How do I use a TAF to determine if an alternate airport is required for a Part 121 flight?

Under 14 CFR 121.619, you must file an alternate if the destination TAF shows a ceiling below 2,000 feet or visibility below 3 statute miles during the period from 1 hour before to 1 hour after your estimated time of arrival. Review every change group (FM, TEMPO, BECMG, PROB40) in that window and use the most restrictive forecast condition to make the determination.

What is the difference between a TEMPO and a BECMG group in a TAF?

A TEMPO group describes temporary fluctuations in conditions that last less than one hour at a time and occur less than half of the stated period — after the tempo event, conditions return to the base forecast. A BECMG (becoming) group describes a gradual change that will be complete within a 2-hour window, and the new conditions then persist for the rest of the forecast period. For dispatch, BECMG changes must be planned for as sustained conditions once the transition is complete.

Does a low scattered cloud layer count as a ceiling for instrument approach minimums?

No. A regulatory ceiling is defined as the lowest broken (BKN), overcast (OVC), or obscured (VV) layer reported in AGL feet. A scattered (SCT) layer, regardless of how low it is, does not constitute a ceiling. This distinction is critical when comparing reported or forecast sky conditions to published approach minimums for dispatch and alternate planning.

Decoding METARs and TAFs for Release Weather Decisions

Every flight release begins with weather. Before an air carrier flight can legally depart under 14 CFR Part 121, the dispatcher and pilot-in-command must jointly sign a release that, among other things, accounts for weather at the departure airport, the destination, and any required alternate. The primary tools for that assessment are the METAR (Meteorological Aerodrome Report) and the TAF (Terminal Aerodrome Forecast). Being able to decode each code group rapidly and correctly — and then translate that raw data into a legally sound, operationally safe release decision — is one of the most fundamental skills of the certificated aircraft dispatcher.

This article walks through the structure of both products, explains every element a dispatcher must evaluate, and connects that knowledge directly to the release decision and alternate requirements that form the backbone of Part 121 domestic and flag operations. All weather product information is grounded in the FAA Aviation Weather Handbook (FAA-H-8083-28B).

Anatomy of a METAR

A METAR is an observation-based, surface weather report issued at regular intervals — typically every hour on the hour, with special METARs (SPECI) issued between scheduled times whenever conditions change significantly (e.g., ceiling drops below 1,000 feet, visibility drops below 3 SM, or a thunderstorm begins or ends). For dispatch, SPECIs can be legally significant: a SPECI that drops conditions below alternate minimums mid-flight changes the calculus for en-route fuel and alternate planning.

A standard METAR follows a fixed code sequence. Consider this example:

METAR KDFW 121753Z 23015KT 10SM FEW040 BKN080 OVC150 28/18 A2992 RMK AO2 SLP132 T02830183

METAR — Report type. SPECI indicates a special observation.
KDFW — ICAO station identifier (K = contiguous United States).
121753Z — Day of month (12th) and time in UTC (Zulu); 1753Z.
23015KT — Wind from 230° true at 15 knots. Gusts are appended as G##KT (e.g., G25KT). Variable winds below 6 knots are coded VRB.
10SM — Prevailing visibility 10 statute miles. Values below 1/4 SM are reported as M1/4SM.
FEW040 BKN080 OVC150 — Sky condition layers in hundreds of feet AGL. Few = 1–2 oktas (eighths), Scattered = 3–4, Broken = 5–7, Overcast = 8. Ceiling is the lowest broken or overcast layer, or the lowest obscured layer when vertical visibility is given. Here the ceiling is 8,000 feet BKN.
28/18 — Temperature/dew point in Celsius. The spread here is 10°C, meaning moderate moisture but not immediate fog concern. A spread of 2–3°C or less signals fog risk.
A2992 — Altimeter setting in inches of mercury.
RMK AO2 — Remarks; AO2 indicates an automated station with a precipitation discriminator (can distinguish liquid vs. frozen).
SLP132 — Sea-level pressure 1013.2 hPa (prepend 9 or 10 to make a four-digit pressure).
T02830183 — Hourly temperature/dew point to tenths: +28.3°C / +18.3°C.

Dispatchers must also know how to read vertical visibility (VV), coded as VV### when the sky is obscured and a slant-range ceiling cannot be determined. VV004 means vertical visibility of 400 feet — critical for ILS approach minimums decisions.

Anatomy of a TAF

A TAF is a point forecast for the terminal area (typically within 5 SM of the airport reference point) covering either a 24-hour or 30-hour forecast period. TAFs are issued four times daily (0000Z, 0600Z, 1200Z, 1800Z) for major airports. Amended TAFs (TAF AMD) are issued when conditions deviate significantly from the current forecast — dispatchers should always check for amendments before releasing a long-haul flight.

TAF code groups mirror METAR syntax but add forecast-specific change indicators:

FM (From) — A rapid, permanent change. The new conditions completely replace the previous forecast from that time forward. Example: FM1800 28015KT 6SM -RA BKN030 means from 1800Z, expect those exact conditions.
TEMPO — Temporary fluctuation lasting less than one hour at a time and occurring less than half of the indicated period. Conditions revert to the base forecast after the tempo period.
BECMG (Becoming) — A gradual change expected to be completed within the indicated time window (max 2 hours). The new conditions persist after the window closes.
PROB30 / PROB40 — Probability of 30% or 40% that specific conditions will occur. Note: PROB30 alone is not sufficient to plan for those conditions as a certainty, but PROB40 groups must be considered during dispatch planning and alternates analysis because they represent a meaningful likelihood.

