Exploratorium
Exploratorium: Science Snacks: Blue Sky
Through this science demonstration, students discover why the sky is blue, and the why sunset is red.
Synopsys
Synopsys: A Gentle Intro to Optical Design: Why Is the Sky Blue?
Discusses Rayleigh scattering in plain language and attributes the blueness of the skies to this phenomenon. Discussion of scattering is just one topic of many on this lengthy page of optical topics.