Rainbows are beautiful but static decorations in the sky. They are not like meteors - inconstant, edgy entities that may suddenly decide to streak across the heavens, or fall to earth and cause a wondering Yeats to ask:
When shall the stars be blown
about the sky
Like sparks blown out of a
smithy, and die?
No, rainbows as a rule demurely appear and gently fade away, and any sudden movement is to them anathema.
Or is it? Now and then close watchers of the sky have seen a very strange phenomenon: a rainbow that "vibrates". The boundaries of the colours become ill-defined and blurred, and it seems as if the bow is oscillating to and fro. It is said to be a strange and quite disturbing sight, and meteorologists admit they do not know exactly why it happens.
We do know that it happens with a thunderstorm. It occurs, not simultaneously with the flash of lightning, but seems to coincide with the time we hear the thunder. And we believe that it is caused by a change in the size of the water drops by which the bow is formed.
Millions of raindrops, acting like tiny mirrors, form a rainbow. Rather than being reflected on the surface, however, the light passes into each drop to be reflected from the back, and as the sunlight passes through the water it is separated into its constituent spectral colours.
Moreover, the appearance of a bow depends greatly on the size of the water drops from which it forms. Very small droplets give a wide and nearly colourless rainbow, while large drops, more than about a millimetre in diameter , produce a narrow, bright bow with sharp, well-defined colours. And if the drops change rapidly in size for any reason, and then revert, the result will be a shudder of the rainbow. This is what sometimes happens in a thunderstorm.
The accepted wisdom is that the thunderstorm causes certain clusters of smaller drops to coalesce into a single larger one. One might first imagine that the sound vibrations in the air might cause the drops to merge like this with one another, but such a perceptible effect from so slight a cause is regarded as improbable.
Another theory is that the electrical discharge associated with the lightning brings about a change in the surface tension of the water drops, so that they more readily join together; but this, equally improbably, would require a close coincidence between the time for this charge to take effect and the length of time it takes the sound of thunder to arrive. The truth is, no one knows.