Have you seen a perfect halo around the moon recently? Here’s why. – SFGate

If you’ve seen an angelic-looking moon in the Bay Area sky lately, there’s a fun scientific reason for the celestial sight.

“Light from the almost full moon is refracting through ice crystals in the high thin cirrus clouds over the Bay Area this evening to create a well-defined 22 degree lunar halo,” Golden Gate Weather Services meteorologist Jan Null posted on Twitter on Saturday.

Although remarkable to behold, halos are actually more common than rainbows.

Moon halos occur when there are high, thin clouds across the sky. The clouds are filled with countless ice crystals, each of which is shaped like a tiny hexagon. Like a million small lighthouses, the crystals each refract light through eachother. The light refracts 22 degrees, which is the radius of the moon halo.

The phenomenon is visible both during the day and at nighttime, and Bay Area residents looking up on Saturday night were lucky enough to experience it.

The thin clouds are expected to be replaced by stormy ones later Sunday. A bit of rain is expected in the Bay Area overnight, and it’s cold enough that snow is possible at higher elevations.

“Because of temps expected to be at/near freezing above 4000 ft, some of highest elevations have the potential for seeing some minor snow accumulations!” the Bay Area’s National Weather Service office tweeted Sunday morning. “We’ll be keeping a close eye on the Santa Lucia and Diablo Ranges tonight.”


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