Why the Sky Looks Blue and Sunsets Look Red

Sunlight may look white, but it contains the visible colours of the rainbow. These colours have different wavelengths. Red light has a longer wavelength than blue and violet light, while blue and violet have shorter wavelengths. A prism makes this visible by spreading the colours apart, but the colours were already present in the sunlight before they reached the glass.

When sunlight enters the atmosphere, it meets tiny molecules, mainly nitrogen and oxygen. Light can continue forward, be absorbed, or be scattered in a new direction. The size of the particles and the wavelength of the light affect how much scattering occurs.

The scattering caused by molecules much smaller than the wavelength of visible light is called Rayleigh scattering. It affects shorter wavelengths more strongly than longer ones. Blue light is therefore scattered across the sky much more than red light.

When you look upward, you see blue light arriving from many directions after being scattered by air molecules. Violet light scatters strongly too, but the sunlight reaching us, the atmosphere, and the sensitivity of human vision combine so that the sky usually appears blue rather than violet. NASA gives the same central explanation: blue light is scattered more because its waves are shorter. Human eyes are also less sensitive to violet, and some violet light is absorbed in the upper atmosphere, so the scattered colour we notice most strongly is blue.

When the Sun is high, its light takes a shorter path through the atmosphere to reach you. Near sunrise or sunset, the light travels through much more air. More of the shorter blue light is scattered away from the direct path.

The remaining direct sunlight is richer in reds, oranges, and yellows, so the Sun and nearby sky can look warm-coloured. Dust, smoke, water droplets, and other particles can change the exact display, which is why two sunsets in the same place may look different.

On a clear day, compare several directions. This turns a familiar sky into a small observation exercise, and it helps you connect the explanation to evidence you can see for yourself:

• •Look overhead and notice the deeper blue.
• •Look near the horizon and notice the paler colour.
• •Watch how the colour changes as the Sun lowers.
• •Never stare directly at the Sun.
• •After sunset, notice which warm colours fade first.

Clouds are the exception that proves the rule. Cloud droplets are far larger than air molecules and scatter every visible wavelength about equally, so the colours stay mixed and a thin cloud looks white. A thick cloud looks grey because less light gets through and what does has been scattered many times in every direction. Dust, wildfire smoke and volcanic ash add particles of in-between sizes, which is why a smoky evening can produce an unusually deep red sunset, although colour alone cannot tell you what is in the air.

The atmosphere edits position as well as colour. Near the horizon, refraction bends sunlight through layers of air of different density, which flattens the Sun's disc and can keep it visible for a minute or two after it has geometrically set. The sky you see at dusk is therefore a small physics demonstration: scattering has removed the blue, refraction has moved the Sun, and both effects are strongest exactly when the light's path through the air is longest.

A common explanation says the sky is blue because it reflects the ocean. It cannot be, because the sky is just as blue over deserts and inland plains, and much of the sea's own colour comes from water absorbing red light and from reflected skylight. Other worlds make a good test. The Moon has almost no atmosphere, so there is nothing to scatter sunlight across the sky. Astronauts standing in full daylight saw a black sky, with the Sun a sharp, intensely bright disc.

Mars has a thin atmosphere full of fine reddish dust, and photographs from rovers show a daytime sky with a pale butterscotch or pinkish tint. At sunset the pattern is reversed. The dust particles there are a size that scatters blue light more efficiently in a forward direction, close to the line of sunlight, so the area around the setting Sun takes on a cool blue glow. The same physics of scattering applies on both planets; what changes is which particles are in the air.

Scattered skylight has a property your eyes barely notice. It is partly polarised, meaning its waves vibrate more in one direction than in others. The effect is strongest in a band of sky at right angles to the direction of the Sun. You can test this with polarising sunglasses or a camera polarising filter. Look at that part of the sky and slowly rotate your head or the filter, and the blue darkens and lightens. Photographers use this to deepen the sky and make clouds stand out, and it works least well near the Sun or directly opposite it.

Many animals can sense this pattern directly. Honeybees and desert ants use the polarisation of the sky as a compass, which lets them navigate even when the Sun itself is hidden, as long as a patch of blue is visible. Some researchers have suggested that Viking sailors may have used crystals such as Iceland spar to find the Sun's position on overcast days, although this remains a hypothesis with limited direct evidence. Either way, the polarised sky is a reminder that scattering changes the direction in which light vibrates, not only its colour.

Rayleigh scattering grows very steeply as wavelength shrinks. The amount of light scattered depends on one divided by the wavelength raised to the fourth power, so halving the wavelength would make scattering sixteen times stronger. Compare blue light at about 450 nanometres with red light at about 700 nanometres. The ratio of the two wavelengths is roughly 1.56, and 1.56 multiplied by itself four times is close to 6. Blue light is therefore scattered roughly six times more strongly than red. That is a big difference but not a total one, which is why direct sunlight at midday still looks almost white.

The explanation took time to settle. In the 1860s John Tyndall shone light through tubes containing very fine particles and found that the scattered light looked bluish. In 1871 Lord Rayleigh worked out the mathematics for particles much smaller than the wavelength of light, producing the fourth power rule. Rayleigh later argued that the molecules of the air themselves were enough to do the scattering, without any need for dust or water droplets. Because the brightness of the sky then depends on how many molecules are present, measurements of skylight were even used to estimate the number of molecules in a given volume of air.

The daytime sky looks blue because small air molecules scatter shorter blue wavelengths strongly in many directions. At sunset, sunlight travels through more atmosphere, so much of that blue light is scattered away and warmer colours remain in the direct beam. One scattering process, two familiar skies.