Ever seen a ring appear around the moon in the night sky? Ever wonder what that sometimes occurs?
Really, the meteorological setup for a ring to become visible around the moon is pretty simple. The key ingredient is having a layer of cirrus clouds streaming high overhead.
Cirrus clouds are high-level clouds, generally located at least 20,000' in the air. They are composed of ice crystals where super-cooled water droplets have frozen.
When you have a bright moon in the night sky, the light from the moon is bent as it passes through the ice crystals in such a way that a visible ring is formed. This is a similar phenomena to light passing through a lens.
Also, as a fun aside, often times when you see a ring around the moon, it can be an indication that a storm system will arrive within the next 24-48 hours. Reason being, cirrus clouds often precede incoming areas of low pressure.