Atmospheric scattering is the effect that distant objects become more blueish and lose contrast. It is caused by particles in the sky between the object and the observer scattering light and it is especially visible on hazy days with the sun high up in the sky. The fact that the distant objects turn blueish is because blue light is scattered most efficient.
(2005-06-05)
(2005-06-16)
[ before sunset || after sunset ] ⇛ In the first photo the last direct sunlight is scattered in the air, making the distant mountains look orange. In the second photo the sun is below the horizon, and only the blue light from the sky is scattered, making the mountains look darker (since there is less light being scattered) and blue (2013-09-11)
(2013-09-13)
(2013-09-13)
The forest in the back is covered with a blue haze (2013-09-21)
In this case the light of the sunset is scattered, making the distant objects more orange in stead of blue (2014-09-19)
The iconic Herðubreið mountain in Iceland turning blue (2015-09-25)
(2017-08-05)
The mountain on the left lies far in the distance, thus turning blue. The one to the right lies a lot closer by, therefore this one is not turning blue (2019-08-25)