A TAF also contains an AMD NOT SKED notation when the issuing office does not plan amendments — dispatchers should treat this as a caution flag and cross-reference other products (PIREPs, SIGMETs, prog charts) for situational awareness.

Applying METARs and TAFs to the Release Decision

Under 14 CFR Part 121, the dispatcher has equal legal authority with the PIC to initiate, continue, or halt a flight. Weather assessment is central to that authority. Here is how both products feed the release:

Departure Airport

There is no regulatory takeoff minimum imposed by Part 121 on visibility alone for departure (operators set their own in their Operations Specifications), but the dispatcher must ensure the aircraft can return or reach an alternate if an emergency occurs shortly after departure. Low-visibility takeoff ops are governed by OpSpecs — many carriers authorize Cat II/III takeoffs under specific conditions, but the dispatcher must confirm current METAR conditions are at or above the authorized minimums.

Destination Airport

The TAF is the legal document for destination planning. Under 14 CFR 121.619 (domestic), a dispatcher must show that the weather at the destination airport is forecast to be at or above the applicable instrument approach minimums for the ETA. If the TAF shows conditions below minimums at ETA, a filed alternate and appropriate fuel are required. The dispatcher must examine the TAF for the ETA ±1 hour window and identify the most restrictive forecast condition in that period — including any TEMPO or BECMG groups that overlap the window.

Alternate Airport

14 CFR 121.619 requires an alternate when the destination TAF shows conditions below 2,000 feet ceiling and 3 SM visibility during the period from 1 hour before to 1 hour after the estimated time of arrival (commonly called the 1-2-3 rule for Part 91; Part 121 uses similar logic but references OpSpecs and the specific rule). The selected alternate must itself have weather forecast at or above the alternate minimums defined in the operator's OpSpecs — often 600-2 for a precision approach airport or 800-2 for a non-precision alternate.

Key Numbers and Rules

METAR ceiling: lowest BKN, OVC, or VV layer in AGL hundreds of feet.
TAF validity: 24 or 30 hours; issued 4 times daily at 0000, 0600, 1200, 1800Z.
SPECI trigger criteria: ceiling at or below 3,000, 1,500, 1,000, or 500 ft; visibility at or below 3, 2, 1, or 1/2 SM; onset/end of thunderstorm, freezing precipitation, or tornadoes, among other criteria.
Temperature-dew point spread: 2–3°C = high fog/low stratus risk; used to cross-check TAF fog forecasts.
PROB40: must be operationally considered; PROB30 typically is advisory only.
Alternate ceiling/visibility (typical Part 121 precision): 600 feet and 2 SM (verify against specific OpSpecs).
TAF ETA window: examine TAF for at least ±1 hour around filed ETA for destination and alternate planning.

Memory Aid

For ceiling layers in a METAR or TAF, remember FEW-SCT-BKN-OVC mapped to oktas: Few = 1-2, Scattered = 3-4, Broken = 5-7, Overcast = 8. A ceiling does not exist until you reach BKN or OVC — scattered layers, no matter how low, do not constitute a regulatory ceiling. This distinction is tested repeatedly on the dispatcher knowledge exam.

Why It Matters for Dispatcher Authority

The dispatcher certificate carries legal responsibility. If a dispatcher releases a flight to a destination where the TAF clearly shows conditions below minimums at ETA without a proper alternate, that dispatcher has violated 14 CFR Part 121 and may face certificate action. More critically, an aircraft may find itself without a legal, fueled alternate during an in-flight emergency. The METAR and TAF are not just bureaucratic hurdles — they are the dispatcher's primary tools for protecting passengers, crew, and the carrier's operating certificate. Staying current on TAF amendments throughout the flight (via ACARS or flight following) is as important as the pre-release check.

Additionally, understanding the limitations of these products is part of dispatch professionalism. A TAF is a forecast — it can be wrong. Dispatchers must integrate TAFs with PIREPs, SIGMETs, AIRMETs, radar imagery, and prog charts to build a complete weather picture. A TAF showing VFR conditions at ETA means little if an active convective SIGMET covers the destination terminal area. FAA-H-8083-28B emphasizes using multiple weather products together rather than relying on any single source.

Common Test Traps

Scattered ≠ ceiling: A layer reported as SCT010 is NOT a ceiling. Only BKN, OVC, or VV layers constitute a ceiling for regulatory minimums purposes.
TEMPO vs. BECMG: TEMPO conditions are temporary and the forecast reverts; BECMG conditions persist after the change window. Confusing the two can lead to underestimating how long bad conditions will last.
PROB30 vs. PROB40: Many students assume both must be planned for equally. PROB40 carries more operational weight in dispatch planning; PROB30 is generally advisory. Verify what your carrier's OpSpecs require.
UTC vs. Local time: METARs and TAFs are always in Zulu (UTC). Students regularly miscalculate ETA windows by forgetting to convert local scheduled departure/arrival times to UTC before comparing to the TAF valid period.
VV vs. BKN/OVC: Vertical visibility (VV003) means the sky is obscured. The VV height is used as the ceiling for approach minimums — do not confuse it with a broken or overcast cloud layer at the same height.