Unable to make terrains with high elevations

I had an idea for a terrain including a big mountain, but there is one problem. If the terrain is really high up, the sky is black in sections. If the terrain is too low, it is really foggy. In a mountain terrain, it covers both of these sections. Is there a way to remove the black sky and the fog?

 

for the fog, at home, it's like this
change the height and the density