🌅Why Is the Sky Blue - Rayleigh Scattering, Sunset Colours, Mie, Polarization

How to use Light Scattering & Sky Colour

1. 1On Sky colour simulator, pick a scene or set Sun elevation, turbidity and ozone, and look at the panorama and the Sun colour
2. 2Read the direct transmission, air mass and the spectra above the atmosphere, in the direct beam and at the zenith
3. 3On Why blue, not violet, see how the scattered spectrum and the eye response combine into sky blue
4. 4On Rayleigh vs Mie, drag the particle size to compare phase functions and efficiency; on Sky polarization, rotate the polarizer

FAQ

Why is the sky blue?

Air molecules are much smaller than the wavelength of light, and they scatter with an intensity proportional to 1/λ⁴: blue at 450 nm is scattered about 4.4 times more than red at 650 nm. As sunlight crosses the atmosphere the short wavelengths are thrown in every direction, so wherever you look in the sky you see scattered light dominated by blue. This is Rayleigh scattering.

If violet scatters even more, why is the sky not violet?

Three reasons: sunlight is already weaker at the violet end; skylight is a broad continuous spectrum, and the eye sees its mixed colour, not its peak wavelength; and the eye is very insensitive to violet, with a luminance response at 400 nm of only about 0.04% of its peak. Multiply the scattered spectrum by the three cone responses and the result is pale blue.

Why are sunrises and sunsets red and orange?

With the Sun low, its light crosses the atmosphere at a slant and the path (air mass) is 10 to almost 40 times longer than overhead. Blue light is scattered out along the way, so the direct beam left over is red-orange, and clouds and haze lit by it glow the same colour. More aerosol (dust, volcanic ash) deepens the colours. After sunset, the blue overhead partly comes from ozone absorbing orange-yellow light.

Why are clouds white?

Cloud droplets are about 10 μm in radius, far larger than visible wavelengths (size parameter about 100). That is the Mie / geometric-optics regime, where scattering efficiency barely depends on wavelength, so all colours are scattered equally and the cloud looks white. Very thick clouds scatter light many times so less reaches the bottom, which looks grey.

Is skylight polarized?

Yes. Single Rayleigh scattering gives a degree of polarization P = sin²Θ/(1 + cos²Θ), largest 90° from the Sun: 100% in theory, about 70–80% in practice because of multiple scattering. Rotate a polarizer or polarized sunglasses and that part of the sky darkens. Photographers use it to deepen blue skies, and bees and other animals navigate by it.