A blue sky is a journey made by light

The sky’s blue is not paint spread above the Earth. It is light reaching our eyes after encountering the atmosphere, the layer of gases surrounding our planet. Sunlight looks white but contains many colours. NASA explains that, as sunlight enters the air, some of it scatters in different directions. Not all of it continues along its original path directly from the Sun towards the ground. [1]

Different colours have different wavelengths. Imagine the distance between two crests of a wave: blue light has a shorter wavelength than red light. The tiny molecules in atmospheric gases scatter short wavelengths more effectively than long ones. This sends plenty of blue light towards us from many directions across the sky. The process is called Rayleigh scattering. It helps explain why the clear daytime sky usually looks blue. [2]

Here is an example invented by our editors. Imagine a route marked with arrows of different colours. Some continue straight ahead; others turn towards people watching from the sides. The arrows are not real light, and molecules are not people sorting them. They simply represent a change in direction. Looking at a patch of sky means receiving light that the air has scattered along its journey towards us. [1]

If wavelength matters, why do we not mainly see violet, which has even shorter waves? The National Weather Service explanation also considers the Sun’s light and the sensitivity of our eyes. We are more sensitive to blue than to violet, and sunlight supplies more energy in blue light than in violet. The colour we perceive comes from the interaction between light, the atmosphere and our way of seeing. [2]

At sunset, the route changes. When the Sun is low, its rays pass through more atmosphere before reaching the observer. More blue light scatters away from the direct line of sight, allowing reds and oranges to stand out. This does not make every sunset identical: particles and conditions in the air influence the result. The same starting idea can therefore help explain two very different colours in one sky. [1][2]

An activity from our editors: on a clear day, compare the sky above you with the sky near the horizon, keeping your gaze away from the Sun. NASA describes how the blue is often paler near the horizon, where light has passed through more air and scattered again. Note what you actually see, including clouds or haze. You do not need to force the view to match a diagram: real conditions are part of the explanation. [1]