A viewer writes: “I saw the biggest, most beautiful ring around the moon long before dawn this morning… almost like a “sun dog”. What makes this ring? What is it reflecting light from? Is it a portent of a storm, or rain, as so many used to say?”

moon halo 2

Actually, likening it to a sun dog is not a bad comparison. What you saw was a moon halo or a “moon dog.” Most of these rings occur at an angle of 22 degrees from the moon, although there are rarer versions that can occur at twice that angle, usually in conjunction with an inner 22 degree ring.

moon halo physics

Why is the angle so consistent from one moon halo to the next? The reason for that has to do with how halos are formed. They occur when light coming from the moon passes through ice crystals that form at very high altitudes. Typically, the crystals are hexagonal in cross section, and when the light passes through them, the mathematics works out in such a way that the halo appears to be 22 degrees away from the moon. If it is an especially bright moon, and the crystals are clear enough, you can get some double bounces inside of the icy prisms, and you can make out a fainter additional halo out at 44 degrees from the moon.

moon halo 3

So what does it all mean weather-wise? Since the halo is caused by moonlight refracting through ice crystals, it means that the upper level of the atmosphere must have some moisture in it… even if the skies appear to be clear otherwise. High level moisture often creeps out well ahead of storm fronts, and therefore moon halos (and sun halos in all their forms as well) are often a sign that a storm might be coming in the next couple of days. It’s not fool proof, but in the absence of a satellite loop, it can be helpful.

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