Look at hills or buildings far away on a hazy day. The nearest ones are dark, sharp and full of color. Each row farther back looks paler, bluer and softer, until the farthest almost melts into the sky. Artists call this atmospheric perspective (or aerial perspective).
Key idea
Faraway things look lighter, bluer or grayer, softer at the edges, lower in contrast and less detailed than near things.
Why it works
Between you and a far hill is a great deal of air, with tiny bits of water and dust. They scatter light. Some of the sky's blue light is scattered toward you from all that air, which veils the hill in pale blue, and the hill's own darks and details get washed out. The more air in between, the stronger the veil.
Worked example: Push it back
You want a tree in your drawing to look far away. What do you change?
- Draw it smaller and higher on the page (the depth cues from earlier).
- Use lighter values and less contrast: no deep blacks.
- Keep the edges soft and leave out small details like single leaves.
Answer: Smaller, lighter, softer and simpler.
Common mistake
A common mistake is drawing far hills with the same dark lines and crisp detail as the foreground. They then jump forward. Save your darkest darks, sharpest edges and finest details for the front.
Leonardo da Vinci wrote about this effect in his notes on painting, and the hazy blue landscape behind the Mona Lisa is a famous example. Bruegel's Hunters in the Snow shows it in winter: the far valley and mountains are much paler than the dark figures and trees in front.
With pencil alone, atmospheric perspective is all about value: see Contrast and Key.