Rare rainbow phenomena

In December 2012, Deborah Carter of Maun, Botswana, was visiting a friend in Cairns, Australia, when the “sky started to darken and the wind got up, and we knew we would have to quicken our step in order to escape the incoming rain. So we walked a little faster, and then I noticed a rainbow forming.” Carter snapped a few photos of the sight with her iPhone and later asked me about their details. What especially fascinated her was the “difference in the color of the sky below and above the rainbow.”

In contrast
What Carter had actually seen was a bright double rainbow. And one of the most notable characteristics of such a sight is that the sky between the primary and secondary arcs appears darker than regions surrounding it. Alexander of Aphrodisias first described this curiosity in a.d. 200 in his commentary on the fourth volume of Aristotle’s Meteorology. Today, skywatchers continue to rediscover it whenever two rainbows appear at once.

Now popularly known as Alexander’s Dark Band (after Alexander rather than a ragtime ensemble), this cheerless segment of the daytime sky’s most colorful and uplifting display is largely a contrast phenomenon. In addition to the raindrops that form the two spectral rings, there are other raindrops that send light rays either toward the center of the primary bow or to the outside of the secondary bow. This leaves a region largely devoid of reflected light, which looks dark in contrast to the two light regions.

Rainbows are concentric arcs of multicolored light that appear in rain-washed skies and are centered directly opposite the Sun at a place known as the “antisolar point.” The brightest displays associated with Alexander’s Dark Band have primary bows 42° from the antisolar point and secondary bows that stretch 51° away from it. The highest bows occur at sunrise and sunset. The primary always has blue/violet light on the inside and red light on the outside, while the opposite is true for the secondary bow.

Radial streaks

Aside from the phenomenon Carter described, one of her images also caught my attention because it captured an uncommon occurrence — a “rainbow wheel” or “spoked bow” — one that I have yet to see myself. Let’s look closely at this rare sight.

In her close-up shot of the primary bow, Carter recorded dark and light radial streaks like the spokes of a wheel descending from the primary toward the rainbow center.

“The darker spokes are produced by small clouds outside the rainbow and nearer the Sun that cast long shadows across the sky,” explains Les Cowley, a retired physicist and an expert in atmospheric optics (www.atoptics.co.uk). “Their shadows are normally invisible, but when they block sunlight from reaching rainbow-forming raindrops, we see them as dark spokes. The spokes can be even more dramatic when seen in real life or on video, for they sometimes rotate around the rainbow, turning it into a wheel.”

Paying attention to the sky, even in the daytime, can yield views of rare events like wheels spinning in the sky. While we are all familiar with regular, single rainbows, different weather and atmospheric conditions make the variants of this phenomenon fascinating to observe.

As always, let me know what you see and think at someara@interpac.net.