Sailing aboard the Swan Hellenic exploration ship SH Diana in the Antarctic we were lucky enough, on one occasion, to witness this halo around the sun.
We were on a Zodiac cruise when we looked up at the sky and spotted a coloured halo around the sun. It was as if a rainbow was surrounding the sun.
The phenomenon is technically not a rainbow. However, just like its colourful counterpart, it is caused by sunlight refraction.
Mixed with ice crystals
Sunlight mixed with ice crystals at just the right altitude and angle, can produce these stunning displays of atmospheric optics.
Some people might call it a rainbow around the sun, but it is actually caused by ice crystals.
Raindrops are spherical, but ice crystals have six sides, similar to snowflakes. This refracts sunlight at a different angle.
This rare optical phenomenon occurs when light from the sun passes through ice crystals in the atmosphere. This causes the light to refract and form a ring around the sun. The ice crystals need to be aligned by uniform winds in the upper atmosphere.
The ice crystals need to be perfectly aligned to refract the light at the right angle.
Absolutely beautiful
Rainbows are absolutely beautiful. But they are somewhat boring when compared to other kind of atmospheric optic phenomena. That is because the raindrops are spherical. And so, the angles are always going to be the same. No matter what angle that raindrop is oriented in.
Ice crystals, on the other hand, are of different shapes and sizes. And that is how a halo is formed and also explains why a sun halo is such a rare phenomenon.
Just like rain, sunlight will go into the ice, and it will be refracted. So, the red and blue light will be refracted at slightly different angles. And in this case, because it’s ice, it happens to be at a 22-degree angle.”
Those crystals are in all sorts of angles and orientations. That means you get the light coming from all around the sun rather than, say, one particular spot.
That’s how you get a halo that encircles the sun, provided those icy clouds surround it. The ring sits at a 22-degree angle, and it’s how that ring gets its name: A 22-degree halo.
