When looking up at the night sky, a striking visual phenomenon sometimes appears as a large, bright circle surrounding the moon. This luminous ring often appears white or only faintly colored, spanning a significant portion of the visible sky. The reason this happens involves a complex interaction of light and frozen particles high above the Earth’s surface. This spectacle is not caused by anything physically surrounding the moon itself, but rather by an atmospheric process occurring much closer to home.
Identifying the Lunar Halo
The optical event is formally known as a Lunar Halo, which is a member of the family of ice crystal halos. More precisely, it is classified as a 22-Degree Halo, referencing its consistent angular radius in the sky. This phenomenon is an atmospheric event, taking place entirely within Earth’s atmosphere. Halos form specifically in high-altitude, thin clouds known as cirrus or cirrostratus, typically at altitudes between 5 and 10 kilometers above the ground. These clouds must be composed entirely of tiny, frozen water particles rather than liquid droplets.
The Role of Ice Crystals and Refraction
The mechanism behind the halo relies on the presence and structure of the ice crystals suspended within these high-altitude clouds. These crystals naturally grow into a specific hexagonal prism shape, featuring six equal sides and flat top and bottom faces. When moonlight encounters these frozen structures, the light undergoes a process called refraction. Refraction is the bending of light as it passes from one medium, like air, into a denser one, like ice. The hexagonal ice crystal acts like a miniature prism, causing the light to bend twice: once upon entering the crystal and again upon exiting it.
Light rays from the moon enter one side of the hexagonal crystal and exit through a side that is 60 degrees away from the entry point. This double bending directs the light toward an observer on the ground. Because the light is bent by the ice, the halo often appears predominantly white, though a faint color separation can sometimes be observed. The inner edge of the ring may appear slightly reddish, while the outer edge is often a subtle bluish hue.
Why the Circle Measures Exactly 22 Degrees
The fixed size of the halo is determined by the unvarying geometry of the ice crystals and the physics of light refraction. The 22-degree measurement represents the minimum angle of deflection for light passing through a hexagonal ice crystal. Light entering one face of the hexagonal prism and exiting a face 60 degrees away will be deflected by a minimum of approximately 21.84 degrees. This specific minimum angle creates the sharp, distinct inner edge of the ring.
Light cannot be bent at an angle smaller than this minimum deviation, which is why the sky appears noticeably darker inside the halo than outside it. Since this angle is a function of the ice’s refractive index and the crystal’s 60-degree corner, the size of the halo remains constant. The ring will always have an apparent radius of 22 degrees from the moon.
Halos as Indicators of Approaching Weather
The presence of a lunar halo can provide a clue about changes in the weather. The cirrus and cirrostratus clouds responsible for the halo often form high in the atmosphere ahead of a warm front. A warm front is a boundary between a mass of warm air and a retreating mass of cold air, which typically brings moisture. The high, thin ice clouds are often the first visible signs of this advancing weather system.
Observing a lunar halo historically suggested that rain or snow was likely approaching within the next 24 to 48 hours. While modern meteorology offers more precise forecasting tools, the underlying principle holds true. The appearance of the halo confirms that a layer of moisture-laden air has moved into the upper atmosphere, which frequently precedes precipitation. The beautiful ring is thus an atmospheric indicator of future weather changes.