Which Vis is Which?
Flight Visibility is defined as:
Ground visibility is defined as:
VFR weather minimumsFlight visibility is used to determine basic VFR weather minimums per 91.155.
It's noteworthy that whether you're legally compliant with VFR weather minimums is determined by your judgment, as pilot, of the flight visibility. This is significant because in areas of ground fog, ground visibilities could be reported as significantly lower than the flight visibility you're seeing at altitude.
However, note 91.155(d):
(1) Unless ground visibility at that airport is at least 3 statute miles; or
(2) If ground visibility is not reported at that airport, unless flight visibility during landing or takeoff, or while operating in the traffic pattern is at least 3 statute miles.
Special VFRPer 91.157, special VFR ops are only allowed when flight visibility is at least 1SM, but to takeoff or land under special VFR you need reported ground visibility of 1SM. If ground visibility isn't reported, then flight visibility may be substituted under certain circumstances (a little complicated, see the reg).
Instrument Approaches under IFRPer 91.175, one of the conditions for descending below the MDA/DH, and for landing, is for the flight visibility to meet the minimums specified in the IAP.
Again, since this is flight visibility, it's up to the pilot to make this assessment, and decide whether to descend or else go missed. A part 91 pilot can begin an approach regardless of the reported ground visibility, and descend to minimums, and they decide for himself if he has the visibility required by the IAP. Of course, while this is what the regs say, if the field is reporting ground visibility well below minimums, I can't guarantee that the FAA won't "take an interest" in your explanation that you determined that there was sufficient flight visibility, they may suspect that you're fibbing.
How do you judge flight visibility, any way? Well, you'll sometimes encounter suggestions that you can try to evaluate visibility by knowing the dimensions of the approach lights, or the dimensions of standard runway markings, and use these to estimate flight visibility.
IFR takeoff minimumsThese don't apply to part 91 aircraft, but 91.175 does specify minimums for part 121, 135, etc. It doesn't explicitly mention what kind of visibility is referred to, but I'd assume that's ground visibility (shrug).
Part 135 OpsI'm a part 91 guy, but it's worth mentioning that part 135 has its own rules regarding VFR operations and required flight visibility, per 135.205.
There are also special requirements for meeting visibility requirements for IFR approaches. In particular, part 135 pilots are not allowed to begin an approach if weather reporting at the airport indicates that it's below minimums, per 135.225. Clearly, this necessarily refers to reported ground visibility.
DisclaimerNote that I am not a lawyer, just a guy with a copy of the FAR/AIM, and a super-cool FAA Regs propeller beanie cap.
Harry Mantakos / firstname.lastname@example.org