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Circumhorizontal Arc
10/18/2002
Listen in RealAudio 
Hi, I'm Bryan Yeaton. Today on The Weather Notebook Robin White talks about wierd things to
look for in the sky.
Atmospheric scientist, Paul Doherty calls himself a noticer and a teacher. He works at the
Exploratorium Science Museum in San Francisco, but he's irrepressible and everywhere he goes
he's pointing things out to people. Recently, he was at Windy Hill in Palo Alto and he spotted
a halo around the sun caused by ice crystals in the atmosphere, but at the same time he saw a
second phenomenom called a circumhorizontal arc.
PD: I was up there and I'm a teacher and an observer. Everyone I met I said, "Did you look at
that?" and pointed up to this halo around the sun and they went, "WOW! I didn't see that!",
but the ones who said, "Yeah, I saw the Halo". "Well then, Did you see that," and I pointed at
the circumhorizontal arc and they went, "Wow, no what's that?"
What it is is a band of color up in the air parallel to the horizon. It's caused by ice
crystals falling in the air and bending the sunlight.
PD: Ice crystals can have slightly different shapes. They're usually hexagonal, but sometimes
they get to be flat plates and when they fall, they fall with their faces parallel to the
ground and when the sunlight goes through those it bends in a prismatic way at a different
angle.
Doherty says he only sees horizontal arcs once every couple of years.
PD: It was neat also because I could see those people that I talked to stopping other people
and pointing it out to them. There'd be this little group and they'd all point.
For The Weather Notebook, I'm Robin White.
Robin White submits stories from San Francisco. The Weather Notebook is produced in New
Hampshire by the Mount Washington Observatory and is supported by the National Science
Foundation and Subaru of America.
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