The Winds and Temperatures Aloft Forecast, commonly called the FB (from its ICAO product code, FD winds aloft or FB in modern NWS nomenclature), is one of the most operationally critical weather products a certificated aircraft dispatcher uses every day. It provides a model-derived prediction of upper-level wind direction, speed, and free-air temperature at a series of standard pressure altitudes across the contiguous United States, Alaska, Hawaii, and selected oceanic waypoints. For anyone involved in high-altitude flight planning — airline dispatch, Part 135 operations, or advanced cross-country flying — understanding exactly how to decode, interpret, and apply FB data is not optional; it is a fundamental professional skill.
This article walks through the structure of the FB product, how each data group is decoded, the meteorological principles that make the data meaningful, and the operational decisions that flow directly from proper use of upper-wind forecasts. We also address the common traps that appear on the Aircraft Dispatcher written examination and in practical evaluations.
What the FB Forecast Contains
The FB is issued four times daily by the Aviation Weather Center (AWC) and covers forecast periods of 6, 12, and 24 hours (and in extended forms, up to 48 hours). Each issuance carries a valid time and a based-on time (the synoptic observation time the model used). Dispatchers must always check both: an FB valid for 1800Z issued at 1100Z is based on morning model runs, and conditions can evolve significantly before the valid time arrives.
The forecast is presented as a table with station identifiers across the top and altitude levels down the side. Standard levels in the domestic FB are 3,000, 6,000, 9,000, 12,000, 18,000, 24,000, 30,000, 34,000, and 39,000 feet MSL. The extended high-altitude product adds 45,000 and 53,000 feet for jet and turboprop operations at the highest certified altitudes.
Decoding the Data Groups
Each data cell is a 7-character group (or fewer at low altitudes — more on that below). The format is: DDSSsTT, where:
- DD — Two-digit wind direction in tens of degrees true. A value of 23 means 230° true.
- SS — Two-digit wind speed in knots. A value of 45 means 45 knots.
- s — Sign of the temperature (plus is omitted; a minus sign appears explicitly).
- TT — Two-digit temperature in degrees Celsius.
Example: 2345-08 decodes as winds from 230° true at 45 knots with a free-air temperature of −8 °C.
Several important encoding conventions require memorization:
- Calm or light and variable winds below a usable threshold are encoded as 9900 (no wind value applicable), with the temperature still appended where relevant.
- Wind speeds of 100–199 knots are encoded by adding 50 to the direction digits and subtracting 100 from the speed. So 7308 decodes as direction (73 − 50) × 10 = 230°, speed 08 + 100 = 108 knots. This convention ensures the two-digit direction field does not exceed 36 (representing 360°) during high-speed jet-stream encoding.
- Speeds of 200 knots or greater are encoded as 199 knots (the product is capped).
- No temperature data is provided at 3,000 feet and below when those levels are within 1,500 feet of the station elevation, since the surface temperature renders the figure meaningless. Similarly, temperatures are omitted at 3,000 feet because that level is considered too close to the surface for a reliable free-air value.
Meteorological Context: Why Upper Winds Look the Way They Do
Upper-level winds in the mid-latitudes are predominantly westerly and increase with altitude as a consequence of the thermal wind relationship — the horizontal temperature gradient between the tropics and the poles drives geostrophic flow that strengthens aloft. The jet stream core, typically found between FL250 and FL390, can produce winds exceeding 150 knots, which is why the 100-knot encoding convention exists.
Temperatures reported in the FB are free-air (static air) temperatures, not total air temperatures (which include ram rise). Free-air temperature at altitude decreases with height through the troposphere at an average environmental lapse rate of approximately 2 °C per 1,000 feet, though actual lapse rates vary considerably with weather systems. The FB temperature is used directly in performance calculations that require outside air temperature (OAT), density altitude computations, and turbine engine performance analysis.
