why is the sky blue?
This is yet another example of where the Internet is full of useful information that is very easily found. In my day, you needed to visit the library to look this up in various encyclopedias or other resource books. So the simple solution is to type the question and pick one or more of the websites to find the answer.
My choice was sciencemadesimple.com/sky_blue.html.
Personally, I like the concept of keeping it simple but this topic is a little bit more complicated. My understanding is that gas molecules and water droplets, in the form of vapor and ice crystals, reflect the waves of light that contain the blue spectrum and that is why the sky comes back blue.
If you have ever been in the mountains in say Colorado or British Columbia in the winter, you have noticed how much deeper the blue is there because there would be less dust and less interference for the water droplets in the air at that time. That is also why the sky isn't as blue around cities because of the obvious pollution and other particles in the air that are reflecting the other wavelengths of the color spectrum of light.
So the clearer the sky (less stuff in the air), the better the chances are that the sky will be blue. The time of day is also a factor as well. The sun shines red, then orange, then yellow in the morning because of the angle to the Earth where you are watching the sunrise in relation to the Sun.
Nature is truly magnificent, especially in areas where we haven't had as much impact on the quality of the air. Hopefully, future generations will still learn about Blue Skies by seeing them for themselves and not from a song or historical picture.