You may occasionally notice a ring or halo around the moon, which in folklore is taken as a sign of rain or other precipitation. There are even sayings about rain prediction: “A ring around the moon, means that rain will come real soon”. “If the moon shows a silver shield, don’t be afraid to reap your field; but if she rises haloed round, soon you’ll walk on flooded ground.
The ring is caused by light from the moon passing through clouds made of ice crystals in the upper atmosphere. These six-sided crystals refract extra light toward you that would otherwise disperse elsewhere. People in different places see a different set of ice crystals, so when you see a ring, it was made just for you. Halos mostly occur when high cirrostratus clouds are present, which are called “mare’s tails”.
So do moon halos foretell rain? Yes, but you need to check out the wind too. If you see a well-formed ring and the wind is coming out of the northeast to south range, unsettled weather is not far away. A surer bet is if the ring is accompanied by the moon appearing as if it’s behind frosted glass, caused by thin, sheet-like clouds called altostratus. These clouds usually travel 15 to 20 hours ahead of thicker, heavier cloud cover that usually bring some enduring precipitation. I personally have seen this happen several times and it sure enough did rain the next day.
Rings also appear around the sun because of the same ice crystals, and some feel that a sun halo is a better predictor of bad weather. Just remember to check the wind. If it’s blowing from any other direction besides out of the northeast to south range, it will probably only cloud up and not rain.