Very interesting thread, particularly comments from GLS and 212 on when approach lights can be seen from above in very low vis situations.
GLS - if vis is say 100m (330ft) and cloud OVC000, is your experience (if you have any of such conditions!) that the approach lights can be seen from directly above at say 100ft? On the face of it, presumably if the horizontal vis is 330 ft and the cloud is on the deck, the vertical vis should be the same, ie 330ft, yet practice seems to indicate this is definitely not the case, though this may be because we tend to be looking at slant visibility at the bottom of the ILS. Does cloud density tend to increase above the ground in cloud fog? Assuming it is genuine OVC000 and there is no element of vis beneath the cloud, don't see why it necessarily should.
How about ability to see approach lights through cloud/fog in night compared with day - what experience does anyone have of this?