Why the moon above the Bay Area had a halo on Saturday night - San Francisco Chronicle

Bay Area stargazers may have noticed something unusual on Saturday night: a distinct halo surrounding the moon.

Twitter was abuzz. “Moon’s haunted,” wrote one user. “Moon looking crazy tonight,” wrote another.

What’s causing it?

One local meteorologist, Jan Null, said on Twitter that, “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.”

Null shared a photo of the phenomenon on Twitter, showing the bright halo surrounding the moon in a dark sky.

A halo is a “ring or light that forms around the sun or moon as the sun or moon light refracts off ice crystals present in a thin veil of cirrus clouds,” according to National Weather Service officials.

It is an optical effect that happens when the atmosphere’s “conditions are right,” weather service officials said.

Yes, it's a moon ring or a lunar halo. This effect occurs when light is refracted by ice crystals in high, thin clouds called cirrostratus.

— NWS Bay Area (@NWSBayArea) December 27, 2020

“In the atmosphere, under certain conditions, water drops and ice crystals can act as a prism, allowing us to see the various colors that make up visible light,” weather officials said on its webpage explaining halos and other forms of optical effects. “It’s because of these properties that we get the various atmospheric optical effects.”

The halo was discernible on Saturday night, when it was mostly clear and dry in the Bay Area, but rain is expected to come to the region on Sunday and into Monday.

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. pic.twitter.com/QHwuHymA8O

— Jan Null (@ggweather) December 27, 2020

Lauren Hernández is a San Francisco Chronicle staff writer. Email: lauren.hernandez@sfchronicle.com Twitter: @ByLHernandez

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