photo #4705834

Nienke Lantman (Cloud Appreciation Society Member 24,009) was sharp eyed enough to pick out an atmospheric optical effect hiding among patches of Cirrus in this sky over Erm, Netherlands. It appears dead centre here, as a bright colourless section of arc with a brighter dot at one point. This is a short section of what’s known as a parhelic circle, a light effect caused by sunlight reflecting off the vertical sides of ice crystals tumbling through the sky. When fully formed, this effect appears as a complete circle of white light that extends as a ring right around the sky, level with the Sun and centred on the zenith (the point directly overhead). It only appears like this, however, if the right shapes and orientations of ice crystals happen to be covering the whole sky. More often, only a small part of the sky has clouds with the right sort of ice crystals, and so just a fragment of parhelic circle like this appears.And what about the bright spot? It is a 120-degree parhelion, a brighter point that can sometimes appear at a couple of specific positions along the parhelic circle. If you’re looking towards the Sun, through which a complete parhelic circle always appears to pass, you’d see this brighter patch of the parhelic circle when turning an angle of 120 degrees to the left or the right. The 120-degree parhelion is quite rare since it only appears when the cloud’s ice crystals are particularly pure and of a particular shape and orientation. Rarer still is for anyone to notice, as Nienke did, this subtle light effect that likes to hide among the Cirrus.