Were you up around 11pm last night (29th December, 2020) and were you lucky enough to have clear skies overhead? If so you would have been fortunate enough to enjoy views of a spectacular phenomenon called a 22 degree halo, otherwise known as a moon ring or a winter halo.
It just so happened that last night was a freezing cold evening with perfectly clear skies here on the West coast of Scotland, and it was a full 'Cold Moon' to boot. It all lined up perfectly for us to stand outside and enjoy the best light show I've seen this year and we were lucky enough to be able to photograph it to show you in case you missed it.
What is a Winter Halo & Why Do They Form?
The winter halo is an ice crystal optical phenomenon which results in a perfect ring of apparent radius of 22 degrees, which forms all the way around the moon (or the sun).
They occur when the light from the moon, (it can happen with sunlight too) is both refracted (splitting of light) and reflected, by millions of hexagonal ice crystals which are suspended in the air in high altitude cirrus clouds which form around 20,000 feet above the earth. The ice crystals must be suspended in just the right orientation and position with respect to your eye in order for you to see the halo effect.
The Halo You See Is Personal To You
Curiously, that means that no two people will ever see the same halo; each halo, just like each rainbow, is viewed differently by each one of us through their own particular set of ice crystals, which are different to the set of ice crystals viewed by others, even standing close by.
When you look at a winter halo, like the photo we show here of the 29th December's winter halo, notice the sky inside the halo appears darker that the rest of the sky, the inner edge of the halo appears to be sharp and the outer edge appears to be more diffuse.
Another interesting feature to notice is that the moon doesn't give off very bright light, even on a full moon in a clear sky, this results in winter halos around a cold moon appearing to be largely white often with no other colours. In contrast if you are lucky enough to see a halo around a wintery sun, you may notice more red colour on the inner edge of the halo and more blue colours on the outer edge of the halo.
There's an old farmer's saying "ring around the moon, rain soon" and it's likely to be true more often than not, as the high altitude cirrus clouds, which are responsible for refracting the moon or sun light and forming the halo, often precede a storm.
If you also saw the December winter halo and photographed it, please share your photos, we'd love to see your images of this beautiful phenomenon.