What makes a halo around the sun or moon?........Part I

We get many messages throughout each year from people who’ve just spotted a large ring or circle of light around the sun or moon. Scientists call them 22-degree halos. Why? Because the ring has a radius of approximately 22 degrees around the sun or moon.

People always ask, what causes these gigantic rings?

There’s an old weather saying: ring around the moon means rain soon. There’s truth to this saying, because high cirrus clouds often come before a storm. Notice in these photos that the sky looks fairly clear. After all, you can see the sun or moon. And yet halos are a sign of high, thin cirrus clouds drifting 20,000 feet or more above our heads.

Eliot Herman wrote on May 5, 2018: “This shows the change that occurred over 7 minutes as a lunar halo emerged. It then persisted for about 40 minutes and disappeared with increasing clouds. Although it appears the halo is forming from an odd shape what is actually illuminated is the edge of the clouds just before the halo formed as the clouds drifted in front of the moon. But it does have a nice illusion of an odd-shaped halo then becoming round.”

These clouds contain millions of tiny ice crystals. The halos you see are caused by both refraction, or splitting of light, and also by reflection, or glints of light from these ice crystals. The crystals have to be oriented and positioned just so with respect to your eye, in order for the halo to appear.

That’s why, like rainbows, halos around the sun – or moon – are personal. Everyone sees their own particular halo, made by their own particular ice crystals, which are different from the ice crystals making the halo of the person standing next to you.

If you see a halo, notice this! Because moonlight isn’t very bright, lunar halos are mostly colorless, but you might notice more red on the inside and more blue on the outside of the halo. These colors are more noticeable in halos around the sun. If you do see a halo around the moon or sun, notice that the inner edge is sharp, while the outer edge is more diffuse. Also, notice that the sky surrounding the halo is darker than the rest of the sky.

A moon halo in Mandan, North Dakota, by Marshall Lipp. February, 2015.

Sun halo and sundogs. Captured 37,000 feet up, over Bhopal, India, on October 3, 2015.

Doug Waters in New Bern, North Carolina, caught this photo on February 4, 2015. He wrote: “High cirrus clouds from an approaching weather system presented the perfect opportunity for a halo to form around the sun, which appeared to be pierced by the contrail of a passing airliner.”

Jupiter and full moon witin lunar halo on February 3, 2015. Seen from Estonia by EarthSky Facebook friend Jüri Voit.