What is the difference between a METAR and a TAF?

A METAR (Aviation Routine Weather Report) is an observation of actual, current surface weather conditions at an airport at a specific time, while a TAF (Terminal Aerodrome Forecast) is a forecast of expected weather conditions within a 5 statute mile radius of an airport, typically covering a 24- or 30-hour period. METARs are issued routinely every hour (or as special reports when conditions change significantly), whereas TAFs are issued four times daily by the National Weather Service. For IFR flight planning, pilots use the TAF to anticipate future conditions at departure and destination airports, then cross-check the most recent METAR to confirm actual conditions before and during flight, as described in the FAA Aviation Weather Handbook.

How do you determine flight category from a METAR?

Flight category is determined by evaluating two key elements in the METAR: ceiling (the lowest broken or overcast cloud layer reported in hundreds of feet AGL) and visibility (reported in statute miles). The FAA defines four categories: Visual Flight Rules (VFR) is ceiling greater than 3,000 feet AGL and visibility greater than 5 SM; Marginal VFR (MVFR) is ceiling 1,000–3,000 feet and/or visibility 3–5 SM; Instrument Flight Rules (IFR) is ceiling 500–999 feet and/or visibility 1–3 SM; and Low IFR (LIFR) is ceiling below 500 feet and/or visibility below 1 SM. Accurately reading sky condition groups (such as BKN008 indicating a broken ceiling at 800 feet AGL) and the visibility group is essential, and this skill is directly tested on the FAA Instrument Rating — Airplane Knowledge Test.

Why does a TAF use terms like TEMPO, BECMG, and PROB, and what do they mean?

TAF change indicators describe how and when forecast conditions are expected to evolve during the valid forecast period. BECMG (becoming) indicates a gradual, permanent change in conditions expected to occur within a specified two-hour window; TEMPO (temporary) signals fluctuating conditions lasting less than one hour at a time and occurring during less than half of the indicated period; and PROB followed by a percentage (such as PROB30 or PROB40) indicates the probability of a thunderstorm or other phenomenon occurring, with values below 50% used in TAFs. Understanding these groups is critical for IFR pilots because a TEMPO drop in visibility or ceiling below minimums can make an alternate airport legally required under 14 CFR Part 91 and 14 CFR Part 135 alternate requirements, as outlined in the Aeronautical Information Manual.

TAF and METAR Decoding for IFR Flight Planning

Before any IFR flight departs, the pilot-in-command must build a clear mental picture of the weather that exists now and is expected to exist at the destination and alternates during the time of flight. Two products do the heavy lifting for that picture: the METAR (Meteorological Aerodrome Report) and the TAF (Terminal Aerodrome Forecast). Both are coded in an internationally standardized format that, once decoded, delivers ceiling, visibility, wind, and precipitation information in a compact, unambiguous string. For instrument-rated pilots — and for those studying for the instrument knowledge test — fluency with these codes is not optional. Every IFR alternate-airport decision, every 1-2-3 rule calculation, and every go/no-go judgment hinges on correctly reading these reports.

This article walks through every major code group in both products, explains the regulatory context that makes them matter for IFR flight, and flags the decoding traps that cost points on the FAA knowledge test and — more importantly — cause real-world planning errors.

METAR: Decoding the Current Conditions Report

A METAR is an hourly (or special, coded SPECI) surface observation issued from an airport's weather-observing system. It reflects actual, measured conditions at the time of the observation, not a forecast. The report reads left to right in a fixed sequence of groups.

Report Type and Station

The message begins with METAR or SPECI. SPECI is a special observation triggered by significant weather changes — a sudden drop in visibility, a wind shift, or ceiling changes that cross important thresholds. Following the type is the four-letter ICAO station identifier (e.g., KORD for Chicago O'Hare) and the observation time in UTC (Zulu), formatted as a six-digit group followed by Z (e.g., 152253Z means the 15th of the month at 2253 UTC).

Wind

Wind is reported as a five- or six-digit group. The first three digits are the true direction the wind is blowing from, in tens of degrees (e.g., 270 means wind from the west). The next two or three digits are the speed in knots. A gust is indicated by G followed by the peak gust speed (e.g., 27018G32KT means wind from 270° at 18 knots, gusting to 32 knots). Variable wind direction below 6 knots is coded VRB (e.g., VRB04KT). When the wind direction varies by 60° or more and is 6 knots or greater, a variable group is appended showing the range (e.g., 210V280).

