This shows an implementation of Aerial Perspective as explained in the Preetham paper.
The aerial perspective helps to simulate attenuation of light due to particles in the air, as well as increase of light intensity due to reflection of the light on these same particles and gives an impressive and natural sense of depth.
Use Alt+Mouse to manipulate the camera using MAYA mode.
Use Alt+Shift+Mouse to manipulate the camera using UnrealEd mode (i.e. first person).
If you are under Linux and can't use the Alt key, push that button so everything works without Alt.