SL, the assumption that fog has a uniform distribution would be ill-founded.
Fog has many different structures, particularly variations between forming and dispersing.
Stratiform cloud-like 'onion layers' in compression (forming) with significant vertical variation in visibility in each layer. Alternatively 'pork-pie', mini cumulus with Horizontal variation in visibility and ambient light levels during dispersal.
A critical time for variable fog structure in wet fog is around Cat 2 RVRs, where a 'land' decision above DH can turnout to be poorly judged. Whereas Cat 3 RVRs are more likely to be a stable fog with little variability.
Then there is a range of visibility distributions associated with different particles; wet / dry cloud, snow - type or rate of snowfall, dust, smoke, etc.
RVR originated from extensive research into these areas and is a compromise between what can be measured accurately, and the pilots need for slant visual range - what might be seen at different heights.
Last edited by safetypee; 13th Jun 2020 at 16:54.