While out walking last week we noticed what at first glance appeared to be a rainbow forming just before dusk. However there was no rain anywhere near us and having studied the image above a little closer it has become apparent that there was more going on than first met the eye. Some intense googling later has led me to a series of similar images which identified the features as a twenty two degree halo, upper tangent arc and the hint of at least one sundog. I’ve labelled them up below for clarity.
The main feature is obviously the halo and is formed as a result of the sun shining through ice crystals contained within the thin clouds which were prevalent at the time. Apparently it is always the same size no matter its position in the sky and by stretching out your hand and holding it at arms length your thumb should sit over the sun with your little finger against the halo. Interesting stuff I’m sure you’ll agree.
The upper tangent arc is formed in a similar way but this time relies on the presence of hexagonal rod-shaped ice crystals drifting with their long axes orientated horizontally. Its overall shape varies depending on the height of the sun and we caught it at just about its most distinctive here.
Finally we come to the sundog which, unsurprisingly, is again caused by light shining through ice crystals. I’ve seen these before but on those occasions they were isolated features in the sky. If only we’d known we could have climbed the hill and got an even better view of it and its partner on the opposite side of the sun.
If you fancy reading a little more on the subject there is plenty of material out there so delve in and remember to keep your eye’s to the sky. These features are fairly common apparently but are easily overlooked.