Watch Out for Moon Halos

Watch Out for Moon Halos

This week the Moon is waxing full, which means now is the time to watch out for Moon halos. Bright moonlight shining through ice crystals in the air can create a luminous network of arcs, pillars and 'dogs around the lunar disk. James Helmericks photographed this example on Dec. 3rd from the Colville River Delta in northern Alaska:

"At 3 o'clock in the morning in northern Alaska, I went out to film the Moon," says Helmericks. "At first, the Moon was surrounded by little more than a slight corona. After a while, though, all the thicker clouds departed, leaving behind a layer of high icy clouds, and we were also engulfed in thin freezing mist. When this happened a whole new set of phenomena appeared, and I snapped the picture above."
The produce such a complex display, the clouds and mist must have contained a variety of ice crystals, with many shapes and orientations. Atmospheric optics expert Les Cowley was also amazed by the brightness of the halos:
"This ice halo display would stand out if made by the sun. Made by the very much dimmer Moon 'as black as coal' it is truly remarkable," Cowley says. "The parselenic circle crossing through the Moon (only the sun makes parhelic circles) is hugely bright. Moondogs to the left and right of the Moon are intense, too. Because everything is so bright, we can see many halos that often go unnoticced. An upper tangent arc sits atop the familiar 22 degree halo.  There is an outer more rare 46 degree halo.  We know it is that rather than the more common supralateral arc because it does not quite touch the circumzenithal arc at the picture's top."
"The temperature was -7F with 10 knots of wind, so one needed to be bundled up to enjoy the view," adds Helmericks. That's why they call them "ice halos."

From SpaceWeather.com