On holiday the sky may look a deeper shade of blue than even the clearest summer day at home. Some places, including Cape Town in South Africa and Briançon in France, pride themselves on the blueness of their skies. But is there really any difference?
The blue of the sky is the product of Rayleigh scattering, which affects light more at the blue end of the spectrum. The blue we see is just the blue component of scattered white sunlight.
High humidity and dust, as well as smoke and pollutants, also scatter light, but over a wide range of wavelengths. This is known as Mie scattering and results in a whiter or milkier shade across the sky. At higher altitudes there is less atmosphere above to scatter light, so the sky is darker and the blue may appear more intense.
Spectroscopic studies show there are differences in different parts of the world but there has been no thorough global survey.
Antarctica is the likely winner for deepest, most saturated blue sky, as it combines high altitude with extremely low humidity and pollution. Visitors are often struck by the sapphire-blue quality of the Antarctic sky. Contenders for second place include the Atacama desert in Chile and the Tibetan plateau, both notable for their altitude and dryness.
