Decoding a METAR
A METAR gives weather observed at an airport at a specified time, from the ground (TC AIM MET 8.0). Fixed field order, unlabelled: read by position.
Field order: station · day/time · wind · visibility · sky · temperature/dew point · altimeter
Station
- Four-letter ICAO identifier:
CthenW/Y/Z(e.g.CYYZ).
Date/time
251830Z= day 25, 1830 UTC (Z).- On-the-hour report official for any observation within 10 min of the hour.
AUTO / CCA
AUTO: automated observation.CCA: corrected report. Second correctionCCB.- Both can appear together.
Wind 24015KT = from 240° TRUE at 15 kt
- Direction: 2-min mean, rounded to nearest 10° (last digit always 0).
- Speed in
KT. Other countries useKMHorMPS.
| Code | Meaning | Used when |
|---|---|---|
24015G25KT |
Gust | 5-sec gusts exceed mean by 5 kt or more and peak reaches 15 kt or more |
VRB |
Variable direction | Below 3 kt; at higher speed only if variation 180° or more or no single direction determinable |
260V340 |
Direction swing, extremes clockwise (appears instead of VRB) |
Swing 60°-179° over preceding 10 min, mean speed above 3 kt |
00000KT |
Calm | n/a |
- Sensors down at staffed site: wind estimated,
WND ESTin remarks.
Wind reference
| Source | Reference | Cite |
|---|---|---|
| METAR | True | n/a |
| ATC (tower) | Magnetic, matches runways numbered by magnetic heading | RAC 4.2.5 |
| FSS broadcast, at own airport | Magnetic | TP 1102 |
| FSS, sites other than its own airport | True | TP 1102 |
Visibility
- Greatest horizontal distance a prominent object can be seen with naked eye.
3SM= 3 statute miles. Not km or nm. Canada and US use SM.- No maximum value ever reported.
- Sector visibility at half or less of prevailing value: added as remark.
Sky: each layer = cover + height in hundreds of feet, always AGL (above ground level), never ASL (TC AIM MET; sample exam Q66).
| Cover | Meaning | Ceiling? |
|---|---|---|
FEW |
Few | No |
SCT |
Scattered | No |
BKN |
Broken | Yes |
OVC |
Overcast | Yes |
- Ceiling = lowest BKN or OVC layer (more than half sky covered, TC AIM MET ceiling definition). Caps usable height:
BKN025= 2,500 ft. - Layers cumulative from surface up. Steps: 100 ft to 10,000 ft, then 1,000 ft.
- Significant convective cloud appended without space (
SCT025TCU). Layer with both CB and TCU at common base: reported as CB only. - Sky totally obscured: vertical visibility (
VV004) replaces cloud group and is itself the ceiling. Ceiling = lesser of cloud-base height or vertical visibility (GEN 5.1).
Temperature/dew point M05/M12 = −5 °C / −12 °C
- M = minus; no
-sign. - Both round to nearest whole degree. Exactly .5 rounds to warmer whole degree (
-0.5→M00,-12.5→M12). - Spread = temperature minus dew point, M-signed:
M20/M24= −20 − (−24) = 4 °C.
Altimeter
A2992= 29.92 inHg.
Trap: reading wind as magnetic, visibility as km/nm, a scattered layer as a ceiling, or dropping the M on sub-zero temperature.
References
- TC AIM MET 8.1
- TC AIM MET 8.2
- TC AIM MET 8.4.4 (Table 8.3)
- TC AIM GEN 5.1 (ceiling)
- TC AIM RAC 4.2.5 (ATC passes wind in magnetic)
- TP 1102 (FSS wind reporting)
- TC sample exam Q78 (temperature/dew point spread)