Blue skies: The real reason behind their colour

We see the blue sky every day, yet rarely stop to wonder why it looks that way. It's such a familiar part of life that the reason behind its colour is often taken for granted. While many assume it simply reflects the ocean, the truth lies in a fascinating interaction between sunlight and Earth’s atmosphere.

Breaking down sunlight: More than just white
Sunlight might seem white, but it’s actually a mix of many colours, each with its own wavelength. “The visible light spectrum contains a variety of colours, ranging from red to violet. When all of these colours are mixed, the light appears white,” explained Marc Chenard, a meteorologist at the National Weather Service, in an interview with Live Science.


As this white light from the sun enters Earth’s atmosphere, some colours begin to interact with air molecules and tiny particles. Each colour travels in waves of different lengths– red and orange have longer wavelengths, while blue and violet have much shorter ones, Chenard added.

The role of Earth’s atmosphere in colouring the sky

It’s the shorter wavelengths, like blue and violet, that are more likely to be scattered or absorbed and then re-emitted in different directions by the air and gas molecules in the atmosphere. These molecules, mostly nitrogen and oxygen, scatter light through a process known as Rayleigh scattering . Since blue light scatters more easily and in all directions, it’s what we mostly see when we look up.

Blue over violet: What our eyes see
Despite violet light being scattered even more than blue, we see the sky as blue. As quoted by Live Science, Ed Bloomer, an astronomer at the Royal Observatory Greenwich, explains that this happens because the sun emits more blue light than violet, and our eyes are more sensitive to blue wavelengths. Together, these factors make blue the dominant colour we notice.


Why sunrises and sunsets turn red and orange
At sunrise and sunset, sunlight has to pass through more of the atmosphere to reach us. Along the way, most of the blue and violet light gets scattered out, allowing the red, orange, and yellow tones to shine through. That’s why the sky looks warmer and more colourful during these times of day.

A colour unique to Earth’s atmosphere
As mentioned in the Live Science report, Bloomer also notes that Earth’s blue sky is the result of a very specific combination of sunlight, atmospheric composition , and scattering behaviour. On other planets with different gases or lighting conditions, the sky might look completely different. The blue we see above us, then, is not just familiar– it’s uniquely Earth’s.