Anomalous rainbows

Although the scientific theory of the rainbow is more or less complete, it has to be applied cautiously to natural rainbows. The idealised assumptions on which the theory is based seldom, if ever, exactly match the conditions in which rainbows are formed in nature. Hence, very occasionally, you may see, or hear about, a rainbow with features that appear to defy explanation. Such bows are known as anomalous rainbows.

Broadly speaking, anomalous rainbows fall into three broad categories.

• Most reports of unusual rainbows are the result of a lack of knowledge of the full range of optical phenomena in nature. Circumzenithal arcs, circumhorizontal arcs (section 7.7), and glories (section 6.4), are all multicoloured arcs that are frequently mistaken for rainbows by inexperienced observers.

• A second source of anomalies is the result of careless observers who report incorrectly the details of what they see. Over the years this has given rise to a pernicious body of phenomena that cannot be explained, or easily dismissed because the observer is no longer around to establish the correct details of what was really seen.

• Finally, from time to time, genuine anomalies are seen that challenge our understanding of rainbows. In these cases, it is usually a matter of accounting for small, though important, details of the circumstances that would be necessary for such an unusual feature to occur.

Reflection bows, particularly those limited to a short vertical shaft next to the foot of the primary bow, are probably the type of bow most often mistaken for an anomalous bow.

The major difficulty with genuine anomalies is not that rainbow theory cannot explain them; the problem is usually how to account for the circumstances that would give rise to such a bow. R.W. Wood, who was an eminent authority on optics, claimed to have seen a rainbow of variable curvature 'the left half appearing to be pulled out into an arc of greater curvature'. Similar bows have been seen on other occasions. It is known that the diameter of a rainbow depends on the size of the raindrops in which it is formed. So the change in curvature could be explained by assuming that the average diameter of drops in which one part of the bow was seen was not the same as that in which the other part was seen. The difficulty here is to explain how drops can be significantly larger in one part of a shower than in another.

Another unusual observation is that the bow appears to vibrate, with colours fading and reappearing in rapid succession. This has been seen very rarely and may be due to the effect of the shock wave of a lightning stroke on the shape of the raindrops. Recent investigations into the effect that the shape of raindrops have on the appearance of rainbows seem to support the view that a shock wave from a lightning stroke may cause larger raindrops to oscillate. As a drop oscillates, its cross-section changes. A rainbow ray emerges in the usual direction only when the cross-section of a drop is spherical, or nearly so. So, as drops oscillate, portions of the bow may periodically fade from view. However, many details remain to be worked out and a complete explanation for these oscillating bows has yet to be given.

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