Winds Aloft, officially known as the Winds and Temperatures Aloft Forecast (These reports were known as "FD" in the US, but are becoming known as "FB", following the World Meteorological Organization (WMO) nomenclature.), is a forecast of specific atmospheric conditions in terms of wind

Wind

Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...

 and temperature

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 at certain altitude

Altitude

Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...

s, 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: DD ss +/- TT.

• Wind Direction (DD) and Wind Speed (ss), displayed as a 4-digit number, e.g. 3127, indicating a wind direction of 310 degrees True North
  True north
  True north is the direction along the earth's surface towards the geographic North Pole.True geodetic north usually differs from magnetic north , and from grid north...
  
   and a wind speed of 27 knots. Note that wind direction is rounded to the nearest 10 degrees and the zero is excluded.
• Temperature (TT), displayed as a +/- two-digit number, e.g. -12, which means -12 degrees Celsius
  Celsius
  Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
  
  .

Example

FBUS31 KWNO 190440

DATA BASED ON 190000Z DATA VALID 190600Z FOR USE 0600-1200Z.

TEMPS NEG ABV 24000

FT      3000        6000       9000     12000     18000    24000    30000    34000    39000  (feet)

ABR   1027   1326+01  1428-03  1431-09  1639-21  1749-34  175849  175957  174758

ABI               2849+04  2857-02  2760-07  2670-20  2682-32  760344  760648  259451

ALS (etc.)

(ABR, ABI, and ALS are the station abbreviations used in this report)

Wind speeds over 99 knots are decoded by subtracting 50 from the direction and adding 100 to the speed. Thus, for example, the wind forecast for Abilene

Abilene Regional Airport

Abilene Regional Airport is a public airport located three statute miles southeast of the central business district of Abilene, a city in Taylor County, Texas, United States. It is within the Abilene city limits and owned and operated by the City of Abilene.The majority of aircraft operations at...

 (ABI) at 30,000 feet, shown above as 7603, indicates a forecast wind of 260 degrees at 103 knots.

This forecast is now made four times a day based on 0000Z,06000Z,1200Z and 1800Z data (the Z stands for Zulu and indicates Coordinated Universal Time

Coordinated Universal Time

Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...

). 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.

9900 / If there is no velocity, then there is no direction, so light and variable is coded as 9900