As a good reference point, here is a late medieval Italian painting that didn't use atmospheric perspective:
Look at how flat it looks! And confusing because there are shadows on those mountains that make us want to believe they're 3-D, but... And the people on them are clearly much smaller than those on horses, but... There is no background because the mountains jump forward along with everything else (now it clearly wasn't meant to be completely realistic because the sky is gold, which is seen a lot in Medieval art, so we could be really generous and assume they flattened the scenes on purpose for some reason? Or possibly that they didn't know how to make the background recede so they embraced it and painted the sky gold to look like it was on purpose. Which do you think was the case? Or did they even think of it or care? And if the latter is the case, kudos to them for working with what they had but it's an example of how learning the rules can improve your own art or at least give you more options to work with)!
If you want to follow-up, you're welcome to use my photos as reference:
Or go out and take some of your own! You'll probably notice this marvelous effect more now and wonder why it's "purple mountains majesty" instead of blue. Ah well.
For further study...
Really cool site that shows the "gradual change of color and tone with distance."An in-depth discussion about atmospheric perspective from The Helpful Art Teacher.
Want to look more into the science of atmospheric or aerial perspective? Go here.
Can you find other artists that really nailed it when it comes to atmospheric perspective? How about any that ignored it/broke the rules? Here is a start. Have fun!