These clues are derived from human visual perception. This is why the daytime sky is blue and sunsets are reddish. The atmosphere is not perfectly clear. We usually see some dust and haze. Therefore, when distances are considerable, the farther away an object is from the viewer, the bluer it appears from blue light scattering and the less contrast it has because of dust and haze.
Spatial perspective is a scale effect.
For a well known object, say a person, the smaller the person appears in a scene, the farther away they are perceived to be. Objects are perceived to have height and width when nearby or at middle distance. But they seem to get smaller and smaller with increasing distance, finally vanishing as tiny points. The classic example of this effect is the view of the rails of a railroad track, seeming to converge and vanishing at a point on the horizon. Aerial perspective Notice the blue cast to the upper part of the image, with the water and farthest trees.