MEMPHIS, TN (WMC) - Have you ever looked up into the night sky and noticed a ring around the moon? What you were seeing is an optical illusion, caused by reflections of ice crystals in the upper atmosphere. This is called the “halo effect,” or a Lunar Halo, and it is caused by light rays diffracting around the moon.
Twenty thousand feet above your head are thin wispy Cirrus clouds. You may or may not be able to see them, but you will when the light from the moon reflects on those tiny hexagon-shaped ice crystals. The light rays coming from the sun that are then reflected on the moon are bent by the crystals 22° away from the moon. This creates a halo that has a 22° radius (44-degree diameter).
This phenomenon is most common during winter months and in the mid-latitudes, due to the frequency of systems in the cool season. This is applicable to tropical and hurricane systems too, because there are high thin clouds surrounding the outskirts of those systems.
These halos can even resemble pale rainbows, with red color on the inside, and a blue outside. The full process is due to refraction, reflection, and dispersion. They are caused by the same effect, because the ice crystals are 6-sided prisms.
So how does this predict precipitation? Cirrus and cirrostratus clouds are often forerunners for those storm clouds behind them, because they are most often the first cloud layer seen.
In rare circumstances you could see a double halo, caused by less-than-perfect crystals. These crystals wouldn’t have the hexagon shape and would rather take on a different form (such as a triangle), causing a different sized halo.
“Moon dogs” are also not the same. These are rarer and are bursts of extra light (often seen as arcs or even blobs) on the outside of the halo. Moon dogs would occur at the same time as the Lunar halo’s, but a fuller moon is necessary for this outcome.
Downfalls: Some stray cirrus clouds could cause the halo to occur, and in these cases precipitation may not occur.