Tuesday, 19 May 2009

Rainbows, Moonbows and the Alexander Dark Band Effect

Driving over the Sydney Harbour Bridge on a stormy afternoon recently, the sun broke through momentarily and a brilliant double rainbow formed, arcing east over the city toward the sea. The clouds behind were nigh on black so the twin bows stood out starkly against this background.

We are all familiar with this glorious phenomenon of nature and appreciate them from the time when we were kids and marvelled the first time we saw one. My mind turned to thinking about them when at sea when most times they are more spectacular than when on land.

The first thing to notice about twin bows is that the colour spectrum of the outer one is the reverse to the inner one - this is a result of the refraction process. The lesser known thing is that the band in between the two bows is slightly darker and appears to flatten out the colour of the background clouds. This is known as the 'Alexander Dark Band', the 'Alexander Band' or the 'Alexander Effect'. Alexander of Aphrodisius was the first to describe this effect way back in 200AD and today it carries his name.

Because the raindrops have already been refracted to form the reflected second rainbow, all the rain in between cannot reflect any light to the observer, so that band appears darker. Next time you see a double rainbow take a close look and you will see the 'Alexander Dark Band' effect which also tends to flatten out the colour in that banded area.

This brings us to the far more rare phenomenon of a Moonbow. They are observed far less frequently and to view a good one, very rare indeed. The cause and structure is identical to a rainbow except that they occur at night using the light from the moon. They look exactly the same as a rainbow except they are in black and white and grey, no colour. Having observed a near flawless exhibition (see extract below) in a South Seas squall, I can confirm this. The bands are quite sharply delineated in their various shades of grey, dark to light, but no colour. This is because night light does not have sufficient strength to activate the human eye colour cone receptors.

To find more technical data on these fascinating and spectacular natural manifestations you can go to http://www.atoptics.co.uk/

Meanwhile, the following is an extract from my ebook 'Voyage of the Little Ship 'Tere Moana' downloadable from my website for sailors http://www.sailboat2adventure.com/

'...........It is on one such other similar moonlit night watch, the captain, glancing over his shoulder into a disappearing rain squall is transfixed by a rare sight. There, sitting dead centre of the fast retreating raindrops, and between them and the moon behind him, is a perfect Moonbow. This rare phenomenon is created exactly the same way as a daytime rainbow with this particular one forming a perfect hoop from sea surface to sea surface, absolute even brightness all the way round, with the bands quite distinctly separated.

They are not in colour, just various shades of grey matching the daytime rainbow band sequence. Night light has not sufficient strength to activate the colour cone receptors of the human eye so they don’t recognise colour. Staring with wonder, it goes through the captains’ mind to rouse the others, but rather selfishly decides to let them sleep on. If he woke them and they were anything less impressed than he, then it would diminish his own satisfaction and fulfilment of witnessing one of natures’ marvels – what a privilege to view this unique spectacle.'

Images reproduced courtesy atoptics and Wikipedia

You can read more about weather patterns, reading and interpreting the weather whilst cruising/passagemaking in my ebook 'Voyage of the Little Ship 'Tere Moana' downloadable from my website http://www.sailboat2adventure.com/ - see you there.

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