FB upper-wind decode
FB upper-wind forecast: wind and temperature at fixed levels. Replaces FD, four issues per day (TC AIM MET 1.3.7). Canadian FBCN...CWAO levels: 3000, 6000, 9000, 12000, 18000 ft ASL. Group: ddff or ddff±tt.
Issue times (TC AIM MET 3.1 Table 3.1):
| Forecast | Levels | Issue times (Z) |
|---|---|---|
| Canadian FB | 3000-18000 ft | 0330, 0930, 1530, 2130 |
| US | 24000-53000 ft | 0200, 0800, 1400, 2000 |
Inputs (both Canadian FB and US; TC AIM MET 9.2):
- Upper-air soundings: pressure, temperature, relative humidity, wind velocity
- Taken at 32 sites
FB header (TC AIM MET 9.2):
| Field | Gives |
|---|---|
| Data time | Data used to build the forecast |
| Valid time | Time the forecast is valid for |
| FOR USE | Period the forecast is to be used |
Wind digits:
| Part | Digits | Unit | Example |
|---|---|---|---|
| Direction | first two | tens of degrees TRUE (no variation applied) | 25 = 250° |
| Speed | next two | knots | 2523 = 250° at 23 kt |
Special wind codes:
| Code | Meaning |
|---|---|
9900 |
Light and variable (forecast speed < 5 kt) |
| Coded direction 51-86 | Speed 100-199 kt: coded direction +50, speed -100. Decode: direction -50, speed +100 (directions 510-860 do not exist). 7711 = 270° at 111 kt |
Speed 199 |
200 kt or more coded as 199. 7799 = 270° at 199 kt or greater |
Temperature (°C):
| Level | Sign | Example |
|---|---|---|
| At and below 24000 ft | Printed, + or - |
-10, +05 |
| Above 24000 ft | Dropped, always negative | 50 = -50 °C |
Missing data:
- No temperature at 3000 ft.
- Whole 3000 ft level dropped (shown as a dash) where terrain elevation is above 1500 ft.
Trap: missing a 51-86 direction means 100+ kt (subtract 50, add 100); temperature above 24000 ft is always negative, never positive; Canadian (0330Z...) and US (0200Z...) issue times are not interchangeable.
References
- TC AIM MET 1.3.7
- TC AIM MET 3.1 Table 3.1
- TC AIM MET 9.2