(Moon halo in the Bay Area. Credit Gado Images).
Residents in the San Francisco Bay Area this week noticed a strange halo around the moon this week. The halo--which appeared as a perfect circle--combined with patchy, low clouds to create a mystical, ominous look in the night sky.
Bay Area citizens took to social media to comment on the halo. Many noted that the effect began just after Christmas. Charis Tang of Cuptertino, for example, wrote on Twitter “Absolutely GORGEOUS night sky last night in the Bay Area with a phenomenon I’ve never seen before! A winter halo, also known as a moon ring or 22–degree halo, lit up the sky the night after Christmas.”
Elizabeth Gopal of Oakland, California wrote “I found a glorious Moon Halo directly above home (10:30 pm...) I inhaled the beauty and welcomed the magic”.
What’s going on with the Bay Area moon halo? According to SFGate and meteorologist Jan Null, there’s a perfectly normal--and totally earthly--explanation for the strange halos Bay Area residents have been seeing.
As users of Twitter noted, the halo is called a “22 degree halo.” It forms when a full moon shines through ice crystals at high altitudes, and is also associated with the presence of cirrus clouds. Light refracts through these crystals at 22 degrees, creating the halo effect around the moon.
Notably, the halo does not actually surround the moon itself--it is created in Earth’s atmosphere, and is a trick of moonlight filtering through ice crystals, not a feature of the moon.
Because the effect is associated with cold temperatures at high altitudes, it means that snow may be possible on some of the Bay Area’s peaks. Mount Diablo occasionally sees snow, and this is possible over the coming days if temperatures remain low.
In the meantime, if you’re in the Bay Area at night, take a look out your door and see if the halo is there. The effect is relatively common, but sustained halos like the ones the Bay has seen over the last few nights are still striking to see and to photograph.
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