WRAL WeatherCenter Blog

At different times of the year rings are visible in the sky around the moon. I have been told they are caused by ice in the atmosphere. Why do the circles vary in size?

Posted January 24, 2007
Updated March 5, 2007

MIKE MOSS SAYS:       Bill,    Rings around the moon can be caused by ice crystals or by small water droplets in clouds that are shallow enough to allow the moon to be seen behind them, and the ring size depends in some cases on the thickness of the clouds along with the size distribution of droplets or crystals, and in the case of ice crystals, also depends on the shape and orientation of the crystals themselves.

The primary types of rings seen around the moon are the 22-degree halo, the 46-degree halo and the corona. Both halos are caused by hexagonal ice crystals in a thin sheet of high altitude cirrus clouds, with the ice crystals oriented randomly. Light that passes into one of the six faces on the side of the ice crystals and then out of a face having a 120 degree angle to the first is refracted, or bent, in a way that produces the 22-degree halo. This ring is seen at about the distance from the thumb to the tip of the little finger of a spread hand held toward the moon with your arm fully extended.

The 46-degree halo is formed in a similar way, except that the light passes into one of the six sides of the crystals and then out of either the top or bottom face. These are much less commonly seen, because in many cases a sizable portion of the crystals in a cloud have cleanly formed sides, but poorly formed ends that are not sufficiently flat and clear. As you would surmise, these rings are located a little more than twice the distance away from the moon as the 22-degree halo.

Finally, coronas are  ringlike phenomena in which a brightened disk of light appears to extend outward from the moon. This disk contains a series of prismatic colors in a concentric pattern produced by a diffraction process that yields a series of constructive and destructive light wave intensity and color modifications. The size of the corona and the pattern of light, dark and color depend on the number and size distribution of cloud droplets or ice crystals filtering the moonlight.

Some of this is easier to grasp by way of diagrams and photographs than to describe clearly in a few sentences. For more information, you may want to visit these addresses, part of an extensive site covering atmospheric optical phenomena:

http://www.atoptics.co.uk/halo/circ1.htm

http://www.atoptics.co.uk/halo/46form.htm

http://www.atoptics.co.uk/droplets/corona.htm
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