With the last full moon of 2012 I was flooded with photos of moon halos. These halos or rings around the moon are not very rare at all but often when people see them for the first time they are amazed. They form through a pretty simple process thanks to high cirrus clouds at around 20-30,000 feet. Cirrus clouds by their nature are made up of tiny ice crystals or super cooled water droplets. When these particles form hexagonal shaped ice crystals then we are in business.
These six-sided ice crystals act like tiny prisms and refract the incoming moonlight or during the day sunlight. Then due to the shape of the ice crystal you get a 22° halo or ring around the light source. In this case the moon or in some cases the sun.
The key is the hexagonal shaped ice crystal:
If you get really lucky you can sometimes see a secondary halo or ring around 44° away. Now that is a rare treat indeed. Because moonlight is not as intense as sunlight this ring or halo is usually white in appearance. On rare occasions and during the day when it’s around the sun this halo can appear to be a rainbow.
There is an old wives tale that when you see a moon halo snow is on the way. This really isn’t entirely true but is based on a bit of actual meteorology. When you have high cirrus clouds moving in you typically have a storm system approaching. The high level clouds move in first and thus you tend to get halos before a storm system approaches. Usually though it’s just rain and not snow because what are the chances that a full moon, cold air and an approaching storm system all line up at the same time? Not very likely but you can bet a weather change is on the way the next time you see a moon halo.
Great shot of Jupiter as well near the edge of the halo!