CA.I.C.K3jFAA-S-ACS-7B · Nov 2023Knowledge
j. Fog/mist
Meteorology applicable to the departure, en route, alternate, and destination under visual flight rules (VFR) in Visual Meteorological Conditions (VMC), including expected climate and hazardous conditions such as: j. Fog/mist
Bottom line
Fog is a visible aggregate of minute water droplets based at the Earth's surface that reduces horizontal visibility to less than 5/8 sm (1 km). Fog forms when the temperature and dewpoint of the air become identical or nearly so. Mist reduces visibility to less than 7 sm (11 km) but greater than or equal to 5/8 sm (1 km), so it does not obstruct visibility to the same extent as fog.
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 | Fog spread |
| deepens up to ~15 kt; >15 kt lifts to stratus | Advection wind |
| temperature ≤ 32 °F (0 °C) | Freezing fog |
FAA references
- AWH ch. 18 (pp. 18-2–18-4) — FAA-H-8083-28A
- AWH ch. 18 (pp. 18-6–18-9) — FAA-H-8083-28A
- PHAK ch. 12 (p. 12-15) — FAA-H-8083-25C (2023)
- AC 91-92, para. 7.6 — AC No: 91-92
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