Winds aloft, officially known as the winds and temperatures aloft forecast, (known as "FD" in the US and Canada, 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).
When the forecast wind speed is calm, or less than 5 knots, the data group is coded “9900,” which means light and variable.
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.
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.