Note that the tropopause interrupts the lapse rate: above it temperatures become roughly isothermal or slightly increase. Because FB levels extend well into the lower stratosphere (45,000 and 53,000 ft), dispatchers working very high-altitude turbine aircraft must recognize that temperatures at those levels may be warmer than at FL390 — a counterintuitive but operationally important fact.
Operational Applications for Dispatchers
A working dispatcher applies FB data across several interconnected planning tasks:
Fuel Planning and Flight Time
Upper winds directly determine true airspeed vs. groundspeed. A headwind component at cruise altitude increases fuel burn per nautical mile; a tailwind reduces it. Modern flight planning systems ingest FB data automatically, but a dispatcher must be able to sanity-check the output manually. Given a true airspeed of 460 knots and a direct headwind of 80 knots from the FB, the groundspeed is 380 knots — a 17% penalty in fuel per mile. For a transcontinental flight this can mean a significant fuel reserve difference and may trigger alternate fuel requirements under 14 CFR Part 121.
Optimum Altitude Selection
Because wind speed and direction change with altitude, there is often a best altitude where headwinds are minimized or tailwinds are maximized. The dispatcher compares FB groups across multiple flight levels, computes the net wind component for the planned route, and selects the altitude that minimizes trip fuel or time. This must be balanced against aircraft performance limitations, airspace structure (RVSM airspace, special use airspace), and MEA/MOCA requirements on the route.
Turbulence Avoidance and Ride Quality
Strong wind-speed shear between adjacent FB levels is a reliable indicator of potential clear-air turbulence (CAT). A large change in wind velocity over a small altitude increment suggests mechanical or thermal instability. Dispatchers use this information in conjunction with Significant Meteorological Information (SIGMETs) and Turbulence AIRMETs (Sierra/Tango) to recommend ride-improving altitude changes in the flight release or to coordinate with the crew en route.
Temperature for Performance
FB temperatures feed directly into density altitude calculations at cruise: higher-than-standard temperatures reduce engine and aerodynamic performance. For turbine aircraft this manifests as a lower available thrust rating and higher fuel flow for a given power setting. Dispatchers working weight-and-balance limited releases must confirm that cruise temperatures support the planned power settings and fuel burn assumptions.
Key Numbers and Rules
- FB standard domestic altitudes: 3,000 through 39,000 ft MSL (plus 45,000 and 53,000 in extended product).
- Issued 4 times daily; valid periods of 6, 12, and 24 hours.
- Direction always in degrees true — never magnetic.
- Speed always in knots.
- Temperature in degrees Celsius; sign is explicit (minus shown; plus omitted).
- Encoding for 100–199 kt: add 50 to direction digits, subtract 100 from speed.
- Light and variable calm encoding: 9900.
- No temperature provided at 3,000 ft (and no wind data at altitudes within 1,500 ft of station elevation).
- Use time: dispatchers should use the FB whose valid time most closely brackets the planned en-route time, not simply the most recently issued forecast.
Common Test Traps
- Forgetting the 50/100 convention. When the direction digits exceed 36, the exam expects you to subtract 50 from DD and add 100 to SS. Many students decode 7308 as winds from 730° at 8 knots — an impossible direction — rather than applying the convention to get 230° at 108 knots.
- Confusing true vs. magnetic direction. FB winds are always true north referenced. Pilots convert to magnetic using variation; the exam sometimes asks whether a given FB direction is true or magnetic and expects the answer: always true.
- Misidentifying the valid time vs. the based-on time. The FB header gives both; using the wrong time for planning (e.g., using the observation time as if it were the forecast time) leads to applying a forecast to the wrong segment of a flight.
- Assuming temperatures are given at all altitudes. No temperature at 3,000 ft is a deliberate convention, not a transmission error. Attempting to read a temperature from a 4-character group at 3,000 ft is a common decoder mistake.
- Using the most recent issuance instead of the best valid-time issuance. A newer issuance whose valid time has not yet arrived may cover a period that does not match the planned flight. The dispatcher's duty is to select the FB product whose valid time best corresponds to the actual en-route period.