A ring stood round the sun over Kenwood on Saturday. Two bright spots flanked the sun this week. Both are made of ice, and both sit exactly as far from the sun as the other.
Start with a trick you can use tonight. Hold your arm straight out and spread your hand. Thumb tip to little-finger tip covers about 20 degrees of sky. That span, near enough, is the radius of the ring in the photograph.
The size never changes. Not with the season, not with the weather, not with where you stand. Every ring like this one sits about 22 degrees out from the sun. Work out why, and you have the whole thing.
First, the ice
Five miles up, cloud is ice rather than water.
Those crystals grow six-sided, like a pencil. That shape is what fixes the size of the ring.
Now, the wedge
Take a six-sided pencil. Pick one flat face, skip the next, and take the one after that. The two faces you chose lean away from each other at 60 degrees.
A 60-degree wedge of clear material is a prism. Light bends when it enters and bends again when it leaves. Anyone who has held a glass prism to a window has seen it.
Why the light piles up at 22 degrees
How far the light bends depends on the angle the crystal happens to be sitting at. Up in the cloud the crystals sit at every angle, so light leaves them at all sorts of angles. You might expect a smear of light rather than a ring.
Two things stop that.
The first is a floor. Turn the crystal any way you like, and the light will not bend less than about 22 degrees. It can bend more. It cannot bend less.
The second is what happens near the floor. Tilt the crystal a good deal, and the bend hardly changes. A whole range of crystal angles all send light out at close to 22 degrees. Think of a ball in a valley. It can sit anywhere on the slopes, but it settles at the bottom, and the bottom is flat and wide.
So light spreads thin everywhere else and stacks up at 22 degrees. That stack is the ring you see.
Why the inner edge looks rusty
Red light bends the least and blue light the most. The gap is small, about a degree, but it is enough. Red lands on the inside of the ring and blue on the outside.
That is the reverse of a rainbow, where red rides the outer edge. Look at a bright halo and the inside rim carries a faint rust colour.
The two bright spots are the same ice, hung differently
Sun dogs sit to the left and right of the sun, at that same 22 degrees. The books call them parhelia. The ice is the same and the wedge is the same. What changes is how the crystals hang.
A full ring needs crystals tumbling every which way. Every angle gets covered, light arrives from all round, and you get a complete circle.
Sun dogs need flat crystals, wide and thin, drifting down level. Picture leaves that have settled flat on a pond. They all face the same way, so instead of a ring you get two bright patches, one on each side.
Does a ring mean rain
Sometimes. The high ice cloud that makes halos often runs a few hundred miles ahead of a warm front, so a ring can turn up a day before the weather does. The old saying has a real mechanism under it.
That cloud also drifts over on plenty of days that stay dry. Treat a ring as a hint. Our dashboard carries the actual forecast.
One more to look for
There is a second, bigger ring. It uses a different pair of faces, meeting at a right angle instead of 60 degrees, and it sits about twice as far out. It is fainter and much rarer. Most people who see one had no idea to look.
Both photographs were taken in Kenwood this week. If you catch one, send it to [email protected] and we will run it.