Tuesday, 11 October 2011

What makes a halo around the moon?



Halos around the moon – or sun – are a sign of thin cirrus clouds drifting high above our heads.
Photo credit: normalityrelief
Sometimes you look up on a clear night and see a huge circle, or ring, of light around the moon. This circle is called alunar halo. We get many messages each month from people who’ve seen rings around the moon. They’re pretty common, but they’re so mysterious looking that people often express amazement upon seeing them.
Notice in the photos that the sky looks fairly clear. After all, you can see the moon. And yet halos are a sign of high thin cirrus clouds drifting 20,000 feet or more above our heads. These clouds contain millions of tiny ice crystals. The halos you see are glints of light from these ice crystals, which 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 moon – or sun – 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.
The lunar halo photo at right is by master sky photographer Dan Bush.
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.
By the way, 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.