When a large, luminous circle appears around the Moon, you are witnessing a common atmospheric optical phenomenon known as a lunar halo, or more precisely, a 22-degree halo. This bright ring occurs when the conditions in the upper atmosphere are just right to interact with moonlight. The halo has long served as a natural, though imperfect, indicator of changing weather. This article explains the science behind this striking event and what its presence can suggest about upcoming conditions.
How Ice Crystals Create the Halo
The formation of the lunar halo depends on the presence of tiny ice crystals suspended high in the Earth’s atmosphere. These crystals are typically found within thin, wispy cirrus or cirrostratus clouds, which exist at altitudes where temperatures are always below freezing. The ice crystals usually take the shape of hexagonal prisms, similar to a six-sided pencil, and float randomly in the air.
These ice crystals act like miniature glass prisms, refracting the moonlight as it passes through them. A ray of light enters one face of the crystal and exits through another face angled at 60 degrees. This specific geometry causes the light to be deflected by a minimum angle of approximately 22 degrees from its original path.
Millions of these randomly oriented crystals collectively bend the light, concentrating it into a ring that is always seen at the same 22-degree distance from the light source. This bending of light is the physical reason why a perfect circle is formed around the Moon, and why the phenomenon is officially called the 22-degree halo.
Visual Characteristics of the 22-Degree Ring
The 22-degree halo is a very large circle in the night sky, and its angular radius is constant regardless of the Moon’s position. To visualize its size, holding your hand outstretched with your fingers spread, the distance between your thumb and little finger tip spans about 20 degrees, meaning the halo extends slightly wider than this span. This fixed size is a direct result of the 22-degree minimum angle of light deviation.
The ring often appears bright white, but close observation can reveal faint colors due to the dispersion of light, much like a prism separating white light. The inner edge of the halo is typically marked by a subtle reddish or orange tinge, while the outer edge may show a hint of blue or violet. This color separation occurs because red light bends slightly less than blue light as it passes through the ice crystals.
A noticeable feature is that the sky immediately inside the halo appears conspicuously darker than the sky just outside the ring. This darker inner area exists because the hexagonal ice crystals do not refract any moonlight toward the observer at an angle less than 22 degrees. This lack of light makes the halo look distinct and sometimes gives the impression of a darker “hole” surrounding the Moon.
Predicting Weather Using Lunar Halos
The presence of a lunar halo is often associated with the traditional saying that a ring around the Moon means rain or snow is coming. This folklore has a basis in meteorology, as the atmospheric conditions required to create the halo are linked to approaching weather systems. The ice crystals are contained within high-altitude cirrostratus clouds, which often form at the leading edge of an extensive weather pattern.
These thin, high clouds are often the first visible sign of an approaching low-pressure system, such as a warm front. Warm fronts typically move slowly and bring widespread, prolonged precipitation as they pass over an area. As the warm front advances, the high cirrostratus clouds progressively thicken and lower into mid-level and then low-level clouds, eventually bringing rain or snow.
Seeing a lunar halo means the high-altitude moisture is already present, suggesting that the entire weather system is likely moving closer. While not a guarantee, the halo can act as a natural indicator that precipitation may arrive within the next 12 to 24 hours.