Visibility

Prevailing visibility is reported in statute miles in the United States (most of the rest of the world uses meters). The value 10SM means 10 or more statute miles — the maximum reported. Values below 1/4 mile appear as M1/4SM (M = less than). Runway Visual Range (RVR) follows visibility when it applies; it is measured by a transmissometer and reported in feet for the specific runway (e.g., R28L/2400FT). RVR is critical for CAT II and CAT III ILS approaches where visibility falls below 1/2 mile.

Weather Phenomena

Present weather uses a two-to-six-character code combining intensity, descriptor, and phenomenon. Intensity is coded as - (light), no symbol (moderate), or + (heavy). Common descriptors include TS (thunderstorm), SH (shower), FZ (freezing), and BL (blowing). Phenomena include RA (rain), SN (snow), BR (mist — visibility 5/8 to 6 miles), FG (fog — visibility less than 5/8 mile), HZ (haze), GR (hail), and DZ (drizzle). So -FZRA means light freezing rain, and +TSRA means heavy thunderstorm with rain.

Sky Condition and Ceiling

Sky condition groups are listed in ascending order of height above ground level (AGL). Each group contains a coverage abbreviation followed by height in hundreds of feet AGL. Coverage codes are: FEW (1–2 oktas), SCT (scattered, 3–4 oktas), BKN (broken, 5–7 oktas), and OVC (overcast, 8 oktas). BKN and OVC layers are considered ceilings because they obstruct or obscure the sky. A broken layer at 1,500 feet AGL appears as BKN015. The special designation VV (vertical visibility) replaces sky coverage when fog or obscuration prevents individual cloud layer reporting; VV004 means vertical visibility of 400 feet into the obscuring phenomenon. CLR or SKC (sky clear) appears when no clouds exist below 12,000 feet AGL (CLR is used by automated stations).

Temperature and Dewpoint, Altimeter

Temperature and dewpoint are given in whole degrees Celsius separated by a slash. Minus values are prefixed with M (e.g., M05 means −5°C). The spread between temperature and dewpoint is a quick fog and icing-potential indicator — a spread of 2–3°C or less suggests near-saturation. The altimeter setting follows as a four-digit group prefixed with A in the U.S. (e.g., A2992 means 29.92 inches of mercury).

Remarks (RMK)

The RMK section contains additional information not captured in the coded body — pressure tendency, lightning reports (e.g., LTGIC for lightning in clouds), recent weather, variable ceiling (CIG), sea-level pressure, and more. Do not ignore remarks: a note like CIG 005V010 tells you the ceiling is varying between 500 and 1,000 feet, which is critical for an approach decision.

TAF: Decoding the Terminal Forecast

A TAF is a forecast for conditions within approximately 5 statute miles of an airport, valid for 24 or 30 hours and issued four times daily by the National Weather Service. Where a METAR tells you what is happening, a TAF tells you what is expected to happen — and that distinction is essential for IFR alternate planning and fuel requirements under 14 CFR Part 91.

Header and Validity Period

A TAF begins with the identifier TAF, the station ID, the issuance time, and the validity period. For example, KDEN 151130Z 1512/1612 means the TAF for Denver was issued on the 15th at 1130Z and covers from 1200Z on the 15th to 1200Z on the 16th. The validity window is in the DDHH/DDHH format (day-hour/day-hour). All times are UTC.

Forecast Weather Groups

The initial conditions line uses the same wind, visibility, weather, and sky-condition format as a METAR. What makes the TAF unique is its use of change indicators that divide the forecast into time segments:

FM (From): A permanent change beginning at a specific time. FM151800 means from 1800Z on the 15th, conditions abruptly shift to what follows. Everything before FM is superseded.
BECMG (Becoming): A gradual change expected to occur within a specified two-hour window (e.g., BECMG 1516/1518 means the change will be complete by 1800Z). Unlike FM, both before and after conditions overlap during the change window.
TEMPO (Temporary): Conditions expected to last less than one hour at a time and to occur during less than half of the specified period. Temporary conditions do not replace the base forecast — they fluctuate around it. A ceiling of BKN010 in a TEMPO group means brief periods of broken clouds at 1,000 feet, not sustained.
PROB (Probability): TAFs use PROB30 or PROB40 (30% or 40% probability) for conditions that are possible but not predominant. PROB30 is the minimum used; you will not see PROB10 or PROB20. PROB groups combined with TEMPO are also common (PROB30 TEMPO).

