A luminous ring often observed around the Moon, known as a lunar halo, is an atmospheric optical phenomenon. This bright, circular band typically appears white, though faint colors can sometimes be discerned.
The Role of Ice Crystals
Moon halos form when moonlight interacts with tiny ice crystals suspended in Earth’s upper atmosphere. These crystals refract the light, bending it as it passes through. The ice crystals are commonly hexagonal in shape, similar to miniature prisms. This specific crystalline structure causes the light to bend at a consistent angle.
As moonlight enters one side of a hexagonal ice crystal, it refracts, and then bends again upon exiting another side. This double refraction results in a total bend of approximately 22 degrees, regardless of the crystal’s orientation. This consistent bending angle creates the characteristic 22-degree radius of the halo around the Moon. The halo is a visual effect of light being bent towards the observer, rather than an object physically obstructing the Moon.
Atmospheric Conditions for Formation
The ice crystals responsible for lunar halos reside in high-altitude clouds, specifically cirrus or cirrostratus clouds. These thin clouds are typically found at altitudes of 20,000 feet (about 6,000 meters) or higher, extending up to 40,000 feet (about 12,000 meters). At these elevations, temperatures are sufficiently cold for ice crystals to form and persist, even during warmer seasons.
For a halo to be visible, these clouds must be thin enough to allow moonlight to pass through them. Cirrostratus clouds, which often cover large areas of the sky like a translucent veil, are particularly known for producing halos. The presence and thickness of these ice-laden clouds determine how distinct the halo appears.
Halos and Weather Prediction
Common folklore associates a ring around the Moon with impending weather changes. While the halo itself does not directly cause weather, the presence of the high-altitude cirrus and cirrostratus clouds that create halos can sometimes precede shifts in weather patterns. These clouds often form at the leading edge of a warm front, which can bring precipitation within 12 to 24 hours.
As a warm front approaches, moist air rises to high altitudes, forming ice crystal clouds. Therefore, seeing a halo indicates the presence of these specific clouds, which can be a sign of an approaching low-pressure system and precipitation. However, this is an observation based on cloud presence, not a foolproof prediction, as cirrus clouds can also occur without a subsequent weather change.