"Daddy (or Mommy), why is the sky blue?"

Someday, you'll have to answer that one....

Sunlight, consisting of all colors (wavelengths) of visible light, enter Earth's atmosphere. The air molecules (mainly N2and O2) scatter all wavelengths, but preferentially scatter the shorter (bluer) ones.


This next one shows how white sunlight scatters and diffuses throughout the lower atmosphere, shading the sky blue.





And finally, when the sun sits near the horizon (as at sunrise or sunset), sunlight must travel through so much atmosphere that a large fraction of the shorter wavelengths are scattered from the direct sun beam (that missing blue light is making the sky blue for somebody standing 50-100 km west of you). The Sun and surrounding sky thus appear a yellow-orange in the direction of the rise and set. To get red and deep red sunrises and sunsets, tiny solids (natural and man-made, e.g., pollutants) called "aerosols" must also contribute in scattering the shorter wavelengths of visible light.


Graphics by Chad Palmer, USA TODAY.