A halo around the Moon is followed by rain within a day surprisingly often, and this is one piece of moon weather lore that is straightforwardly correct.

The ring is sunlight refracted through ice crystals in high cirrostratus, and cirrostratus is what runs ahead of an approaching warm front. So the halo is not predicting anything about the Moon — it is a report on cloud six kilometres up, arriving twelve to twenty-four hours before the weather does. The halo is always 22° across, whatever anybody tells you about counting stars inside it.