Residents around the Denver metro area reported seeing a ring around the moon Wednesday night and wondered what caused it.7NEWS Chief Meteorologist Mike Nelson said it is known in folklore as a sign of an approaching storm -- expected to bring snow to the metro area by Thursday evening.Astronomers say the a ring around the moon is caused by the refraction of moonlight from ice crystals in the upper atmosphere. The crystals cause the moonlight to be focused into a ring. This is similar to the way water droplets in the lower atmosphere can bend sunlight to create a rainbow.As light passes through the hexagon-shaped ice crystals, it is bent at a 22 degree angle, creating a halo 22 degrees in radius -- or 44 degrees in diameter.The ice crystals that cover the halo signify high altitude, thin cirrus clouds that normally precede a front by one or two days. Typically, a front will be associated with a low pressure system which is commonly referred to as a storm.Some people had other ideas about the reason for the ring and Twitter was busy with tweets about sightings. Our favorite tweet was from #lauren_groff: "My mom just yelled at my brother and I for suggesting that the ring around the moon was due to a future apocalyptic episode #MomsTheseDays"