March 2021

A Rainbow-like Wreath in the Sky

The rings of iridescent colours that can appear around the Sun or Moon as it shines through a thin layer of cloud are known as a corona. This one spotted by Susan Carnahan (Member 52,855) was caused by patches of Altostratus cloud over Santa Cruz, Arizona, US. Alas, the name of this optical effect is these days more commonly associated with the coronavirus pandemic. The corona name for both virus and optical effect derives from the Latin word corona, meaning a ‘crown’ or ‘wreath’. In the case of the virus, this is because the proteins protruding from it appear under an electron microscope to wreath the virus body. For the light effect, it simply refers to the circular arrangement of delicate pastel colours around the Sun or Moon.

A corona is caused by the interaction between the tiny water particles of a cloud and the waves of the sunlight or moonlight shining through it. When the cloud’s water droplets (or, more occasionally, ice crystals) are particularly small and of a very consistent size, they bend, or diffract, the light in such a way that it separates into rings of blurred colours. The smaller the water droplets the more effectively they bend the light and the larger the resulting wreath of pastel hues. In Susan’s corona over Arizona, the brighter fringes near the Sun are only part of the effect. In fact, subtle iridescent colours extend right across the broad layer Altostratus. Thankfully, there’s no lockdown for the corona of the sky.

A corona of cloud iridescence caused by Altostratus clouds spotted over Santa Cruz, Arizona, US by Susan Carnahan (Member 52,855).

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