Aerial perspective

the farther things are away from the camera, the less distinct they become. This is because the air affects the light as it travels from subject to sensor. Dust particles and water droplets diffract the light—making the image weaker. This effect is known as aerial perspective and can be used to make an image seem more three-dimensional.

How marked the effect is on the image is largely dependent on distance. Many subjects are within a stone’s throw of the camera and aerial perspective only starts to become noticeable over long distances. It is therefore more useful when shooting landscapes rather than portraits—although the effect can be seen in any picture where the background is far enough away from the camera. Atmospheric conditions are a contributory factor; heat haze, mist, and pollution will weaken the distant part of the image more noticeably. The effect can be accentuated by the

use of long telephoto lenses, since these allow you to juxtapose a clearer foreground with a diffuse background. 

Dust particles in the atmosphere do not just weaken the image. They diffract some wavelengths
of light more—and this causes the distant hills to look bluer than
the foreground landscape. 

Aerial perspective is particularly useful for photographing mountainous landscapes. It helps give a feeling of distance by making each successive peak look increasingly pale and diffused.