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.

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.