Why does the sky change color? Why is the sky blue during the day, red during sunrise/set and black during the night?
Optics – Why the Sky Changes Color: Blue Day, Red Sunrise/Sunset, and Black Night
atmospheric sciencefaqopticsscatteringvisible-light
Related Solutions
The colour you see in the sky on cloudy nights is due to the reflection of city lights off the clouds. In rural areas, a cloudy night is, as you expected, significantly darker.
However, the massive amount of light given off in urban areas reflects back to Earth when there is cloud cover.
And so, you see a red-orange hue, similar to the overall colour of the city lights.
Raleigh scattering is very weak so the vast majority of the light from the Sun passes through the atmosphere without being scattered. That means when we look at the Sun we see the 99% of the light that isn't scattered, and that light has the original 5,700K colour spectrum.
The only light we see directly from the Sun is the light that travels in a straight line from the Sun to our eye - that's the horizontal yellow line in this diagram. If you consider the upper yellow line we can't see this light ray because it misses our eye. However the Rayleigh scattering due to the air scatters in all directions, so some of this scattered light reaches our eye. That means when we look away from the Sun we only see the scattered light and not the direct sunlight.
The Rayleigh scattering depends on the wavelength and blue light is scattered most. That means the light we see coming from directions away from the Sun has a spectrum weighted towards the blue. NB it isn't pure blue light. It's a spectrum of light enriched in blue compared to the direct sunlight. A spectrum of the scattered light from the blue sky is given in this answer:
And that's why the Sun looks yellow and the sky looks blue.
Best Answer
The keywords here are Rayleigh scattering. See also diffuse sky radiation.
But much more simply, it has to do with the way that sunlight interacts with air molecules. Blue light is scattered more than red light, so during the day when we look at parts of the sky that are away from the sun, we see more blue than red. During sunset or sunrise, most of the light from the sun comes towards the earth at a sharp angle, so now the blue light is mostly scattered away, and we see mostly red light.