A moon ring, also known as a winter halo, is a phenomenon that usually appears in conjunction with a full moon. There appears to be a whitish ring, approximately 10 to 20 times the size of the moon, surrounding the moon and centered on it. It is caused by refraction of the light from the full moon in the ice particles floating in the clouds, as opposed to a rainbow, where light refracts in the water vapor that makes up the clouds. Since this happens most effectively at a certain angle, this ring appears at the bottom of the clouds, and since similar triangles must form between the moon, the refracting surface, and the observation point, the "highlighted" clouds are at approximately the same distance from the moon, creating the image of a ring.

In ancient beliefs it is believed that a moon ring means very warm days before the winter storm. It is also said that the number of days can be counted by counting the stars inside the ring. If you count 15 stars then you have 15 days before a winter storm is to come.