Why is the sky blue and why are sunsets red?

Both effects come from one process, called Rayleigh scattering. Sunlight looks white but is a mix of colors, each with its own wavelength. Gas molecules in the atmosphere scatter short blue wavelengths much more strongly than long red ones. Blue light is sent in all directions across the sky, so the sky looks blue. At sunrise and sunset the light crosses far more air, most of the blue is scattered out of the direct beam, and reds and oranges remain. [1] [2]

Sunlight is made of many wavelengths, with red the longest and blue and violet the shortest. When it enters the atmosphere it meets molecules, mainly nitrogen and oxygen, that are much smaller than the wavelength of visible light. Particles that small scatter shorter wavelengths much more than longer ones, so blue is redirected in all directions. [1] [3]

Violet light is actually scattered even more than blue, yet the sky does not look violet. The Met Office gives two reasons: our eyes are more sensitive to blue, and sunlight contains less violet to begin with. [1]

The sky is also paler near the horizon. Light from there passes through more atmosphere and is scattered and re-scattered, which mixes in more white light and weakens the blue. [1] [2]

When the Sun is low, its light has to travel through a thicker layer of air to reach you. Along that long path most of the blue light is scattered away from the line of sight. The longer wavelengths, reds and yellows, pass through, giving sunrise and sunset their colors. Clouds and other objects reflect this reddened light, which is why they glow. [1] [3]

Clouds themselves are white or gray at midday because their droplets are much larger than light waves and scatter all colors about equally. They simply take on the color of the incoming light, so they look orange or red when the light is reddened. [4]

Small particles can enhance sunset colors, but gas molecules remain the primary cause of both the blue sky and the red sunset. NASA notes that dust, pollution and other aerosols also scatter blue light, leaving purer reds and yellows. Intense red sunsets are often seen near forest fires, after volcanic eruptions, and in heavily polluted cities, though the latter indicates increased air pollution. [2] [5] [3]

Large particles behave differently. They scatter light with little change between colors, so a dusty or polluted daytime sky tends to look grayish white rather than blue. After a very large volcanic eruption, sunsets around the world can be more colorful. [3] [6]