(Image: Esther Hevens) The multicoloured halo surrounding the peak of this dark cloud looks heavenly, but it's actually an iridescent pileus cloud, also called a cap cloud. These smooth, round clouds form on top of a puffy cumulus cloud when it rises into higher, colder air.
The pileus cloud is made up of uniformly sized water droplets that diffract sunlight, creating a rainbow of colours. Usually, the effect is outshone by the brightness of the sun, but in this photo taken in Ethiopia, the darker cloud is helpfully blocking the glare to reveal the spectrum of light behind.
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