Why the Sky Changes Colour

Visible sunlight is a mixture of wavelengths that your eyes interpret as different colours. Violet and blue light have shorter wavelengths, while orange and red light have longer wavelengths. When all of them reach your eyes in a balanced mixture, the light looks white.

As sunlight enters the atmosphere, it interacts with tiny nitrogen and oxygen molecules. These molecules scatter shorter visible wavelengths much more strongly than longer ones. This process is called Rayleigh scattering.

Blue and violet light are scattered widely across the sky. Human eyes are more sensitive to blue than violet, the Sun produces different amounts of each wavelength, and some violet light is absorbed in the upper atmosphere. Together, these effects make the clear daytime sky appear mainly blue.

When the Sun is high, its light travels through a relatively short path in the atmosphere before reaching you. Near sunrise or sunset, the light enters at a shallow angle and crosses much more air.

Much of the blue and violet light is scattered away from the direct path during this longer journey. The light reaching you from the direction of the Sun contains a larger share of orange and red wavelengths. Dust, smoke, salt and other particles can strengthen or alter the colours by scattering light in additional ways.

The Sun may look flattened near the horizon because refraction bends its light through layers of air with different densities. The atmosphere can also make the Sun visible briefly when it is geometrically just below the horizon. The sky, apparently unsatisfied with colour alone, also edits position.

Cloud droplets are much larger than air molecules and scatter a broad range of visible wavelengths. Thin clouds often look white because the colours remain mixed. Thick clouds can look grey because less light passes through and repeated scattering sends light in many directions.

Air pollution, wildfire smoke and volcanic particles can produce vivid sunsets, but colour alone does not tell you exactly what is in the air. The main ideas to keep are:

• •Sunlight contains many visible wavelengths.
• •Air molecules scatter short wavelengths strongly.
• •Scattered blue light fills the daytime sky.
• •Sunset light travels through more atmosphere.
• •More blue is removed, leaving warmer direct light.

The sky changes colour because the atmosphere scatters different wavelengths by different amounts. Shorter blue light spreads across the daytime sky, while the long path at sunset removes more blue from direct sunlight and leaves red and orange colours. Follow the light's path, and the changing sky tells a simple scattering story.