A luminous ring encircling the Moon is a common atmospheric optics phenomenon known scientifically as a 22-degree halo. Often called a moon ring or winter halo, this event is caused by the interaction of moonlight with ice crystals in Earth’s atmosphere, not by any change in the Moon itself. The halo requires a specific set of atmospheric conditions to manifest.
The Specific Atmospheric Conditions Required
The formation of a lunar halo is dependent on the presence of high-altitude clouds composed of frozen water. These conditions are found in cirrus or cirrostratus clouds, which drift through the upper troposphere, generally between 6 and 13 kilometers (20,000 to 42,000 feet) above the ground. The air temperature at these elevations is far below freezing, ensuring the cloud composition is entirely microscopic ice crystals.
These ice crystals, rather than liquid water droplets, are necessary for the optical display. The clouds must be thin enough for the Moon’s light to pass through, yet dense enough to contain sufficient crystals to refract the light toward an observer. If the cloud layer is too thick or obscured by lower-altitude clouds, the halo will not form or be visible. The air below the cloud layer must also be relatively clear for the phenomenon to be visible from the ground.
The Optical Physics of the Halo
The ring maintains its fixed size because of the specific geometry of the ice crystals and the physics of light refraction. The crystals responsible for the 22-degree halo are typically hexagonal prisms. When moonlight enters one face of the crystal and exits an opposing face, the light is bent, or refracted, twice.
The angle between the faces of a standard hexagonal ice crystal is 60 degrees. Due to the refractive index of ice, the minimum deviation angle is fixed at approximately 21.84 degrees. Since light cannot be refracted at an angle less than this minimum, the result is a concentration of light rays at precisely 22 degrees from the Moon, creating the visible ring.
The halo is often seen as a bright white circle because moonlight is scattered almost equally across all wavelengths. When colors are visible, the inner edge exhibits a reddish tint, while the outer edge may be slightly bluish. This color separation is due to dispersion, where different wavelengths of light are refracted at slightly different angles. The area inside the 22-degree ring appears noticeably darker than the rest of the sky because no light can be refracted toward the observer at an angle smaller than the 22-degree minimum.
The Halo as a Weather Indicator
The common folklore that a “ring around the moon means rain soon” has a basis in meteorology, though the halo itself is a symptom of a larger system. The high cirrostratus clouds that create the halo often serve as the leading edge of an approaching weather system. These clouds form ahead of an advancing warm front or low-pressure system, which is a large mass of moist air that can bring precipitation.
The visibility of the halo indicates that a moisture-bearing frontal system is moving into the area, with the potential for rain or snow arriving within the next 12 to 24 hours. The cirrostratus clouds are the first indication of this shift in weather patterns, making the halo a natural, short-range forecasting tool. However, the appearance of a lunar halo is not a guarantee of bad weather. The high-altitude clouds can pass overhead without a full storm system developing or reaching the ground. Viewing the halo simply confirms the presence of the specific ice crystals necessary for light refraction.