Rainbows

Tuesday 19th April, 2016

We were happy to welcome back Mike Frost to come and talk to us all about "Rainbows". Not only is he a member of Coventry and Warwickshire Astronomical Society but he is Director of the British Astronomical Society's Historical Section and loves to travel the world to view solar eclipses. He has visited us on several occasions to give us informative and enthusiastic presentations ranging from historical solar eclipses viewed from Warwickshire to the rare green flash of light seen as the Sun disappears below the horizon. On this occasion he came to educate us about where to look for rainbows, the different types and how they are formed.

He began his talk by asking the audience for a show to hands as to how many rainbows they had seen. Many hands were held aloft when he proposed one rainbow. Then fewer were held high when he asked how many of us had seen a double-arched rainbow or double bow. Finally, he asked how many had been lucky enough to see a triple rainbow and only several people were able to raise their hands this time.

He said that every rainbow, no matter how many arches or bows it shows, is unique to its viewer as a rainbow is a virtual image generated by our eyes or a camera. Two people could be standing side by side and yet each would have their own personal rainbow formed by light rays converging on their retinas. The rainbow is not actually "out there" in the sky but is an image formed at the back of our eyes due to our eyes' lenses focussing the incoming coloured light rays.

It turns out that wherever there is moisture in the air in the form of droplets and a light source there is the possibility of a rainbow. Many gardeners will have seen partial rainbows form when using a hose to water a sunny garden but there are also Moon bows, fog bows. These two latter types usually lack the colours of a normal rainbow as moonlight is less intense than sunlight and the water drops in fog bows are much smaller and so do not split up the light into its different colours.

Early morning and late afternoon in spring and autumn are the best time of day and seasons to see rainbows as the Sun is then not too high in the sky. For rainbows form directly opposite the Sun with their centres at the antisolar point, which is a point marking the centre of the Sun's disc but on the opposite side of the sky. If the Sun is too high then the rainbow's image would effectively be below the horizon with the Earth itself getting in the way.

If the Sun is behind you and you are looking towards a rain shower Mike recommended a procedure to increase your chances of spotting a rainbow. He suggested looking for the shadow of your head and then extending your arms forwards and outwards making two diagonal lines. Ideally, your arms should make an angle of roughly 42 degrees but halfway between your sides and straight ahead would be close enough. Perhaps even rarer than a triple bow is a rainbow that forms a whole circle. Mike commented that you have to be extremely lucky to view such a thing as you need to be in a helicopter or plane flying between a low Sun and a rainstorm.