A few days ago, a newspaper received a photograph of a Lunar Halo from one of their readers and asked me to explain what was going on. You can click on the above link to read my answer, but it was by necessity rather short, and incomplete. So what’s the full story?
What we see in the image to the right is a full moon, surrounded by a colourful halo, with the constellation Scorpius faintly visible along the top border. The moon itself is overexposed, which shows us that the halo itself is quite dim. In fact when viewed without a camera, such halos are bright enough to be obvious, but still faint enough that the eye cannot perceive the rainbow colours.
When conditions are right, water vapour in the frozen upper atmosphere condenses into tiny hexagonal ice crystals which catch the moonlight and bend it, like countless tiny prisms. The process by which the light is bent known as refraction, and the amount of bending depends both on the angle at which the light enters the prism, and its colour. Colours towards the blue end of the spectrum bend more than those close to red, so white light will be split into all its component colours and we see a rainbow spectrum coming out the other side.
So the moonlight shines on each individual crystal, and passes through it. On its way, it refracts at an angle of around 22º and gets split into its component colours. To us, standing on the ground and looking at the Moon, certain of those rainbow beams will happen to be aimed in the right direction to hit our eyes.
Solar Halo at Kitumba, Zambia. Credit: Keith Fey Photography
Basic geometry dictates that the beams which reach us can only come from a region which appears to us as a circle around the moon. And so what we actually see is a halo, trimmed in rainbow colours.
Now, if you’ve had even the most basic scientific education in school this should all sound very familiar. It’s a very similar process to what produces rainbows although there are a few important differences. First, rainbows are produced by light passing through raindrops, not ice crystals. Second, the water droplets don’t just refract the light, but also reflect it off their internal surfaces so that the light bounces back in the direction from which it came. And third, the angle of refraction is much greater, at around 42.5º. The effect of all this is that rainbows can only be seen in rain (although the rain does not necessarily have to be falling on the observer), they appear on the opposite side of the sky from the Sun, and they’re twice the size of halos.
Incidentally, both rainbows and halos can be caused by both sunlight and moonlight. A lunar rainbow, or moonbow, is formed in exactly the same way as a rainbow, but is a great deal fainter. In fact to the naked eye, they appear as faint colourless arches. Similarly, solar halos show bright rainbow colours. Solar halos, as it happens, are surprisingly common — almost as common as rainbows. It’s an interesting fact that they are almost never seen by people who aren’t specifically looking for them. Perhaps it is because we think of the daytime sky as a featureless blue dome, compared to the sparkling glory of the night time, or maybe it’s just the sun’s intense brightness which discourages us from ever looking in the right direction?