Human color vision and the unsaturated blue color of the daytime sky

The usual answer to the question “Why is the sky blue?” is based only on Rayleigh scattering from the molecules in the atmosphere and makes little mention of the contribution of color vision. We supplement this answer with a quantitative discussion of the role color vision plays in determining the appearance of the daytime sky. The anatomy of the human eye is reviewed, and its response as a function of wavelength is described via the spectral sensitivities of the cones. Color matching is examined for a mixture of monochromatic lights and for the spectrum of the daytime sky. The spectral irradiance of skylight is shown to be a metameric match to unsaturated blue light. A simple experiment is described suitable for classroom use or a student project.