Why the sky is blue and sunsets are red

An afternoon sky can look blue overhead while the setting Sun colours the horizon orange. Both views begin with sunlight, a mixture of visible wavelengths, and both depend on what happens to that light before it reaches your eyes. ¹

The distinction is between light arriving along a fairly direct route and light redirected towards you from elsewhere in the sky. That difference explains why the sky and the sunlight illuminating the ground need not have the same colour.

The light arriving from the side

When you look at a patch of blue sky away from the Sun, some of the sunlight has changed direction on its way through the air. Molecules scatter it, sending a small amount towards you. In the conditions described by Rayleigh scattering, shorter visible wavelengths are scattered more strongly than longer ones. The blue part of sunlight therefore contributes disproportionately to the light coming from those patches of sky. ²

This wavelength dependence is steep: scattering varies inversely with the fourth power of wavelength. Using representative wavelengths of 450 nanometres for blue and 600 nanometres for red, that relationship gives about 3.2 times as much scattering for blue. This is a comparison within the Rayleigh model, not a claim that every blue sky is exactly 3.2 times brighter than a red one. The light reaching an observer also depends on viewing direction and how much air it has crossed. ²

Why it does not look violet

Violet has a shorter wavelength than blue, so it is reasonable to ask why the whole sky does not look violet. Scattering alone does not determine the colour we perceive. The available sunlight and the sensitivity of human vision matter as well. Our eyes are less sensitive to violet than to blue, and sunlight contains less violet than blue. The mixture is normally perceived as blue. ³

Nor does Rayleigh scattering remove every other colour. Red, green and the wavelengths between them are scattered too, just less strongly. Calling the sky blue describes the overall appearance of a mixture. It does not mean that the air sends only one narrow band of light to our eyes. ³

The longer route at sunset

As the Sun approaches the horizon, the light travelling towards an observer passes through a longer stretch of atmosphere. More of the shorter wavelengths are scattered out of that direct path. The light that remains is relatively richer in the longer wavelengths, producing the yellow, orange and red familiar from sunset photographs. ¹

The red is not newly created by the atmosphere, and the blue has not turned into red. The proportions in the arriving mixture have changed. Looking towards the sunset and looking at a patch of sky overhead sample different routes through the same atmosphere. Warm colours near the horizon can therefore appear alongside blue elsewhere in the sky. ¹

Clouds scatter light differently

A white cloud raises a further question: if scattering makes the sky blue, why does a cloud not become an even stronger blue? Its droplets are much larger than the molecules responsible for Rayleigh scattering. Across the visible spectrum, these larger particles scatter the different colours much more evenly. Combined, the scattered light can appear white. ⁴

A thick cloud can have a dark underside for the same broad reason that its upper surfaces are bright. Light is scattered repeatedly inside it, and much of that light leaves through the top or sides. Less reaches an observer looking at its base. A grey cloud need not contain grey water. ⁴

Cloud colour also depends on the light illuminating it. Droplets lit by the reddened sunlight of a low Sun can produce orange or pink clouds. Whiteness in the middle of the day does not require those same droplets to remain white under every lighting condition. ⁴

A blue sky is not one fixed colour

Air contains more than gas molecules. Aerosols are suspended solid particles and liquid droplets, including material from sea salt, volcanic activity and combustion. Their presence changes how light travels through the atmosphere. With few aerosols, a clear sky can look a deep blue; with more, it can look pale or hazy, sometimes almost white even without an obvious cloud layer. ⁵

This is why the explanation for the usual colour is simpler than a prediction of the exact colour on a particular evening. Sun angle, the mixture of particles and the paths available to the light all matter. A photograph records the combined result, rather than identifying one cause on its own. ⁵

Two photographs of the same landscape, taken at different times, may contain quite different combinations of direct and scattered light. The colours help reveal those differences. The air between the camera and the landscape is part of the photograph, even when it looks empty.