A lunar halo appears over the La Silla Observatory in Chile. (Yuri Beletsky)

Observers at the La Silla Observatory on the border of Chile’s Atacama Desert were studying the stars and constellations last week when they saw something spectacular: A bright lunar halo had formed in front of the Orion constellation and Sirius, the brightest star in our sky.

The technical term for a lunar or solar halo is a “circumscribed halo.” Similar to sun dogs, lunar and solar halos belong to the family of ice halos, which are optical phenomena formed by the refraction of sun on hexagonal ice crystals suspended high up in the atmosphere, most commonly in cirrus clouds. These are also known as 22-degree halos, because the ring of light forms 22 degrees from the sun.

Cirrus clouds are made up of millions of tiny ice crystals. When the light from the sun or moon hits these crystals in just the right way, the light refracts and reflects. It’s this interaction between the light and the hexagonal ice crystals that forms the ring around the moon.

Although solar and lunar halos form in the same way, there’s one noticeable difference. Since moonlight isn’t very bright, lunar halos are typically colorless, compared with their more rainbow-colored solar cousins. However, with the ideal optical setup and perfect camera settings, we can sometimes see color in lunar halos — typically red on the inside and blue on the outside.

That is exactly what photographer Yuri Beletsky was able to capture!

There’s an old weather saying that “a ring around the moon means rain soon.” I mentioned that halos form when the sun reflects off ice crystals found in high cirrus clouds. Because cirrus clouds are the highest in our atmosphere, they move at the fastest speed and are often the first clouds to arrive ahead of a storm system.

If you see this ring around the moon, expect the clouds to thicken and lower — and for rain to arrive within the next 12 to 24 hours.

Weather is awesome. #cwgpicoftheweek