photo #7186732

This Cirrostratus cloud has found something to be happy about. Its colourful smile is the optical effect known as a circumzenithal arc, which was spotted here over Newcastle, Australia by Jennah Haddock (Member 49,413). To separate the sunlight into this geometric arc of colour, the cloud’s ice crystals have to be just right. They must be clear like glass and in the shape of perfect hexagonal plates that act as microscopic prisms in the sky. And for the optics to work, the ice crystals also need to be arranged horizontally, like leaves falling in autumn. No wonder the cloud’s looking pleased with itself. Since a circumzenithal arc only ever appears way up above the Sun, with its curve centred on the point directly upwards, it is missed by most people. Only occasionally, when a sharp-eyed cloudspotter like Jennah is around, does this smile of light high in the sky cause a real one to form miles below.