From an article in The Economist, June 30, 2015:
During our summer in the northern hemisphere, we may get cloudless azure skies, some of the time at least. What makes the sky this colour?
For scientists, the answer is relatively straightforward: Rayleigh scattering. When white light from the Sun reaches the Earth it hits the gas molecules that make up the atmosphere. These molecules—mainly nitrogen and oxygen—are smaller than the wavelengths of light in the visible spectrum and so scatter the light. White light is made up of different wavelengths, which, since Isaac Newton’s experiments with prisms in the 17th century, we think of as a spectrum of seven different colors: red, orange, yellow, green, blue, indigo and violet. Light at the violet end of the spectrum travels in shorter, tighter waves, which are affected more by the molecules in the atmosphere than the longer, lower-frequency waves at the red end. This phenomenon is named after Lord Rayleigh, the British physicist who discovered it in the 19th century. The sky appears blue because shorter wavelengths are scattered more by the atmosphere than longer wavelengths; so the scattered sunlight that reaches our eyes when looking at the sky (rather than at the Sun itself) is predominantly blue.
Once you have got your head around that, it might seem that the answer to the initial question—“Why is the sky blue?”—is rather simple. But there is a catch: not everyone would agree that the sky is blue. In 1858 William Gladstone, better known for being the Prime Minister of Britain four times during the 19th century, published a treatise on Homer. He noted, with astonishment, that the Greek poet did not once use the word blue. He used color words rather oddly—he described the sea as “wine-dark”, iron as violet and honey as green. Further research showed that the Koran, the original Hebrew Bible, the Icelandic sagas and the Vedic hymns, written in India between 1500 BC and 1000 BC, also lack references to this hue, even when talking about the heavens. There are still many languages today that do not have a word that precisely correlates to the English word for the slice of the spectrum between green and purple. Russians might call the sky either goluboe (light blue) or sinee (darker blue); in Japan 青 (ao) encompasses the color of the sky but also apples and grass; the Namibian Himba tribe would describe the sky as zoozou, which roughly translates as “dark” and includes shades of red, green and purple as well as blue.
This is more than a pedantic issue of translation: evidence suggests that language has a huge impact on how people interpret the world. Incredible as it may seem, having a distinct word for a color reinforces and amplifies the perception of it as distinct from other shades. Without the word you don’t perceive it as readily. To prove this scientists showed groups of colored tiles to the Himba, who found it difficult to pick out one blue tile from a group of 11 green ones (although they found it far easier than English-speakers to spot one yellow-green tile hiding amongst some more pine-hued ones). So although it is true that to English-speakers the sky is blue, it is arguably only blue because they say it is.