Stargazers in the Bay Area may have noticed something unusual on Saturday night: a distinct halo around the moon.
Twitter was buzzing. “The moon is haunted,” wrote one user. “Moon looks crazy tonight,” wrote another.
What causes it?
A local meteorologist, Jan Null, said on Twitter that, “Light from the near-full moon will break through ice crystals in the tall thin cirrus clouds over the Bay Area tonight to create a well-defined 22 degree lunar halo.”
Null shared a photo of the phenomenon on Twitter, with the bright halo surrounding the moon in a dark sky.
A halo is a “ring of light that forms around the sun or moon when the sun or moon light reflects off ice crystals contained in a thin veil of cirrus clouds,” National Weather Service officials said.
It is an optical effect that occurs when the atmosphere is “the right conditions,” said weather officials.
Yes, it is a moon ring or a moon halo. This effect occurs when light is refracted by ice crystals in tall, thin clouds called cirrostratus.
– NWS Bay Area (@NWSBayArea) December 27, 2020
“In the atmosphere, water droplets and ice crystals can act as a prism under certain conditions, allowing us to see the different colors that make up visible light,” weather officials said on their webpage explaining halos and other forms of optical effects. “It is because of these properties that we get the different atmospheric optical effects.”
The halo could be seen on Saturday night, when it was mostly clear and dry in the Bay Area, but rain is expected to hit the region on Sunday and Monday.
Tonight, light from the near-full moon will break through ice crystals in the tall thin cirrus clouds over the Bay Area to create a well-defined 22 degree lunar halo. pic.twitter.com/QHwuHymA8O
– Jan Null (@ggweather) December 27, 2020
Lauren Hernández is a staff writer at San Francisco Chronicle. Email: [email protected] Twitter: @BuienRadarNL