Winds aloft, officially known as the winds and temperatures aloft forecast, (known as "FD" in the US, but becoming known as "FB", following the World Meteorological Organization (WMO) nomenclature), is a forecast of specific atmospheric conditions in terms of wind and temperature at certain altitudes, typically measured in feet (ft) above mean sea level (MSL). The forecast is specifically used for aviation purposes.
The components of a winds and temperatures aloft forecast are displayed as DDss+/-TT:
Example
ABR, ABI, and ALS are the weather station abbreviations used in this report.
Wind speeds over 99 knots are encoded by adding 50 to the direction and subtracting 100 from the speed. Thus, for example, the wind forecast for Abilene (ABI) at 30,000 feet, shown above as 7603, indicates a forecast wind of 260 degrees at 103 knots (so that the direction became 76=26+50, and speed became 3=103-100).
This forecast is now made four times a day based on 0000Z, 0600Z, 1200Z, and 1800Z data (the Z stands for Zulu and indicates Coordinated Universal Time). The forecasts are valid 6 (FD1/8), 12 (FD2/9), and 24 (FD3/10) hours after the observation date/times of 0000Z and 1200Z upon which they are based. Depending on station elevation, FD1/2/3 wind forecasts are issued for the following levels: 3, 6, 9, 12, 18, 24, 30, 34, and 39 thousand feet.
The first level for which a wind forecast is issued is 1,500 feet or more above the station elevation. Temperature is forecast for all wind levels, except the 3,000 foot level, that are 2,500 feet or more above the station. As indicated in the third line of the forecast, the temperatures at all levels above 24,000 feet are generally negative and the minus sign is excluded. FD8/9/10 are for the 45,000 and 53,000 foot levels.
Light and variable winds (which by definition have speed close to zero and also lack specific direction) are coded as 9900.