Primary, Secondary & Rare Secondary Supernumerary
By Matt Howker near Castle Howard, Yorkshire, England with the June evening sun 6° high.
©Matt Howker, shown with permission

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"Having gone out for another sunset shoot, I noticed this very bright rainbow. What caught my eye, to start with, was the brilliance of the main bow. It was the 'golden hour' before sunset and a rain storm/shower had just passed over. I was in cloud shade which helped, I think, but this was still the brightest bow I've ever seen. The supernumeraries and secondary bow intensities changed over the 7 minutes these pictures were taken, however the main bow remained full and bright throughout and for some time after. There were a few spots of rain about so I didn't want to change to a wider lens."

A glorious optical display with supernumerary bows as the gems. Multiple supernumeraries are stacked inside the inner primary bow indicating a rainbow origin from small and uniform sized raindrops. But the rarity is the supernumerary outside the secondary bow. It is already faintly visible to the right of the secondary bow where it crosses the dark cloud. The larger of the unprocessed images shows it best. At lower right it is made more obvious by severe enhancement (levels and deep unsharp masking).

A secondary supernumerary is a very uncommon sight, look for one whenever there are bright primary supernumeraries and there is a dark cloud background to enhance contrast.

Descartes’ 1637 first scientific theory of the rainbow involved light travelling in straight lines except where refracted or reflected. It neatly explains and predicts the inner primary and outer secondary. It cannot explain supernumerary bows because they arise from waves overlapping and interfering with one another. Each angular direction of a rainbow is supplied by waves that travel via two routes through raindrops. Their paths are not of the same length and so their wave crests can be in step, partially out of step or completely out of step depending on the position in the rainbow and the raindrop sizes. In-step waves give a bright fringe, out of step waves darkness.

The monument on Bulmer Hill is part of the Castle Howard estate, the private residence for some 300 years of the Howard family. The 120 ft high monument, erected in 1869/70 by public subscription, celebrates George William Frederick Howard the 7th Earl of Carlisle.