A Winds Aloft Forecast, often designated as an FB or FD product, provides a specialized prediction of atmospheric conditions at various flight altitudes. This data is a computer-generated model of future wind and temperature values for a specific time window, not a current observation. The primary audience includes pilots and flight dispatchers involved in aviation planning.
The forecast is a necessary tool for calculating factors such as ground speed, estimated time en route, and fuel consumption for cross-country flights. By understanding the predicted winds, aircrews can determine the most advantageous altitude to fly to minimize headwinds or maximize tailwinds. The core values presented are wind direction, wind speed, and temperature, all encoded in a standardized numerical format.
How Wind Direction and Speed are Coded
The wind data within a Winds Aloft Forecast is represented by a four-digit group, typically referred to as DDss. The first two digits denote direction and the last two denote speed. Wind direction is always given in tens of degrees with respect to true north; for example, a forecast of 27 translates to 270 degrees. Wind speed is expressed in knots.
A critical exception applies when the forecast wind speed is 100 knots or greater. For wind speeds between 100 and 199 knots, the encoding system modifies the direction and speed digits to fit the four-digit structure. The forecaster adds 50 to the wind direction code and subtracts 100 from the wind speed code.
For example, a coded group of 7319 must be decoded by subtracting 50 from 73, yielding 23 (230 degrees), and adding 100 to 19, resulting in 119 knots. This encoding method ensures that the wind direction value will range from 51 to 86, which instantly signals that the actual wind speed is 100 knots or more.
Including Forecast Temperature Data
Temperature is the second core value included in the Winds Aloft Forecast and is always expressed in degrees Celsius. Temperature values are generally provided for forecast levels at 6,000 feet Mean Sea Level (MSL) and above, or at least 2,500 feet above the reporting station’s elevation.
For altitudes between 6,000 feet and 24,000 feet MSL, the temperature is represented by a two-digit number preceded by a plus or minus sign. For forecast levels above 24,000 feet MSL, the negative sign is omitted. This is because all temperatures at those elevations are consistently below freezing.
A temperature value of 45 at 30,000 feet, for instance, is decoded as minus 45 degrees Celsius. This temperature information is used by pilots to predict potential aircraft icing conditions, which are most likely to occur between zero and minus 20 degrees Celsius.
Structuring the Forecast by Altitude and Time
The Winds Aloft Forecast organizes the predicted data by specific altitude levels above Mean Sea Level (MSL). The forecast data is listed beneath the three-letter identifier for the reporting station, providing a localized prediction for each geographical area.
Standard reporting levels typically begin at 3,000 feet and then progress through:
- 6,000 feet
- 9,000 feet
- 12,000 feet
- 18,000 feet
- 24,000 feet
- 30,000 feet
- 34,000 feet
- 39,000 feet
For altitude levels at or below 15,000 feet, the values are referenced as true altitudes above MSL. Levels at or above 18,000 feet are referenced as pressure altitudes, which are equivalent to Flight Levels (FL). The forecast is generated four times daily based on atmospheric model runs conducted at 0000, 0600, 1200, and 1800 Coordinated Universal Time (UTC). Each forecast includes a “valid” time and a “for use” time window, specifying the period during which the predictions are expected to be accurate.
Decoding Special Conditions
Two specific coded groups address conditions that fall outside the standard wind velocity range.
The condition of light and variable winds, where the speed is less than 5 knots, is represented by the code “9900”. This four-digit group indicates that the wind direction is highly changeable and the speed is negligible for flight planning purposes.
Another special condition is forecast when the wind speed is expected to be 200 knots or greater. In this extreme case, the forecast simplifies the wind speed to a maximum value of 199 knots, and the wind direction code is adjusted accordingly. The coded group will present a wind speed of 99, such as “7799,” which translates to a wind from 270 degrees at 199 knots or greater.