Question: Why is the sky usually blue, but then at sunset and sunrise it goes pink, purple, orange, and other colours?

Susan Cartwright answered on 19 Jun 2015:

The air molecules and water droplets in the atmosphere scatter light. Because they are very small, they scatter short wavelengths much more effectively than longer ones. Therefore, the shortest wavelength sunlight – blue – gets bounced around so much that it completely forgets which direction it originally came from, and so the whole sky appears blue.

At sunset and sunrise, the sunlight is coming through much more atmosphere, and the other colours besides blue also get scattered. Therefore the Sun appears red (because the yellow and green light are now coming from everywhere, not just from the direction of the Sun) and the sky will appear orange or pink, particularly close to the Sun (the orange and red light hasn’t completely forgotten its original direction, so the sky in the other direction is usually still blue).

Large volcanic eruptions throw fine dust high into the atmosphere. This increases scattering, and often results in extremely spectacular red sunrises and sunsets, sometimes for several years (it takes very fine dust a *long* time to settle back to Earth). This is probably the explanation for the spectacular sunset scenes painted by the 19th century artist JMW Turner, shortly after the 1815 eruption of Mt Tambora.