systems often produce precipitation within the next 24 to 48 hours. Therefore, the halo is not a direct cause of precipitation but a visible early sign that a moisture-laden system is advancing.