At the early-morning observation, the METAR at the destination reads Runway Visual Range of 1,200 feet — right at a typical CAT II minimum. A dispatcher reads that not as a verdict but as a start point: the forecast calls for the RVR to reach CAT I standard before the arrival, the crew needs a legal ILS category to fly the approach, and the alternate must carry enough fuel to reach it plus a reserve.
If the improvement lands before arrival, the approach continues. If it doesn’t, the captain still owns the go-around or divert call. The dispatcher plans the window; the captain closes it.
Fog doesn’t arrive with a front. Radiation fog forms on clear, calm nights, when the ground radiates heat into space and cools the air above it to the dewpoint. By sunrise, the airport can sit in gray while the weather report flips to the two-letter code FG.
Travelers see a delay. A dispatcher sees a window.
The number a dispatcher reads isn’t the fog line
Pilots learn to read the visibility group in a METAR, where dense fog is reported as the code FG. A dispatcher reads that line and then mostly ignores it for landing decisions. What matters is Runway Visual Range, an instrument-derived measurement of what a pilot can see down the runway, not what an observer reported from the field.
Under 14 CFR §121.655, the latest report’s RVR value controls takeoffs, landings, and straight-in instrument approaches on that runway, according to the FAA AIM/FAR Portable Reference. That is the operational threshold — not the general visibility.
Before the fog even forms, the dispatcher has been watching the temperature-dewpoint spread narrow to a few degrees or less. On calm nights, that is the classic early sign. Radiation fog peaks around sunrise, so the earliest observation is usually the beginning, not the worst.
The improvement window runs on approach category, not hope
The METAR at the destination reports Runway Visual Range of 1,200 feet. The terminal forecast calls for enough improvement for a CAT I approach by the time the flight arrives.
The dispatcher doesn’t ask whether the flight could land right now. The aircraft is still en route. She asks whether the arrival time sits before or after the forecast’s improvement point.
If the aircraft lands before the improvement point, the fog may still be near the low end. If it lands after it, the margin changes entirely.
That is where the ILS category table decides whether there is a legal approach at all.
| ILS Category | Authorized RVR minimum |
|---|---|
| CAT I | Standard 2,400 feet; 1,800 feet with touchdown-zone and centerline lighting |
| CAT II | RVR as low as 1,200 feet |
| CAT IIIa | RVR not less than 700 feet |
| CAT IIIb | RVR less than 700 feet, but not below 150 feet |
| CAT IIIc | No RVR limitation |
The CAT II row is the one to read first: decision height as low as 100 feet, visibility minima as low as RVR 1,200 feet. That matches the current report exactly. If the crew is only CAT I qualified — standard minimum RVR 2,400 feet, reducible to 1,800 with touchdown-zone and centerline lighting — the same number is already below their floor.
FAA rules add another layer. Under Legal Information Institute’s posted text of 14 CFR §121.651, an approach cannot begin when the latest visibility report is below the published minimums.
But once the aircraft is inside that segment, a later drop in reported visibility does not force an immediate abort. The crew may continue to decision altitude or minimum descent altitude; below that point, the required visual references or enhanced flight vision criteria must be present. The plan must put the flight in a position where the number at the start of the approach is legal — not bet on what the crew will see through the fog at decision height.
Flight deals
most people never see
Our AI monitors 150+ airlines for pricing anomalies that traditional search engines miss. Air Traveler Club members save $650 per trip per person on average: see how it works.
Each deal saves 40–80% vs. regular fares:
The alternate is not a backup; it is fuel mathematics
Before pushback, the dispatcher selects an alternate with a real weather margin. That alternate must be reachable, and the fuel load has to prove it. Under U.S. Government Publishing Office’s posted text of 14 CFR §121.639, a domestic flight must leave with enough fuel to reach the destination, fly to the farthest required alternate, and then hold for 45 minutes at normal cruise burn.
That reserve is the fog contingency budget: if the destination RVR does not improve, the crew can hold near the airport for a while and still divert to the alternate with the legal reserve intact. International flag and supplemental operations use different arithmetic — fuel to the destination plus 10 percent of en route time, the most distant alternate, then a reserve equal to the lower of 30 minutes plus 15 percent or 90 minutes. The logic is the same: the alternate must be a real destination at the moment the plan is filed.
Once airborne, the captain owns the commit-or-divert call
The popular image of the dispatcher deciding whether a flight lands is wrong in one specific legal way. 14 CFR §91.3 puts the pilot in command in charge of operating the aircraft and makes that authority final.
The dispatcher builds the plan, chooses the alternate, and ensures legal release. But the captain owns the decision once the wheels leave the ground. That authority does not disappear just because the release paperwork treated a landing as the expected outcome.
In practice, that means the dispatcher is the planner and the captain is the closer. Neither can pass the fog call to the other.
What this means for you when fog is in the forecast
No one is handing out cash for fog. In Europe, dense fog is an extraordinary circumstance under The National Archives’ UK legislation text of EU261, which normally defeats fixed cash compensation. The duty of care survives: meals, an overnight hotel when needed, and a choice between re-routing or a refund.
The US DOT takes a different route. Weather-related delays and diversions do not create a right to cash compensation, but a prompt refund is owed when an airline cancels or significantly changes your flight and you decline the alternatives. When fog is forecast at your destination, the best leverage is not legal — it is to book a connection buffer long enough to absorb the hold-and-divert cycle.
Questions? Answers.
How to read visibility on a METAR?
For landing decisions, the operational number is Runway Visual Range, not the general visibility group. Under 14 CFR §121.655, when the latest report includes an RVR or runway visibility value, that runway-specific figure controls takeoffs, landings, and straight-in instrument approaches to that runway.
What fuel reserve applies when fog could force a go-around or diversion?
For a domestic Part 121 release, the flight must carry fuel to reach the destination, then the most distant required alternate, then 45 minutes at normal cruise burn. Flag or supplemental turbine operations use a different structure: destination plus 10 percent of en route time, the most distant alternate, and a reserve that is the lower of 30 minutes plus 15 percent or 90 minutes at normal cruise.
Who has the final authority to decide whether to land when visibility is near minimums?
The captain has final authority. 14 CFR §91.3 makes the pilot in command directly responsible for operation of the aircraft, while §121.651 restricts starting or continuing approaches when visibility is below minima and §121.655 makes RVR controlling. The dispatcher plans fuel and alternates, but the captain decides in flight whether to land, go around, or divert.
Are airlines required to compensate passengers when fog causes delays or diversions?
In Europe, dense fog is an extraordinary circumstance under EU261, which can defeat the fixed cash compensation but does not remove the duty of care for meals, hotels, re-routing or refunds. In the US, no cash compensation is required for weather-related delays or diversions, though the DOT requires a prompt refund when an airline cancels or significantly changes a flight and the passenger declines alternatives.
