Which Vis is Which?

Okay, let's all don our FAA Regulation propeller beanie caps for a moment, and look at the different measures of visibility, and in which instances they apply.

Definitions

Flight Visibility is defined as:

    ... the average forward horizontal distance, from the cockpit of an aircraft in flight, at which prominent unlighted objects may be seen and identified by day and prominent lighted objects may be seen and identified by night.
How is flight visibility measured? Well, per the definition, it can only be measured from the cockpit of an aircraft in flight, so it's not something the AWOS can give you, it's not something the tower can tell you, it's clearly something only the pilot is in a position to judge.

Ground visibility is defined as:

    ... prevailing horizontal visibility near the earth's surface as reported by the United States National Weather Service of an accredited observer.
Clearly, this is what's reported by an AWOS or tower controller, either from direct observation or on-field sensors, including RVR.

Application

VFR weather minimums

Flight 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):

    (d) Except as provided in 91.157 of this part, no person may take off or land an aircraft, or enter the traffic pattern of an airport, under VFR, within the lateral boundaries of the surface areas of Class B, Class C, Class D, or Class E airspace designated for airport:
    (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.
So the legality of taking off, landing, or entering the traffic pattern, VFR, at an airport in B, C, D, or E surface area airspace, depends on the reported ground visibility, but this rule falls back on flight visibility if there is no reporting of ground visibility.

Special VFR

Per 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 IFR

Per 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 minimums

These 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 Ops

I'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.

Disclaimer

Note 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 / harry@meretrix.com