Why is there sometimes circle of rainbow around the moon?

A circle of rainbow around the moon, known as a lunar halo, occurs when light from the moon is refracted, reflected, and dispersed through ice crystals in high-altitude clouds, typically cirrus clouds. The ice crystals act like prisms, bending the moonlight and creating a circular halo that can appear white or faintly colored, similar to a rainbow. This phenomenon is more likely to occur when the moon is full or nearly full, and the atmospheric conditions are right for ice crystal formation. Lunar halos can serve as an indicator of upcoming weather changes, often signaling that precipitation may be on the way.