Gulli also overlooks the fact that the weather could be clear, blue, and 22 where the pilot is....and IFR where the Tower or weather measurement equipment is located.
Despite the runways being visible for a full half of their length....due to the way fog formed/dissipated.....several places I have worked out of had that as a common occurrence.
The Official weather controls of course....but blanket statements fail to consider reality sometimes.
In Olympia Washington the Tower Cab used to be in the sunshine looking out to Mt. Rainier but the airfield was IFR due to ground fog.
Even if they could see half the runway....they were still IFR.