Fog & Visibility
Fog and other obscurations are the most common cause of weather-related GA accidents — not because they're sudden, but because they're underestimated. The PPL track covers fog formation types (radiation, advection, upslope, steam, frontal — see PPL Weather). The CPL standard adds the reporting codes, the visibility thresholds, and the operational difference between an indefinite ceiling and a defined ceiling. Reference: FAA-H-8083-28 (Ch. 15) and AC 00-45H.
Fog reporting codes
- FG — Fog: visibility < 5/8 SM
- BR — Mist: visibility 5/8 to 6 SM (the same atmospheric condition as fog, but with more visibility — the threshold for the code change)
- VCFG — Fog in the vicinity: fog 5–10 SM from the point of observation but not at the field
- MIFG — Shallow fog: visibility ≥ 5/8 SM, fog less than 6 ft deep
- BCFG — Patches of fog: fog in patches with breaks
- FZFG — Freezing fog: fog with droplets that freeze on contact (rare; severe icing)
If forecast visibility > 6 SM, mist (BR) is not reported. The 5/8 SM threshold between FG and BR is one of the most-tested CPL oral details — note the fraction.
Vertical visibility vs ceiling — operationally critical
When visibility is so reduced that the cloud base cannot be determined, the report uses VV followed by the height in hundreds of feet. Example: VV003 = vertical visibility 300 ft.
- Defined ceiling (BKN, OVC at altitude X) — discrete cloud base; expect normal visibility below the layer.
- Indefinite ceiling (VV) — no clear cloud base; the obscuration is the limit. Operationally worse than a defined ceiling at the same height.
An indefinite ceiling means the visibility is the limiting factor, not just cloud bases. Approaches in indefinite ceilings have higher minima for that reason.
Other visibility hazards
- BLDU / BLSN — Blowing dust / blowing snow: reduced visibility from particulates carried by wind
- HZ — Haze: dry suspension; common in summer high pressure. Visibility may be VFR-legal but slant-range visibility (toward the sun, particularly) is much worse than reported horizontal.
- FU — Smoke: from wildfires; reduces visibility for hundreds of NM downwind. Ground-level smoke pools in valleys overnight.
- SS — Sandstorm / DS — Duststorm: common in arid regions; reduce visibility to near zero, abrade leading edges, ingest into engines. Major hazard for desert ops.
- VA — Volcanic ash: can disable engines and abrade airframe. Visible on satellite imagery and SIGMETs. Avoid by hundreds of NM — ash plumes drift far.
Helicopter operational notes
- Helicopters can fly under helicopter VFR minimums per 14 CFR § 91.155 — clear of clouds with visibility 1/2 SM by day, 1 SM by night in Class G below 1,200 ft AGL. See PPL Navigation.
- Indefinite ceilings (VV) reduce slant-range visibility more than horizontal — a helicopter at 500 ft AGL with VV003 reported has effectively zero forward sight picture.
- Coastal fog (advection) can fill in 15 minutes; brief a no-go altitude for unreliable visibility forecasts.
- Fog burn-off is solar-driven; a confined-area approach into a fog-prone valley can become impossible to depart for 60–90 minutes in the morning.
- Inadvertent IMC in a non-IFR helicopter is a leading cause of GA fatalities. Train for the recovery procedure.
Pre-flight visibility check — the questions
- What is the dewpoint spread at departure, en route, and destination? ≤ 4°F means fog risk; ≤ 2°F means fog likely soon.
- Is the surface temperature falling toward dewpoint? Sunset radiation cooling is the classic radiation-fog setup.
- Is wind onshore? Advection fog forms when warm moist air moves over cooler ground/water.
- Is your alternate fog-prone too? "Coastal-everywhere" mornings happen often along the West Coast.
- What time will sun burn off the fog? Plan a launch window, not a launch time.