A couple of weeks ago I was driving back from dropping my younger daughter off at her football training. It was about 6.30pm and the route took me almost due west through Plymouth. Ahead of me, in the sky, were (high) Cirrostratus clouds blocking out the Sun as it dropped down towards the western horizon. Either side of the sun were two small patches of “rainbow” – in other words not actually portions of full rainbow arcs but small patches of rainbow coloured light glinting through the clouds, horizontally one on either side of the sun. It was a beautiful site and, of course, when I got home I went scurrying to my books and discovered that what I had just seen was an excellent pair of “sun dogs”, also known as “mock suns” or “parhelia”. They are formed by the refraction of sublight through hexagonal ice crystals (hence the observation of high clouds). According to my copy of “The Cloud Collector’s Handbook” a matching pair of sun dogs earns me 35 points! (TCCH is like an I-Spy book on clouds…).
For more on this kind of atmospheric optical phenomenon this Atmospheric Optics website is really excellent and contains explanations and lots of wonderful images. The entry on sun dogs is here.
Tim O'Hare 