FB upper-wind decode

An FB upper-wind forecast gives wind and temperature at fixed levels (3000, 6000, 9000, 12000, 18000 ft ASL for the Canadian FBCN...CWAO messages; the US adds 24000 ft and up). Each group decodes as ddff or ddff±tt:

• Direction = first two digits, in tens of degrees TRUE (no variation applied). 25 = 250°.
• Speed = next two digits, in knots. 2523 = 250° at 23 kt.
• 9900 = light and variable (forecast speed < 5 kt).
• Speed 100-199 kt: 50 is added to the direction and 100 subtracted from the speed. A coded direction of 51-86 flags this (directions 510-860 do not exist), so subtract 50 from the direction and add 100 to the speed. 7711 = 270° at 111 kt.
• Speed 200 kt or more: coded as 199 (7799 = 270° at 199 kt or greater).
• Temperature is in °C. At and below 24000 ft the sign is printed, + or - (-10, +05). Above 24000 ft the sign is dropped and the temperature is always negative (50 = -50 °C).
• No temperature is forecast at 3000 ft, and the whole 3000 ft level is dropped where terrain elevation is above 1500 ft (shown as a dash).

For interpolating the wind to a cruise altitude between two levels, see the upper-winds interpolation lesson.

Trap: not recognizing a 51-86 direction as a 100+ kt wind (subtract 50, add 100), or reading a high-level temperature as positive when above 24000 ft it is always negative.

Practice