A ring around the Moon is caused by moonlight passing through thin, high- altitude ice crystals in cirrus clouds, which refract and sometimes reflect the light into a circular halo. The effect is typically about 22 degrees in diameter and can sometimes produce a secondary ring at about 44 degrees. This phenomenon is weather-linked: halos often precede incoming weather systems, particularly warm fronts, and can be a sign of changing conditions rather than a specific forecast of rain. The same mechanism explains halos around the Sun (solar halos) during daylight.

Key details:

• The ice crystals are usually hexagonal and suspended high in the troposphere, roughly 5–10 km up, with cirrus clouds being the common site. Light is refracted as it passes through the crystals, producing the halo at a characteristic angle of about 22°.

• A lunar halo is often described as a ring around the Moon or a bright circular arc, sometimes with a faint rainbow-like color due to dispersion. In rare cases you may notice a second ring at ~44 degrees.

• Practical observations: halos are more noticeable when cirrus clouds are present, and they can appear in any season since high-altitude ice crystals exist year-round in the atmosphere. They do not indicate immediate precipitation, but they can be associated with approaching weather systems.

If you’d like, I can tailor this explanation to your location or give you tips on how to identify a lunar halo in real time and what weather patterns typically follow halos in your region.