When you observe a large, luminous circle encompassing the Moon, you are witnessing a common and beautiful atmospheric optical event. This natural occurrence results from light interacting with Earth’s upper atmosphere. It is a phenomenon that has captured human attention for centuries, inspiring both scientific inquiry and traditional folklore. This captivating display is a testament to the intricate interplay between celestial light and atmospheric conditions.
What You’re Seeing: The Lunar Halo
The phenomenon you are observing is formally known as a lunar halo, most commonly the “22-degree halo,” or sometimes a moon ring. This optical event manifests as a large, often faint, ring of light encircling the Moon. Its characteristic size, with a radius of approximately 22 degrees, is consistent enough to be roughly estimated by holding an outstretched hand at arm’s length, where the Moon sits at the thumb and the halo extends to the little finger. The halo typically appears as a bright, whitish circle, although subtle colors, with red on the inside and blue on the outside, can sometimes be discerned, especially when the Moon is particularly bright. It is distinct from a rainbow, formed by water droplets, and a corona, a smaller, more colorful ring caused by diffraction from water droplets or tiny ice crystals close to the light source.
The Atmospheric Science Explained
Lunar halos are formed through a specific interaction of moonlight with ice crystals suspended in the atmosphere. This occurs when moonlight is refracted and reflected by tiny, hexagonal ice crystals found in high-altitude clouds. These clouds, typically cirrus or cirrostratus, float at altitudes ranging from about 20,000 to 40,000 feet (6,000 to 12,000 meters) above the ground. The hexagonal shape of these ice crystals is essential for halo formation.
As moonlight enters one side of a hexagonal ice crystal and exits another, it bends, or refracts, at a precise 22-degree angle. This consistent bending angle creates the uniform 22-degree radius of the halo. The optical properties of these crystals also mean that light is not directed back towards the center of the halo, which can make the sky inside the ring appear noticeably darker than the surrounding sky.
The Weather Lore and Beyond
The appearance of a lunar halo has long been associated with traditional weather predictions, most notably the saying, “ring around the moon means rain soon.” This folklore has a meteorological basis. The high-altitude cirrus and cirrostratus clouds that produce lunar halos often precede the arrival of a warm front. Warm fronts are atmospheric boundaries that can bring moisture and a change in weather, frequently resulting in precipitation within 24 to 48 hours.
These high clouds are often the initial indicators of an approaching weather system, appearing before the thicker, lower clouds that eventually bring rain or snow. While a lunar halo does not guarantee precipitation, it signals the atmospheric conditions that frequently precede it, making the old saying an observational tool with scientific backing. This connection between a celestial sight and impending weather allowed people for centuries to use such natural phenomena as a form of rudimentary weather forecasting.