A luminous ring that encircles the Moon on a clear night is a real physical atmospheric phenomenon. This circular glow is officially known as a lunar halo. Its appearance has been a source of fascination for centuries.
The Lunar Halo: Naming the Phenomenon
The most common form of this phenomenon is the 22-degree halo. This designation refers to the precise angular distance between the Moon and the inner edge of the ring. The halo typically appears as a soft, diffuse glow that is mostly white because moonlight is not bright enough for the human eye to perceive subtle colors.
The visibility of the halo can vary dramatically, sometimes appearing bright and distinct, and other times faint and hazy. This difference relates to the concentration of atmospheric elements creating the ring. A dense layer of particles scatters more moonlight, resulting in a more vivid circle. The sky inside the ring often looks noticeably darker than the surrounding sky, making the boundary stand out clearly.
The Atmospheric Conditions Required
The formation of the lunar halo depends on specific atmospheric conditions: high altitude and cold temperatures. Halos occur when moonlight interacts with clouds located 6 to 13 kilometers (20,000 feet or more) above the surface. The responsible clouds are the thin, wispy types known as cirrus or cirrostratus.
At these high altitudes, temperatures are far below freezing, ensuring the water vapor is not liquid. Instead, the clouds are composed entirely of millions of microscopic ice crystals. These frozen particles are essential, as they transform the atmosphere into a temporary optical instrument. Without a layer of these crystals, the light would pass through unobstructed, and no halo would form.
Decoding the 22-Degree Radius
The precise and consistent size of the ring is determined by the specific geometry of the atmospheric ice crystals. The tiny ice particles are almost always shaped like hexagonal prisms, featuring six equal sides and a 60-degree apex angle between alternating faces. When moonlight enters one side of this six-sided crystal and exits through an alternate face, the light is bent, or refracted, twice.
Light passing through this 60-degree prism shape is always refracted by a minimum of about 21.84 degrees. This minimum angle of deviation is consistent regardless of the crystal’s orientation as it tumbles through the air. Since the ice crystals are randomly scattered, the light reaching an observer’s eye is always refracted at this fixed angle, creating a perfect circle with a uniform 22-degree radius around the light source.
Folklore and Halos: Predicting the Weather
The appearance of a ring around the Moon has long been associated with weather prediction. The common saying, “Ring around the moon means rain soon,” reflects an ancient observation that holds a degree of scientific truth. The high-altitude cirrus clouds containing the necessary ice crystals often form at the leading edge of a large, advancing weather system.
This weather system, often a warm front, can bring precipitation within the following 24 to 48 hours. Therefore, seeing a halo suggests a change in weather is approaching, giving the old adage an understandable correlation with atmospheric science. However, cirrus clouds can also appear without an accompanying storm system, meaning the halo is not a perfectly reliable forecasting tool.