It’s often thought it’s because of light reflected back into the sky from the blue sea. In fact, it’s the result of sunlight being scattered when it strikes the air molecules in the atmosphere.
Sunlight is made up of – literally – all the colours of the rainbow, and the complex physics of how light interacts with molecules means shorter-wavelength blue light is more strongly scattered than red light. As a result, it is blue-coloured light that ends up spread most widely across the sky, dominating its appearance.
Robert is a science writer and visiting professor of science at Aston University.