Obscuration codes, haze & blowing snow
Obstructions to vision: non-cloud phenomena that cut visibility (mist, haze, smoke, dust, sand, volcanic ash). Each has a METAR obscuration code.
Reporting rule (TC AIM MET 8.2):
- Generally reported at prevailing visibility 6 SM or less.
- Exception:
VA(volcanic ash) always reported.
| Code | Phenomenon | Reported when |
|---|---|---|
BR |
mist | visibility 5/8 SM or more |
FG |
fog | under 5/8 SM |
DU |
dust | 6 SM or less |
SA |
sand | 6 SM or less |
HZ |
haze | 6 SM or less |
VA |
volcanic ash | any visibility, always |
(TC AIM MET Table 8.1)
BRandFGare the same water-droplet suspension, codedFGbelow 5/8 SM.
Wind-lifted snow, dust or sand, coded by lift height (TC AIM MET 8.2, Table 8.1, qualifier (c)):
| Code | Term | Lift height |
|---|---|---|
DR |
drifting | below 2 m |
BL |
blowing | 2 m or more |
BLSNblowing snow: 2 m (about 7 ft) or more (TC AIM MET 8.2).
Haze (FAA AWH 18.1.3): suspension of particles too small to see individually. Sits in stable air.
| Seen against | Appearance (scatters short wavelengths) |
|---|---|
| Dark background | Bluish tint |
| Light background | Yellowish veil |
- Mist: plain gray only, no color shift.
Haze has a definite top:
| Position | Visibility |
|---|---|
| Below top | Slant-range (air-to-ground) poor |
| Above top | Air-to-air unrestricted |
- Visibility also varies facing toward vs away from the sun.
Trap: VA ignores the 6 SM rule: reported at any visibility. Calling DR (below 2 m) blowing snow; BLSN needs 2 m or more. Surface visibility is not the view from the air; below haze top, air-to-ground is poor.
References
- TC AIM MET 8.2, Table 8.1 (Significant Present Weather Codes)
- TC AIM MET 8.2 (obscuration reporting rule)
- TC AIM MET 8.2, Table 8.1 (qualifier (c))
- FAA AWH 18.1.3 (haze)