How much fog is too much?

[Gerard M]

Had a thought whilst i was driving west on friday morning. When i went past Richmond at 6am i couldn't see past the perimeter fence the fog was so thick. Then when i went past Bathurst airport it was the same story, except 2hours later as well. With Richmond being considered as the second airport for Sydney, and clearly fog is going to be an issue, how much fog is too much before a plane is able/not able to land? Or when planes are diverted from Sydney to Melbourne how much fog has there been? And what measures/procedures do planes use when there is significanly decreased visability when taking off/landing?

Thanks in advance,

Gerard