- The nearly-full moon was surrounded by a ring this evening, an artifact of hexagonal ice crystals in high-altitude cirrus clouds and a harbinger of bad weather to come.
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Open source, procurement, and gov tech.
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Are there ice crystal that aren’t hexagonal when in the clouds?
Yes, but they’re not particularly common. Only hexagonal ice crystals are stable. Sometimes it can be cube shaped, but it requires ridiculously cold temperatures — under -100°F, IIRC. And sometimes it can be pyramid-shaped, which is more of an extruded hexagon, with 14 or 20 faces (one on each end, and 2-3 sets of hexagons). These are not as common as hexagonal crystals, but certainly not uncommon. Hexagonal crystals have consistent angles between the faces, so the ring is always at the same apparent distance away from the moon, but pyramidal ones end up with all kinds of angles, which can lead to halos really close to or really far from the moon.
Nice photo. I noticed that tonight as well, but I didn’t get a chance to photograph it.
I just got a tripod for Christmas, so I was geeked to have a chance to put it to use. :)
A friend called me and breathlessly announced there was a huge ring around the moon.
I told him I had seen your picture online, and proceeded to read him the description of what causes it. He was interested to know the reason.
For Christmas, I asked for a tripod that would fit both my telescope and a camera, but Santa had other ideas and got me a satellite radio instead (among other things).
Very nice photo, Waldo.
Very nice photo, bad weather is indeed on its way
http://www.nnvl.noaa.gov/cgi-bin/index.cgi?page=items&ser=111634
Wow, thanks for that link, Alice — that’s quite an image.