Why the sky is blue and sunsets are red

Sunlight is a mixture of colours

Sunlight looks white, but it is a mixture of light of many wavelengths. The light we can see runs from roughly 380 to 750 nanometres, with violet and blue at the short end and orange and red at the long end. Shine sunlight through a prism and these colours spread out into a band. To understand the blue sky and the red sunset, follow what happens to each of these colours as it travels through the air. The short answer is that air does not let every colour through equally; it scatters short wavelengths sideways much more.

Shorter wavelengths scatter more

The nitrogen and oxygen molecules that make up air are far smaller than the wavelength of light. When light hits particles this small and spreads in all directions, the effect is called Rayleigh scattering, and its strength is inversely proportional to the fourth power of the wavelength. Halve the wavelength and the scattering rises sixteenfold. Comparing blue light at 450 nanometres with red light at 700, blue scatters about 6 times more. So during the day, whichever part of the sky you look at, blue light scattered by air molecules reaches your eye, and the whole sky looks blue.

So why isn't the sky violet?

On wavelength alone, violet scatters most, so the sky ought to be violet. It isn't, because several things overlap. Sunlight contains less violet than blue to begin with. Our eyes are also far more sensitive to blue and green than to violet. The scattered light is actually a blend from violet to green, and the colour we call sky blue is what our eyes and brain make of that blend. That is also why sky blue is paler and brighter than a pure blue.

Why sunsets are red

When the Sun is near the horizon, its light has to cross a much thicker slice of atmosphere at a shallow angle than when it comes from overhead. Near the horizon that path is tens of times longer than at midday. Along the way almost all the blue is scattered out sideways, and mostly the orange and red, which scatter less, survive to reach us. That is why the Sun itself looks red and the sky around it and the undersides of clouds glow red. Sunrise skies are red for the same reason, and a low full Moon looks yellowish by the same principle.

• Midday: short path, blue scattered across the sky
• Sunset: long path, blue lost and red left
• Sunrise: same principle as sunset

Why clouds are white

Cloud droplets are much larger than air molecules, comparable to or larger than the wavelength of light. Particles this large scatter all colours roughly equally regardless of wavelength, so the mixed light still looks white. Rain clouds look grey or dark not because their colour has changed but because they are thick: most light is scattered or absorbed near the top and little reaches the bottom. On humid or dusty days the sky looks pale and milky for the same reason, as larger particles scatter every colour evenly and dilute the blue.

The sky's blue changes from day to day

Even on clear days the blue is not always the same. On dry, clean days, especially after rain has washed out larger particles, the blue is deep. The sky is darkest overhead and paler towards the horizon, because looking towards the horizon you see through a long stretch of air in which light has been scattered many times and mixed. When the air fills with particles such as wildfire smoke or volcanic ash, sunsets can change in colour and brightness. An unusually red sunset does not necessarily mean dirty air, though; the height and position of clouds matter a great deal.

Common misconceptions

Many long-standing explanations of sky colour are wrong. The idea that the sky reflects the sea gets things backwards. The sea looks blue partly because it reflects the sky and partly because water itself absorbs red light. Far inland, with no sea in sight, the sky is just as blue.

• The sky reflects the sea: inland skies are blue too
• Air is naturally blue: it is the result of scattering
• Red sunsets mean polluted air: clouds and Sun height matter a lot

Try it yourself, and a caution

Fill a clear glass with water, add a few drops of milk and shine a torch through it from the side to see the principle in action. Viewed from the side, the water looks bluish; looking through it towards the torch, the light looks orange. The milk particles play the role of air molecules. When watching a sunset, though, do not stare at the Sun with the naked eye or look at it through binoculars, a telescope or a camera viewfinder, even when it is low; it can damage your eyes. If glare or an afterimage lingers, see an eye doctor.