A clear afternoon can offer a deep blue sky, followed a few hours later by a sunset full of orange and red. The Sun has not changed colour on a daily schedule. Its light has taken a different journey through the atmosphere.
To understand the change, it helps to distinguish light arriving from the Sun's direction from light redirected toward us by the surrounding sky. The atmosphere treats different wavelengths differently, and where we look changes what reaches our eyes.
White light carries many colours
Sunlight contains a mixture of visible wavelengths. A prism can separate that mixture into colours, but the atmosphere does something else: its molecules scatter some incoming light in new directions.
For molecules much smaller than visible wavelengths, shorter wavelengths are scattered more strongly than longer ones. This process is called Rayleigh scattering. Blue light is therefore redistributed through the sky more readily than red light.
When you look away from the Sun on a clear day, some of that scattered blue light reaches your eyes. The sky is not a blue surface overhead. It is a direction from which scattered sunlight is arriving.
Why not violet?
Violet wavelengths are shorter still, so a simple statement that the shortest wavelength wins would predict the wrong everyday colour. The light produced by the Sun and our eyes' different sensitivities both matter.
Human colour perception combines the responses of different light-sensitive cells. We are less sensitive to violet than to blue, and the incoming sunlight contains different amounts of different wavelengths. The resulting mixture is normally perceived as blue.
This is a useful reminder that colour depends on both the physical light and the observer. A wavelength chart alone cannot describe every aspect of the scene we experience.
Sunset makes the journey longer
When the Sun is high, its light travels through a comparatively short atmospheric path to reach the ground. Near the horizon, it follows a much longer slanting route through the air.
More of the blue light is scattered out of that direct path before the remaining light reaches us. Longer red and orange wavelengths become more prominent in the sunlight arriving from the sunset's direction.
The same process that supplies a blue daytime sky therefore helps produce a warm sunset. It is not a contradiction: one view emphasizes light scattered toward you, while the other emphasizes light left in a long direct path.
Why every evening looks different
Clouds, dust, smoke and other particles complicate the scene. Their sizes and concentrations change how light is scattered and absorbed. Clouds can catch already-reddened sunlight and spread the warm glow across a larger part of the sky.
More pollution does not automatically produce a better sunset. Thick haze can dull or obscure the view. The arrangement of clouds, clear air and particles along the whole path matters more than a simple clean-versus-dirty rule.
The evening sky is therefore both a colour display and a record of a journey. Light has crossed layers of atmosphere before reaching you, and its final mixture carries clues about everything encountered along the way.
Sources and further reading
NOAA NESDIS: Why is the sky blue?