Atmospheric perspective is a visual phenomenon in which the density of the atmosphere progressively increases through space, creating a 'haze.' Shapes become more ambiguous, colors become less saturated, and overall brightness can increase or decrease depending on the time of day.

Benedito Calixto. Proclamação da República. 1893. (public domain)

Böhringer Friedrich. Fog. View from Oberfallenberg, Dornbirn district, towards the Mountains of Switzerland. 2006.
(Böhringer Friedrich / CC-BY-SA-2.5)