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| Space on May 15, 2013

What makes a halo around the sun or moon?

Halos around the moon – or sun – are a sign of thin cirrus clouds drifting high above our heads. They are a sign of nearby storms.

A ring or circle of light around the sun or moon is called a halo by scientists. We get many messages throughout each year from people who’ve just spotted a ring around the sun or moon. People want to know: what causes a halo around the sun or moon?

Do you live by the moon? Order your EarthSky Lunar Calendar!

A planisphere is virtually indispensable for beginning stargazers. Order your EarthSky Planisphere today!

Solar halo seen May 14, 2013 in Monmouth, NJ, as captured by EarthSky Facebook friend Stacey Baker-Bruno.  Thank you, Stacey!  See more photos of May 14, 2013 photo here.

Solar halo seen May 14, 2013 in Monmouth, NJ, as captured by EarthSky Facebook friend Stacey Baker-Bruno. Thank you, Stacey! See more photos of May 14, 2013 photo here.

Moon halo, with Jupiter on the edge of the halo, seen December 23, 2012 by Danny Crocker Jensen in Wardsville, Missouri. Thanks, Danny!

EarthSky Facebook friend Randy Miller in Anderson, Indiana saw this lunar halo on the night Sandy made landfall on the U.S. mainland, October 29, 2012.

On the night Sandy made landfall, lunar halos were seen as far west in the U.S. as the state of Washington. EarthSky Facebook friend Susan Jensen in Odessa, Washington saw and photographed this delicate halo, as Sandy was carving its path of destruction along the U.S. East Coast.

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.

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.

Lunar halo – with greenish northern lights on the left – as seen on the morning of October 8, 2012 by EarthSky Facebook friend Colin Chatfield in Saskatoon, Saskatchewan, Canada.

Sun halo seen in Tucson, Arizona on September 27, 2012 by EarthSky Facebook friend Sean Parker Photography. Thank you, Sean! Click here to expand image. More from Sean here.

Sun halo seen in Washington state on May 16, 2012. Image via EarthSky Facebook friend Sean Abbasi

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.

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.

Bottom line: Halos around the sun or moon happen when high, thin cirrus clouds are drifting high above your head. Tiny ice crystals in Earth’s atmosphere cause the halos. They do this by refracting and reflecting the light. Lunar halos are signs that storms are nearby.

Learn more about halos at Les Cowley’s great website Atmospheric Optics

Sun halo on May 16, 2012 via EarthSky Facebook friend Nonya Justagirl