A clear cloudless daytime sky is blue because molecules in the air scatter blue light from the Sun more than they scatter red light. This scattering effect is known as Rayleigh scattering. Because the blue light is scattered in random directions, some of it reaches the planet surface, where we see it.
On a clear day with the sun well above the horizon, the sky looks blue because the shorter blue waves are scattered by the air and are reflected back to earth from all parts of the sky. But when the sun drops toward the horizon at evening, its light travels through many additional miles of the earth’s atmosphere to reach us. Thus it strikes many more air molecules and dust particles than when the sun is overhead. The shorter blue rays are blocked and absorbed in the atmosphere before they can reach our eyes. The longer red rays penetrate the atmosphere to reach us, causing the colors we see at sunset and at sunrise.
For more information on this subject and others, please go to jw.org "Online Library."
Also for free downloads, publications or read online.