For an aircraft dispatcher, weather is never merely a background detail—it is a legal and operational constraint that determines whether a flight can legally depart, continue, or land. Among the most precise and consequential weather elements a dispatcher evaluates are ceiling, visibility, runway visual range (RVR), and the concept of slant-range visibility. Each of these quantities feeds directly into the approach minima framework that governs what an aircraft can legally attempt at the destination and alternate airports. A dispatcher who misreads or misapplies any one of these factors risks an illegal dispatch—and, far more importantly, an unsafe operation.
This article provides a thorough treatment of how ceiling and visibility are defined and forecast, what RVR measures and how it differs from prevailing visibility, why slant-range visibility matters operationally, and how all of these elements interact with published approach minima during the dispatch decision-making process. The discussion is grounded in the FAA Aviation Weather Handbook (FAA-H-8083-28B) and relevant portions of 14 CFR Parts 91, 121, and 135.
Defining Ceiling and Visibility
A ceiling is the height above ground level (AGL) of the lowest broken (BKN) or overcast (OVC) cloud layer, or a vertical obscuration. A few or scattered layer does not constitute a ceiling, even if it is lower than a higher broken layer. This distinction is critical: a METAR reporting SCT008 BKN025 has a ceiling of 2,500 feet AGL, not 800 feet. Dispatchers must parse cloud layer reports carefully.
Prevailing visibility is the greatest distance at which objects can be seen and identified through at least half of the horizon circle. It is reported in statute miles (and fractions) in METARs and TAFs in the United States. Visibility restrictions arise from precipitation, fog, haze, smoke, dust, and blowing snow—phenomena that a Terminal Aerodrome Forecast (TAF) attempts to predict using standard weather codes (FG, BR, HZ, SN, RA, etc.) along with visibility values.
The TAF is the dispatcher's primary planning tool for destination and alternate weather. It covers a 24- or 30-hour period for qualifying airports—30 hours is the standard TAF length at major hub airports—and uses TEMPO, BECMG, and probability (PROB) groups to communicate expected fluctuations. A PROB30 group—a 30% probability of specified conditions—does not by itself make a flight illegal, but a dispatcher must account for it in risk management, particularly for alternates. A PROB40 or higher condition may affect alternate selection in a practical operational sense even if not explicitly mandated by regulation.
Runway Visual Range (RVR): What It Measures and Why It Differs
Runway Visual Range is a measured value, not an estimated one. It is produced by a transmissometer or forward-scatter meter positioned at the side of the runway. The instrument measures the extinction coefficient of the atmosphere along the runway sight path and converts it into a distance value representing how far a pilot can see along the runway centerline from the touchdown zone. RVR is reported in hundreds of feet in the United States, typically ranging from 600 feet (R06) to 6,000 feet (R60), though values above 6,000 feet are reported as "P6000" (plus 6,000).
RVR differs fundamentally from prevailing visibility in two important ways. First, it is directional—it measures visibility specifically along the runway axis, not around the entire horizon. Second, it accounts for the runway lighting environment. Transmissometers are calibrated assuming a specific runway light intensity setting. If lights are at a higher setting than the calibration assumption, the effective RVR for a pilot may be somewhat better than reported; if lights are dim or unserviceable, the converse applies. Dispatchers using RVR must understand it as a sensor-based approximation of what a pilot in the cockpit will actually see looking forward along the runway during the approach and landing roll.
At major airports with Category II and III ILS approaches, there may be multiple RVR sensors: one at the touchdown zone (TDZ), one at midfield (MID), and one at the rollout end (RO). Each sensor's value may differ, and approach procedures specify which sensor values are controlling for the purposes of minima. For a CAT III approach, all three sensors may be required to meet minimums; for a CAT I approach, typically only the TDZ RVR is controlling.
Slant-Range Visibility: The Pilot's Actual View
One of the most operationally important—and frequently misunderstood—concepts in approach weather is slant-range visibility. Surface visibility and RVR are measured horizontally, at ground level. A pilot on approach, however, is looking downward at an angle through the atmosphere toward the runway. The line of sight from the aircraft to the touchdown zone is neither horizontal nor vertical; it is oblique.
As a result, a pilot descending through a fog layer or low overcast may break out of the clouds and be able to see the runway at a longer slant distance than the surface visibility figure suggests—or, in some conditions, a shorter one. When fog is shallow and the layer is thin, slant-range visibility can actually be better than surface visibility because the pilot looks through a thinner column of the restricting layer. Conversely, when precipitation or dense fog extends deep into the lower atmosphere with no well-defined top, slant-range visibility may be worse than the surface figure, because the pilot is looking at a greater oblique path length through the obscuring medium.
For dispatch and approach planning purposes, the FAA Aviation Weather Handbook (FAA-H-8083-28B) notes that slant-range visibility is a qualitative consideration that helps explain why reported surface visibility does not always directly translate to what a crew will see at decision altitude or minimum descent altitude. Dispatchers should factor this into go/no-go risk assessments, particularly in conditions involving shallow radiation fog, freezing fog, or ice fog where the difference between surface and slant-range visibility may be significant.