Flight Categories: How METARs and TAFs Define IFR Status

The FAA defines four flight categories based on ceiling and visibility. These drive go/no-go and alternate decisions. VFR: ceiling greater than 3,000 feet AGL and visibility greater than 5 SM. MVFR (Marginal VFR): ceiling 1,000–3,000 feet and/or visibility 3–5 SM. IFR: ceiling 500–999 feet and/or visibility 1–3 SM. LIFR (Low IFR): ceiling below 500 feet and/or visibility below 1 SM. A destination reporting or forecast at IFR or LIFR triggers specific regulatory requirements for alternates and approach minimums.

Why It Matters for IFR Operations

Under 14 CFR 91.169, a destination alternate is required when the TAF for the destination airport — from one hour before to one hour after the estimated time of arrival — forecasts a ceiling below 2,000 feet or visibility below 3 SM. This is the 1-2-3 rule. Misreading a TEMPO or BECMG group can lead a pilot to incorrectly conclude no alternate is needed. For example, if a TAF shows TEMPO periods with BKN012 and 2SM, those TEMPO conditions count: if they appear in the one-hour window, the alternate requirement applies.

For the alternate airport itself, the standard IFR alternate minimums under 14 CFR 91.169 are 600-foot ceiling and 2 SM visibility for a precision approach, and 800-foot ceiling and 2 SM visibility for a non-precision approach. TAFs — and, when TAFs aren't available, pilot reports and surrounding METARs — must be used to assess whether the alternate will be usable at the expected arrival time.

Key Numbers and Rules

METAR issued every hour; SPECI issued as needed for significant changes.
TAF valid 24 or 30 hours; issued 4 times daily (0000, 0600, 1200, 1800 UTC).
Ceiling defined by the lowest BKN or OVC layer (or VV).
Alternate required (1-2-3 rule): TAF at destination within 1 hour of ETA showing ceiling below 2,000 ft or visibility below 3 SM.
Precision approach alternate minimums: 600-2 (600 ft ceiling, 2 SM).
Non-precision approach alternate minimums: 800-2 (800 ft ceiling, 2 SM).
RVR reported in feet; visibility in statute miles (U.S. METARs/TAFs).
PROB30 = 30% probability; lowest probability value used in TAFs.
FG coded only when visibility is less than 5/8 SM; BR used for 5/8 to 6 SM.

Memory Aid: WSFPSC

To remember the order of groups in a METAR/TAF body, use W-S-F-P-S-C: Wind, Statute miles (visibility), Flight weather phenomena, Precipitation/present weather detail, Sky condition, Confirmation (temperature/dewpoint and altimeter). While not an official FAA mnemonic, it mirrors the actual group sequence and helps students decode reports in order rather than hunting through the string.

Common Test Traps

TEMPO doesn't replace the base forecast. A TEMPO ceiling of 800 feet does not mean the forecast ceiling is 800 feet — it means brief intervals of 800 feet are expected. However, for alternate planning, those brief intervals during the arrival window do trigger the alternate requirement.
FM completely replaces previous conditions. Once an FM group begins, everything coded before it no longer applies. Students sometimes add FM conditions to the previous forecast instead of substituting them.
BKN is a ceiling; SCT is not. A report showing SCT015 BKN080 has a ceiling of 8,000 feet, not 1,500 feet. Only BKN and OVC (and VV) define a ceiling.
Wind direction is where the wind comes FROM. A wind of 090 is an easterly wind — blowing from the east toward the west. This matters for crosswind component calculations.
PROB30 vs. PROB40: The test sometimes asks which probability values appear in TAFs. The answer is 30 and 40 only — never 10, 20, or 50. PROB50 or higher becomes the predominant condition and is coded differently (FM or BECMG).
TAF validity time is UTC, not local. A KBOS TAF issued at 1130Z valid 1212/1312 covers noon UTC on the 12th through 1300Z on the 13th. Always convert to local time for practical planning, but record the discrepancy.