The blue end of the visible spectrum is scattered most by oxygen and nitrogen molecules in the atmosphere. This occurs all along the path of sunlight. At sunrise or sunset, sunlight takes a long path through the atmosphere to get to us, and a lot of the blue light has already scattered sideways, leaving a reddish sunrise/sunset. At noon the path straight down through the atmosphere is relatively short, and there is less scattering of blue light.
Work Step by Step
This is discussed on pages 512-513, and shown in Figure 27.18.