Originally Posted by
Equivocal
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I agree.
It comes straight out of DOC4444
7.11.6 Reduced runway separation minima shall be subject to the following conditions:
a) wake turbulence separation minima shall be applied;
b) visibility shall be at least 5 km and ceiling shall not be lower than 300 m (1 000 ft);
It will ensure the succeeding aircraft is able to see the preceding aircraft, and also the controller to see the aircraft involved. <- this is my take on it.
Once the ceiling and visibility get that low, you're also close to the limit of being able to use the separation anyway, since you still need to apply minimal separation between then when they dissapear from view from the tower.