Corona Colours over Braga, Portugal
Cloud of the Month for July is an optical effect known as a ‘corona’. That’s the name for these beautiful rings of pastel colours surrounding a white disk, centred on the sun. The colours are produced by the interaction between the sunlight and the cloud’s tiny water particles. A corona is most often produced by clouds made of water droplets, but sometimes very small ice crystals can do the trick. The sunlight is separated into rainbow colours as it passes around the cloud particles – a process known as ‘diffraction’. It turns out that the different wavelengths of light are bent by different amounts. This has the effect of separating the sunlight into its constituent colours.
The best colours appear when the cloud particles have a very consistent size, rather than a range of sizes. The width of the corona depends upon the droplet (or crystal) size. Smaller particles cause a larger corona. In fact, you can sometimes watch the size of the corona change as different clouds drift across the face of the sun.
A corona can also often be seen at night, when the right clouds are in front of a bright moon. But during the day you need to be careful not to damage your eyes. The central white disk of the corona can be very bright, so you should shield your eyes from the light. Never look for a corona without first covering the sun itself – perhaps with your hand, a tree or the edge of a building. That way, you can see the beautiful colours without frying your eyes in the process.
High Altocumulus clouds producing a corona over Braga, Portugal by Guilherme Silva.
Explaining July’s Cloud of the Month to The Weather Channel
Here, Gavin Pretor-Pinney (Member 0001) explains the optical effect known as a corona to The Weather Channel.