IR.I.B.K3jFAA-S-ACS-8C · Nov 2023Knowledge
j. Fog/mist
Meteorology applicable to the departure, en route, alternate, and destination for flights conducted under Instrument Flight Rules (IFR) to include expected climate and hazardous conditions such as: j. Fog/mist
Bottom line
Fog seldom forms when the temperature-dewpoint spread is greater than 2 °C (4 °F). When fog is a consideration, pilots should plan adequate fuel reserves and alternate landing sites.
Key numbers
| Number | What it means |
|---|---|
| < 5/8 sm (1 km) horizontal visibility | Fog visibility |
| < 7 sm (11 km) but ≥ 5/8 sm (1 km) | Mist visibility |
| 2 °C (4 °F) temperature-dewpoint spread | Spread limit |
| about 15 kt wind speed for advection fog | Advection wind |
| usually –25 °F or colder | Ice fog temp |
FAA references
- AWH ch. 18 (pp. 18-6–18-9) — FAA-H-8083-28A
- AWH ch. 18 (pp. 18-2–18-4) — FAA-H-8083-28A
- IFH ch. 10 (p. 10-24) — FAA-H-8083-15B
- PHAK ch. 12 (p. 12-15) — FAA-H-8083-25C (2023)
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