Aerial Perspective

dictionary.com gives the following definition:

–noun

a technique of rendering depth or distance in painting by modifying the tone or hue and distinctness of objects perceived as receding from the picture plane, esp. by reducing distinctive local colors and contrasts of light and dark to a uniform light bluish-gray color.

Also called atmospheric perspective.

Where I find a student having difficulty with a landscape painting, it's sometimes due to the use of photographic reference material. "Depth of Field" on the camera is often (if not always) the default setting and many of us use the "point and shoot" technique. The photo prints out "less interesting" than the original scene. 

If you squint your eyes way down and look into the distance, you will often see the effects of atmospheric perspective. The strong contrast and sharp edges in the foreground fade and grey (or blue) to  less distinct colors in the distance. Now, even if you don't particularly see this, you should try it in painting a landscape. For example, in this autumn painting, I emphasized the light blue of the background to "push" the distance into the picture plane. Of course, there are other depth cues in the painting, not least of which is the lane which dwindles to a point at the horizon.