When gazing at the night sky, people sometimes witness a luminous ring encircling the Moon. This natural atmospheric optical event, while appearing mystical, arises from specific interactions between moonlight and the Earth’s atmosphere. It is a display of intricate physics high above us, transforming the night sky into a canvas of light.
What a Moon Halo Is
A moon halo, also known as a lunar halo, is a large, bright ring that surrounds the Moon. This ring typically maintains a consistent radius of approximately 22 degrees from the Moon’s center, earning it the name “22-degree halo”. It appears as a soft, diffuse whitish glow without a sharp outline. Faint spectral colors, with red on the inner edge and blue on the outer, can sometimes be observed, though they are usually too dim for the naked eye.
The Science Behind the Halo
Moon halos form from the interaction of moonlight with millions of tiny, hexagonal ice crystals suspended in the Earth’s upper atmosphere. These crystals act like miniature prisms. Moonlight, which is sunlight reflected off the Moon, enters one side of a crystal, refracts, and then bends again as it exits another side.
This double refraction bends the light at a precise 22-degree angle relative to its original path, a specific angle dictated by the hexagonal geometry of the ice crystals. As countless ice crystals are distributed throughout the sky, the refracted light from each combines to form a complete circle around the Moon from the observer’s perspective. This process is similar to how a rainbow forms from water droplets, but with ice crystals creating a circular halo.
Atmospheric Conditions Required
Moon halos require specific atmospheric conditions, primarily the presence of ice crystals at high altitudes. These crystals are typically found within thin cirrus or cirrostratus clouds, located at elevations of 20,000 to 40,000 feet (about 6,000 to 12,000 meters). Even during warmer seasons, temperatures at these heights remain well below freezing, allowing ice crystals to form and persist.
For the 22-degree halo to appear, these high-altitude clouds must be thin enough for moonlight to pass through effectively. The ice crystals within them also need to be randomly oriented. This random orientation ensures the consistent 22-degree refraction that creates the familiar circular halo, as aligned crystals would produce rarer optical phenomena.
What a Halo Might Indicate
While often associated with folklore suggesting impending weather changes, a moon halo provides a scientific indication of high-altitude atmospheric conditions. The cirrus or cirrostratus clouds containing the necessary ice crystals can sometimes precede a warm front. Warm fronts are often associated with low-pressure systems that may bring precipitation, such as rain or snow, within a day or two.
Seeing a halo around the Moon does not directly cause or predict weather, but it signifies that moisture, in the form of ice crystals, is present in the upper atmosphere. The halo is a visual consequence of light interacting with these ice crystals, serving as an indicator of current high-altitude atmospheric composition.