When a ring appears around the Moon, it is not a “rainbow” in the traditional sense, but a phenomenon known as a lunar halo. This optical phenomenon, where a large, bright ring encircles the Moon, has intrigued observers for centuries. Also called a moon ring, storm ring, or winter halo, halos result from interactions between moonlight and Earth’s atmosphere, creating natural beauty in the night sky.
What You Are Seeing
A lunar halo appears as a distinct, circular ring around the Moon. The most common form, a 22-degree halo, has a radius of about 22 degrees from the Moon’s center. This angular distance can be estimated by holding an outstretched hand at arm’s length, with the Moon at the thumb and the halo’s edge reaching the pinky finger. While often appearing largely white or faint, subtle coloration may be visible, with red hues on the inner edge and blue or violet on the outer edge. The sky within the halo can also appear darker than the surrounding sky, creating a “hole in the sky” effect.
The Science of Formation
Lunar halos form from the interaction of moonlight with specific atmospheric conditions. This optical phenomenon occurs when moonlight is refracted, reflected, and dispersed by millions of tiny ice crystals suspended high in Earth’s atmosphere. These ice crystals are typically found within cirrus or cirrostratus clouds, which are thin clouds located at altitudes of 20,000 to 40,000 feet (6,000 to 12,000 meters). The crystals are predominantly hexagonal, acting like miniature prisms.
When moonlight enters a hexagonal ice crystal, it refracts as it enters and exits. This process causes light to deviate by a specific angle, about 22 degrees from its original path, defining the halo’s radius. Light also undergoes dispersion, separating into its component colors, similar to how a prism splits white light. Different wavelengths refract at slightly different angles, leading to subtle color separation. Longer red wavelengths bend less than shorter blue wavelengths.
Understanding Associated Beliefs
Lunar halos have long been associated with beliefs and folklore, especially the saying, “ring around the moon means rain soon.” This weather lore has a scientific basis. The high-altitude cirrus clouds that form halos often precede warm fronts and low-pressure systems, which are associated with impending precipitation like rain or snow.
While a lunar halo indicates atmospheric conditions that may precede a weather change, it is not a direct predictor of specific weather events. Cirrus clouds can appear without subsequent precipitation, making the halo an unreliable forecast. Beyond weather prediction, some cultures interpret lunar halos as signs of divine presence, protection, good fortune, or impending change, reflecting humanity’s fascination with celestial phenomena.