Stimpunks Foundation × More Realms · L★S · Field Guide No. 20
A Field Guide to Rainbows
Sixteen rainbows, sorted by what changed — the drop, the path inside it, the light arriving, the surface it lands on, the instrument at the point of the cone. Every listicle sorts these by how they look and then ranks them by how rare they are. Both sorts fail, and they fail in the same direction: rarity is a fact about where you were standing.
Five things that change
The sort is ours, and it came out of the entries rather than ahead of them. We started meaning to sort the way the genre does — by how the bow looks, from the common ones to the rare ones — and that sort collapsed almost immediately, twice over. It collapsed once because a fogbow and a rainbow look nothing alike and differ only in the size of the drop, while a sea spray bow and a rainbow look identical and differ in the chemistry of the water: appearance sorts them exactly wrong. It collapsed again because "rare" turned out not to be a property of any bow at all. A circumhorizontal arc needs the sun above 58°, which happens for about 140 hours a year in London and about 670 in Los Angeles, and never at all north of 55°N. What actually varies is one parameter at a time, and the physics is the same physics throughout. So the sections below run from changes to the drop, through changes to the light's path inside it, to changes in the light arriving, the surface the cone meets, and the observer standing at the cone's point. None of these ranks above another, which is the whole reason it is a catalogue and not a countdown.
When the water is a different water
Five bows that differ from the everyday one in nothing but the drop: how big it is, what shape the fall has beaten it into, and — in the last card — whether it is water at all. Two of these look nothing like a rainbow and one is indistinguishable from one, which is the first sign that sorting by appearance was never going to work.
When the light takes a longer way round
The tidy schoolroom rainbow — in one face, off the back, out again — counts the bounces, and this section is what happens as you keep counting. Two gives the famous double, one drop making both arcs at once. Three and four put bows on the wrong side of the sky, where they waited until 2011 for a photograph. Then the counting stops working: one entry here is a bow geometric optics cannot produce at all and never could, and the last is still not fully explained.
When what arrives has already been changed
The shortest section, and the count is information rather than an omission: the drop has far more ways to vary than the light does. Both entries here are the same move — something about the beam was settled before it ever reached the water. In the first, the atmosphere edited it on the way in. In the second, the limit was built into the lamp, and it applies to every other bow in this guide.
When the cone meets something
A rainbow is not an arc. It is a cone of directions with your eye at the point, and arc is only what you get when the ground cuts it. Change what the cone meets and the shape changes to a circle, or a hyperbola, or an ellipse — none of which is the rainbow's shape, and all of which get named as though they were.
The instrument at the point of the cone
The other four sections change something in the sky. This one changes the instrument receiving it, and both entries make the same correction: a bow reported as lacking something is not necessarily a bow that lacks anything. A moonbow carries every colour to an eye that cannot report colour that dim. A polarising lens can delete a rainbow outright. That a rainbow is anchored to your own shadow in the first place is not carded here — it is the whole subject of No. 69, and it gets a dashed card below instead of a worse second telling.
Three things we are not going to reframe
These three appear in almost every list of rainbow kinds. They get a card here because leaving them out would look like an oversight — and a dashed one with no reframe table, because the table exists to correct a misreading of a bow, and none of these three is a bow we have misread. One is not a rainbow at all. The other two are arguments rather than mechanisms, and both are made in full elsewhere. The double rainbow was briefly filed here and has been moved into section two, because its mechanism is exactly the kind of thing a catalogue is for — and the handoff survives inside the card.
Every drop in this guide is doing one thing: slowing light down, and slowing the short wavelengths a little more than the long ones. The oxygen in the water was forged in dying stars, the hydrogen has been here since the first few minutes of everything, and the calcium in the eye reading the result came from the same furnaces. The light is old and the water is old and the arrangement is about four milliseconds old.
What the sixteen have in common is not appearance. It is that none of them is a deviation. There is no master rainbow that these fall short of — the everyday one is simply the set of conditions we happen to stand in most often, and the moment you move the drop size, or the liquid, or the receptor, the same rule produces something the genre has to give a separate name and a rarity score. The naming is ours. The rule never changed.
There is no standard rainbow. And there is no standard place to stand, which is the same sentence said from the other end — and the one this guide hands to Your Shadow Is the Centre (No. 69) rather than argue twice. The argument about a single bow having two arcs is in Both Arcs, One Drop (No. 70). This guide is the catalogue for both.