The gist of the BBC News article 'Incredibly rare rainbow' photographed in Highlands is that while it has several familiar features, there are aspects that are still unexplained.

Question: Are there really aspects of this particular multiple rainbow that are currently unexplained by science?

update: there is now a tweet about it

rainbow at Loch Lochy in the Highlands

The rainbow was photographed at Loch Lochy in the Highlands, SAXAPHONEJAN/BBC WEATHER WATCHERS

Unusually, the colours of the third rainbow are in the same order as the main one it was attached to.

BBC Weather presenter Simon King said: "This has scratched a few heads as this is something incredibly rare."

The meteorologist said the rainbow had the characteristics of both of what are known as a twinned rainbow and supernumerary rainbow.

Supernumerary bows are formed by small, similar-sized raindrops which then create ripple-like rainbows.

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    $\begingroup$ Re Are there really aspects of this particular multiple rainbow that are currently unexplained by science? Too short to be an answer: No. Too rude to be an answer: Some of the so-called meteorologists at the BBC are not that smart. $\endgroup$ Jul 19 '19 at 10:34
  • $\begingroup$ @DavidHammen the article says "On Twitter, the presenter has asked: 'Is there a professor of rainbows out there that can help?'" twitter.com/bbc5live/status/1152107473941278720 $\endgroup$
    – uhoh
    Jul 19 '19 at 11:10
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    $\begingroup$ Whenever you have a question about optical interactions between water and air, there's only one site you have to go to, and that is not this site. It is Wes Cowley's atmospheric optics site. It has very nice and detailed explanations of primary rainbows, secondary rainbows, supernumerary rainbows, and many other related phenomena. Wes Cowley is British; the BBC has zero excuse for calling this unexplained. $\endgroup$ Jul 19 '19 at 14:04
  • $\begingroup$ @DavidHammen okay I'll spend some time there this weekend, thanks!!! $\endgroup$
    – uhoh
    Jul 19 '19 at 14:08
  • $\begingroup$ @David Hammen: And WRT "incredibibly rare", not in my experience. Fairly uncommon at most, at least hereabouts. (East side of Sierra Nevada mtns.) $\endgroup$
    – jamesqf
    Jul 19 '19 at 18:25

There's nothing inexplicable about that photo. It's got a few uncommon features, but nothing unknown. In the image below, I've drastically distorted the brightness curve to make things more obvious:

enter image description here

  1. Primary arc. This is the classic "internal reflection" rainbow that you learn about in school.
  2. Secondary arc. This one forms when light is reflected twice within a raindrop, broadening the bow and reversing the order of the colors.
  3. Supernumerary arcs. These are formed by the wave nature of light: light, even of a single color, spreads out as it passes through a raindrop. The arcs are interference patterns formed by the spread-out light from many small raindrops, and because of this spreading, they're usually pastel colors rather than the single-frequency colors of the main arcs. The less variation there is in raindrop size, the sharper the arcs are; these are especially sharp.
  4. Supernumerary arcs on the secondary bow. These are rarely seen, because they're even more sensitive to variations in raindrop size than the primary arc's supernumeraries.
  5. Possible fifth-order bow. This faint green patch is in the right place to be the green stripe from light reflecting five times as it passes through a raindrop. Fifth-order bows are almost never seen because of how faint they are, and are photographed even less often, so this may just be an image artifact.
  6. Primary glow. Only the light passing through the edges of a raindrop forms the rainbow. Most of the light passing through the middle just goes straight through to form the zero-order glow (not pictured), but some reflects back to form a glow in the middle of the rainbow.
  7. Alexander's dark band. The light making up the rainbows has to come from somewhere, and this band is where it comes from.
  8. Reflected rainbow. This isn't a reflection of the primary arc (rainbows aren't real objects, so they don't have reflections), it's the rainbow you'd see if your position had been reflected across the plane of the water.

All of these have been known to science for quite some time. The primary arc was the first one to be explained, by Persian astronomers in the 1200s. Supernumerary arcs were the last one to be explained, by Thomas Young in 1803.

The "third rainbow" the BBC is talking about is probably the first few supernumerary arcs of the primary arc. Supernumerary arcs have the same color ordering as the arc they're attached to, but because the brightness and width of the arcs varies, and because they're usually pastel colors rather than spectral colors, it can give the appearance of a reversed spectrum.

The "third rainbow" isn't a twinned rainbow, because the second arc of a twinned rainbow (caused by two rainstorms at different distances with different raindrop characteristics) wouldn't be concentric with the first arc.

  • $\begingroup$ Wow, thank you for the "rainbow cataloguing"! This is really helpful. As far as what is supposed to be unexplained: "Unusually, the colours of the third rainbow are in the same order as the main one it was attached to." and later in the article "'...you'll notice that inside the main rainbow there is another rainbow on the inside almost attached with the colour order the same as that of the primary.'" is 3 having the same color order as 1 unusual, or is it actually standard? $\endgroup$
    – uhoh
    Jul 23 '19 at 22:07
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    $\begingroup$ In this photo (feel free to use this if it helps) they also appear to be in the same order. $\endgroup$
    – uhoh
    Jul 23 '19 at 22:08

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