Corona – A Perfectly Imperfect Effect

If you see concentric rings of colour around the Sun or Moon, you’re likely spotting the optical phenomenon known as a corona. This is an effect caused by the way light waves bend as they pass around tiny obstacles in the sky such as the droplets in a thin layer of cloud. But the droplets have to be just right for the effect to appear.

The rings of iridescent hues that make up coronas form only when the droplets (and, more rarely, ice crystals) in the cloud layer are not only minuscule but also very similar in size. The droplets can be like this when the cloud’s just forming or it’s dissipating away. That’s when formations like the Cirrocumulus spotted over Colfosco, Italy by Paulus Hochgatterer (Member 30,413) lead to the most vivid corona displays. Since the different wavelengths of light bend, or diffract, by different amounts as they pass around the tiny obstacles of the cloud droplets, the light is dispersed into beautiful rings of colour. They often look like perfect concentric circles of colour, but Paulus’s corona is a little bit wonky, isn’t it?

The regularity of a a corona’s shape is all to do with how the size of the droplets varies across the length of the cloud. Only when the droplets are around the same size throughout the cloud does the corona appear evenly circular. The smaller the droplets, the larger the rings of light appear. Slightly wonky coronas like Paulus’s happen when the size of droplets varies from one part of the cloud to another. Paulus’s Cirrocumulus must have an even distribution of droplet size for the rings of light to bulge out like this. Clearly, the droplets are slightly larger off behind the mountain and slightly smaller up towards the top of his image.

Such irregularities and distortions make coronas feel quite different from the geometric precision of optical effects like ice halos that can appear when light shines through a cloud of prism-like crystals. Who wants precision and regularity? We embrace the Japanese aesthetic of ‘wabi-sabi’ (侘寂) that finds beauty in imperfection, and we say long live wonky coronas!

Corona in Cirrocumulus stratiformis spotted over Colfosco, Italy by Paulus Hochgatterer (Member 30,413). View this image in the photo library.