Smoke: FU code, toxicity & clearing
Smoke: small combustion particles suspended in air: forest fires, industrial burning, other sources (AWH 18.1.4).
- METAR code:
FU, reported when visibility 6 SM or less (TC AIM MET 8.2, Table 8.1). - Visibility: can reach zero.
- Toxic: many compounds highly toxic or irritating. Most dangerous: carbon monoxide, sometimes with hydrogen cyanide and phosgene.
- Travel: particles go far. Larger ones settle, rest scatter widely, smoke thins to haze.
Clearing (slower than fog):
| Phenomenon | Clears by |
|---|---|
| Fog | Evaporation |
| Haze, smoke | Air movement (dispersal) |
Daytime effect:
| Above surface-based layer | Result |
|---|---|
| Clear sky | Heating drives convective mixing: layer spreads higher, thins near surface |
| Thick cloud layer | Sunlight blocked: visibility barely improves |
Trap: expecting smoke to burn off by day like fog. Only air movement clears it; cloud deck above blocks even daytime mixing.
References
- FAA-H-8083-28_aviation-weather-handbook.md 18.1.4 (Smoke)
- TC AIM MET 8.2, Table 8.1 (Significant Present Weather Codes)