How Approach Minima Are Structured
Published instrument approach procedure (IAP) minima are expressed in terms of ceiling (or decision altitude/height) and visibility (or RVR). These minima are categorized by aircraft approach category, which is based on 1.3 times the aircraft's stall speed in the landing configuration at maximum certificated gross weight (Vso × 1.3). The categories are:
- Category A: less than 91 knots
- Category B: 91–120 knots
- Category C: 121–140 knots
- Category D: 141–165 knots
- Category E: 166 knots or more
Higher approach categories generally face higher (worse) minima because faster aircraft require longer reaction distances and flare zones. A dispatcher must know the aircraft's approach category to correctly read the applicable minimums from the approach plate.
Approach minima also vary by approach type. Precision approaches (ILS, LPV with DA) use a Decision Altitude (DA)—the height at which, if the required visual references are not in sight, the missed approach must be executed immediately. Non-precision approaches (VOR, NDB, LNAV) and circling approaches use a Minimum Descent Altitude (MDA)—the crew descends to MDA and levels off until either the runway environment is in sight or the missed approach point is reached.
For ILS approaches, minima are typically expressed as a DA paired with an RVR value. For example, a standard CAT I ILS might have a DA of 200 feet HAT (Height Above Touchdown) and RVR 1800 (1,800 feet). CAT II approaches extend to DAs of 100 feet with RVR as low as 1,200 feet, and CAT III approaches can go to DA of 50 feet or no DH at all, with RVR as low as 600 feet, depending on the specific authorization.
Dispatch Weather Minima vs. Approach Minima
A critical dispatcher distinction: the weather minima required for legal dispatch (under 14 CFR Part 121) at destination and alternate airports are not the same as the published approach minima. For a destination under Part 121, the TAF (or most recent forecast) must show that weather will be at or above the operating minimums for the planned approach at the estimated time of arrival (ETA) plus or minus one hour. Alternate airport weather must meet specific regulatory thresholds under 14 CFR 121.625 (paralleling 91.169 for Part 91 IFR alternates)—for a precision approach alternate, the TAF must forecast a ceiling of at least 600 feet and visibility of at least 2 statute miles, and for a non-precision approach alternate, a ceiling of at least 800 feet and visibility of at least 2 statute miles (these are standard alternate minima, not values added on top of the published DA or MDA; actual operator ops specs may vary). These are planning minima; operations can still be conducted at the alternate if weather is above approach minima at the time of actual arrival.
Why This Matters for Operational Dispatch Decisions
A dispatcher's legal and ethical responsibility is to not release a flight into conditions that make a safe completion reasonably improbable. Understanding RVR versus prevailing visibility is essential: an airport may report 1/4-mile visibility (about 1,320 feet) but if RVR is R2400, it may still meet CAT I minima of RVR 1800. Conversely, a 3/4-mile prevailing visibility does not guarantee that RVR is at or above 4,000 feet, particularly if lighting conditions differ from transmissometer calibration assumptions. Dispatchers should use the actual RVR value when it is available, as it is controlling for approaches where RVR is specified in the minimums.
Slant-range visibility considerations remind dispatchers not to be lulled into complacency by reports that appear marginally above minimums. Shallow fog can dissipate quickly or thicken rapidly, and a TAF will often use TEMPO groups to capture brief fluctuations. A flight dispatched with destination weather just above minimums—especially in conditions favorable to rapid fog formation (clear sky, calm winds, high relative humidity, surface cooling)—deserves careful monitoring and, where authorized, early alternate selection reassessment.
Key Numbers and Rules
- Ceiling definition: lowest BKN or OVC layer (or obscuration), in feet AGL.
- RVR range: 600 to 6,000 feet reported; values above 6,000 reported as P6000.
- CAT I ILS standard minima: DA 200 feet HAT, RVR 1800 feet (1,800 ft).
- CAT II ILS standard minima: DA 100 feet HAT, RVR 1200 feet.
- CAT III ILS minima: vary by subtype; can extend to RVR 600 or lower with special authorization.
- Alternate minima (precision approach alternate): forecast ceiling 600 feet, visibility 2 SM.
- Alternate minima (non-precision alternate): forecast ceiling 800 feet, visibility 2 SM.
- Approach category speed: 1.3 × Vso at max gross weight.
- PROB30 in TAF: 30% probability; does not typically create a regulatory violation but requires risk assessment.
Common Test Traps
- SCT layers are not ceilings. Only BKN and OVC layers (and vertical obscurations) define the ceiling. A METAR showing SCT005 OVC040 has a ceiling of 4,000 feet, not 500 feet.
- RVR and visibility are not interchangeable. When an approach specifies RVR, use RVR. When only visibility is listed on the approach plate, use prevailing visibility. Do not convert between the two without understanding the operational context.
- Dispatch minima ≠ approach minima. A destination weather forecast must meet the planning minima threshold (typically approach minimums for Part 91; augmented thresholds under Part 121). An alternate must meet the regulatory forecast minima, which add hundreds of feet and miles beyond the published approach minimums.
- Slant-range is qualitative, not a reported value. No weather product directly reports slant-range visibility; it is a conceptual tool for interpreting why surface-reported visibility may not match pilot experience during descent through a layer.
- PROB30 vs. PROB40 groups in TAFs are not equivalent, and neither is the same as a TEMPO group. A TEMPO indicates conditions lasting less than one hour; PROB groups indicate probability of occurrence. Exam questions often test whether a student understands that PROB30 does not make a TAF non-compliant but demands dispatcher attention.