Photo credit: Sue McGaughey
If you happened to see a halo ring around the moon in the San Francisco Bay Area, you’re not alone. Also called a moon ring, winter halo, or a 22-degree halo, this phenomenon happens when ice crystals suspended high above in thin wispy cirrus clouds refract the moon’s light to form a perfect circle, according to meteorologist Drew Tuma. Believe it or not, these winter halos may be visible on as many as 100 days per year, which is much more frequent than rainbows.
Traditional beliefs say that this phenomenon warns of approaching storms, appearing only when the sky is covered by thin cirrus or cirrostratus clouds that often come a few days before a large weather front. However, the sesame clouds can also appear without any associated weather change, making these winter halos unreliable as a sign of bad weather.
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