Air scatters short wavelengths of visible light more strongly than long ones.

Rayleigh scattering applies when the scattering particles are much smaller than the wavelength of the light. For a fixed set of such scatterers, its strength varies approximately as the inverse fourth power of wavelength. This gives shorter visible wavelengths a substantial advantage over red light.

Looking away from the Sun, you are not seeing empty space lit from behind. You are seeing sunlight scattered toward you by the atmosphere. Blue light is redirected into these viewing directions more efficiently than red, so a clear sky usually has a blue appearance.

The perceived color is not determined by the scattering rule alone. The spectrum of incoming sunlight, absorption in the atmosphere, and the responses of the eye all matter. Although violet has an even shorter wavelength than blue, the combined result is not a sky that normally looks violet.

Near midday, when the Sun is high, its direct light passes through a comparatively short atmospheric path before reaching an observer. Some blue light is scattered away, but the remaining direct beam still contains a broad mixture of visible wavelengths. Meanwhile, scattered light arriving from other directions makes the surrounding sky look blue.

At sunrise or sunset, sunlight follows a longer slanting route through the atmosphere. More short-wavelength light is removed from the direct beam along that route. The light arriving from the solar direction can consequently be enriched in longer wavelengths, producing yellow, orange, or red appearances.

This does not mean that sunset creates new red wavelengths. The atmospheric path changes the balance among wavelengths already present in sunlight. Nor does it guarantee a dramatic red sunset every day: clouds, aerosols, atmospheric clarity, and viewing geometry affect the result.

The fourth-power relationship makes the wavelength dependence steep. Comparing wavelengths of 450 and 650 nanometers under the same Rayleigh conditions gives a scattering ratio of approximately (650/450) to the fourth power, or about 4.4. This compares scattering strength, not the exact brightness ratio your eyes will measure in the real sky.

Cloud droplets are much larger than air molecules and cannot be treated as ordinary molecular Rayleigh scatterers. Their scattering is less strongly selective across the visible spectrum, helping clouds appear white or gray. Haze also changes sky color because suspended particles introduce scattering behavior beyond the simplest clear-air explanation.

The sky's color is not the color of a blue material coating the atmosphere. It results from sunlight being redistributed toward the observer. Looking at the sky away from the solar disk samples scattered light, while looking toward the Sun involves a beam altered by its passage through air.