What causes a rainbow near or around the moon?
There has been lots to see in the night sky recently, including the October full moon, also known as the Hunter’s Moon, during the Thanksgiving long weekend.
I have received some beautiful photos of that moon, including one from Chris Young, who was also curious about what caused the rainbow colour in the cloud below the moon.
We can link this back to similar causes of optical phenomena we see during the daytime with sunlight.
In this case, the light is encountering small water droplets or ice crystals of the same size and is causing the light to be diffracted or spread out. The moon was bright enough to allow it to occur and be partially visible in the cloud.
It’s also common to see full rings form around the moon and sun. For a moon or sun halo, the light refracts in the ice crystals found within high-level cirrus clouds and is then reflected, displaying as a white or rainbow-coloured ring which forms about 22 degrees away from the sun or moon.
While a full halo is not on display in this photo, if you spot one around the sun or moon, it can signal future weather.
According to folklore, “A ring around the sun or moon means rain or snow is coming soon.” This is often true as high clouds arrive first before unsettled weather systems.
Something else interesting to watch for is a lunar rainbow or moonbow, which is essentially just like a daytime rainbow at night but need specific conditions to form, making the chances of seeing one quite low. Thanks for your question, Chris. Have a weather question you would like answered? Send your weather photos and questions to: weather@ saltwire.com