The short answer
Near sunset, sunlight travels through more atmosphere before reaching us. More of the shorter-wavelength blue light is scattered away from that path, leaving warmer colours more prominent in the light that continues through.
Some evenings the sky seems to change while you are looking at it: pale blue gives way to peach, then a strip of orange appears under the clouds. The colours are clues to what light has travelled through.
Start with the blue daytime sky
Sunlight includes many visible wavelengths. Air molecules scatter shorter wavelengths more strongly than longer ones, sending blue light into different directions. This process, called Rayleigh scattering, helps explain the blue sky. The National Weather Service introduces it in Why Is the Sky Blue?.
A longer journey near the horizon
When the Sun is low, its light takes a longer route through the atmosphere than when it is high overhead. More blue and violet light is redirected out of that route. The remaining light can look orange or red. The National Weather Service explanation connects this longer path to sunrise and sunset colours.
Why the clouds join in
Clouds receive light that has already passed through the atmosphere. When that incoming light is warm-coloured, the clouds can take on those colours too. Their water droplets scatter light differently from the tiny gas molecules responsible for Rayleigh scattering. NOAA describes the two processes in The Color of Clouds.
Clouds also vary in thickness and illumination. Less light getting through can make them look gray; different lighting can produce different appearances. A sunset is therefore more than a single red stripe. See NOAA’s cloud-colour guide.
Try a quiet observation
Our suggestion: choose a comfortable, safe place and watch the clouds and sky after the Sun has dropped below the horizon. Never look directly at the Sun or point binoculars at it. You can enjoy the colours without doing either.
- Note the first colour you notice and where it appears.
- Look at a higher cloud and one near the horizon. Do they match?
- Make another note a few minutes later. What changed?
Repeat on another evening if you like. Your notes do not need to prove a theory or predict the weather. They are a way of paying attention to a familiar scene with a new question in mind.
Sources reviewed October 22, 2026. Explanations checked against NOAA and National Weather Service resources. The observing activity is an editorial suggestion; the artwork is illustrative.