//take a look at your gluperspective call or otherwise determine your far plane for assistance in this endeavor

  glEnable(GL_FOG);
  glClearColor(0.5,0.5,0.5,0.0);  //should match fog color
  GLfloat fogColor[4] = { 0.5, 0.5, 0.5, 1.0};
  GLfloat density = 0.3;
  glFogi(GL_FOG_MODE, GL_LINEAR); //exp2 //exp //linear
  glFogfv(GL_FOG_COLOR, fogColor);

  glHint(GL_FOG_HINT, GL_NICEST); //GL_DONT_CARE, GL_FASTEST


  glFogf(GL_FOG_DENSITY, density);

  glFogf(GL_FOG_START,5); // linear only
  glFogf(GL_FOG_END,9); //linear only //match the far plane (or what looks right) here

  //linear:  (end -c)/(end-start)
  //   exp:   e^-(density *c)     
  //  exp2:   e^-(density *c)^2