On a full moon night you may notice that everything on the ground looks brighter with the added illumination. You may also notice when you look up that you see a ring around the moon. This is normally called a halo. Why does it happen? It occurs because the light from the moon is passing through ice crystals in high cirrus clouds in the upper atmosphere. The ice crystals act as prisms and bend (refract) and reflect the light from the moon. If the circle you are seeing is closer to the moon that means that the moisture is little closer to ground. The farther the circle is away is the higher the moisture is. The clouds usually are 15 to 30 thousand feet above the surface or 3 to 6 miles.
There are many folklores attached to the halo. One of them being "Ring around the moon, rain is soon". This is usually true because when you begin to see halo that means we are seeing high clouds. Usually we see these clouds move in ahead of a storm that will bring us low clouds and rain.
This is the same phenomenon that we see from rainbows. In meteorology these are called atmospheric optics. The rainbow occurs as a refraction of sunlight from water droplets in the lower atmosphere.
Meteorologist Kristen Connolly