The skybox has a proper gradient between the white and blue parts of the skybox (customizable)
The skybox has a harsh color change between the blue and white part of the skybox, leading to an unnatural and unpleasant skybox
A user submitted screenshot showing the skybox and fog transitioning perfectly with a value of 0
Another 0 fog distance screenshot showing how it looks with sea level terrain
Smooth Skies lowers the void darkness effect to be at the world's minimum build height, which fixes issues like this and in Skyblock-like void worlds.
Smooth Skies fixes an issue on lower render distances where the skybox can get clipped and form holes. The screenshot here uses night vision to further show the issue.