i agree with text famguy on the fog.
If you want to further up the graphics, go to the launch options and put in +r_lod 0
This disables the source engines feature of rendering lower poly models at distance which means you will see hi quality models at any distance. Next, go to the main ‘Black Mesa’ folder, then go to bin. Find a file called dxsupport.cfg.
Open the file and for the first big cluster of lines up until it stops at vendor id and device id, look for any entry of the commands “cl_detaildist” and “cl_detailfade” under directx 90, 95, and 98, and change the default values from 1200 and 400 to 12000 and 4000. Save the changes and close the file. What those commands do is set the distance which any grass in outside levels cut off and fade in the distance. This should have a drastic change in the quality of the topside levels.