What is the halo moon?
In the study of meteorology, the term halo can refer to several optical phenomena whereas sunlight or moonlight is being either reflected or refracted (or a combination of both) by high altitude cirrus clouds, resulting in bright rings.
A ring of light 22 degrees from the moon (or sun) is the most common type of halo observed from earth and is formed by millions of hexagonal ice crystals with diameters less than 20.5 micrometers within the clouds.
A halo around the moon is often a warning that stormy weather is coming. The ice crystals that create halos come in cirrus clouds, which often come before warm fronts or other weather systems that can bring rain.
—The Fact Monster