depends on your decision if you want to have fog in your map. I would say leave it out for now and maybe add fog later to give the map more depth. Just set "Fog Enable" to "No", then compile and test if you still have those silouettes. Some good values for fog could be:
Fog Start: 8000
Fog End: 30000
but it really depends on how far you can see in the map and how it looks together with lighting. My oil rig map has open sea in it so I used 12000 and 50000 as values for the fog distance.