A. The scattering of sunlight by air molecules
B. The presence of water vapor
C. Absorption of blue light by the air
D. Emission of blue light by the atmosphere
A clear cloudless day-time sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light. When we look towards the sun at sunset, we see red and orange colors because the blue light has been scattered out and away from the line of sight.
