Q: I have observed what I call a smogbow. It is similar to a rainbow in that it is an arc-shaped spectrum of colours in the sky. It appears occasionally at sunset when you can see a short segment of bow either side of the setting Sun in the orange, dirty lower part of the sky. Unlike a rainbow, which you see when the sun is behind you, the smogbow is seen while looking towards (though not directly at) the Sun. Is this a phenomenon of London summer smog? And why is it seen towards, instead of away from, the Sun?
A: The description fits one of the most common forms of optical effect visible from the Earth鈥檚 surface, namely a parhelion 鈥� often termed a 鈥渕ock sun鈥� or a 鈥渟un dog鈥�.
Light passing through opposite sides of a hexagonal ice crystal 鈥� which behaves as a 60掳 prism 鈥� exhibits a minimum deviation angle of 22掳 which results in a concentration of rays deviated by angles near 22掳. The result of such a refraction of rays from the Sun by the vast number of tiny ice crystals tumbling through the upper troposphere in random orientation would be to produce a circle of light whose inner edge has an angular radius of 22掳. This is known as the 22掳 halo which, although uncommon in its entirety, is partially visible at some time on about one day in every three in southern England.
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On the occasions when falling ice crystals are oriented with their bases flat and the sun low in the sky, they can refract light to the observer only from the sides of the halo and not the top. This produces bright spots or bands at a 22掳 angular distance from the Sun (about a hand-span at arm鈥檚 length) although at higher solar elevations they will lie a little outside the main halo. Often only one will be visible, and I suspect that these are the 鈥渟mogbows鈥� referred to.
The more evenly oriented the crystals, the smaller and brighter the resultant parhelion will be, and a rather smeared spectrum is often visible. Spreading aircraft contrails can produce very even fields of ice crystals and occasionally result in startling colours.
Check the spectral order of the parhelion if it is visible 鈥� if red is nearest the sun, the phenomenon is caused by refraction. In a rainbow, caused by reflection, red is furthest from the sun.
For further reading, I recommend Rainbows, Halos and Glories by Robert Greenler, Cambridge University Press, 1980.
A: Mock suns are not confined to smoggy London sunsets: they can be seen anywhere with the right clouds. Smog simply makes them easier to see because the Sun is less bright.