7.3 Fog, Low Ceilings, and Visibility
Key Takeaways
- Fog is a surface-based cloud that forms when air near the ground cools to its dew point, or when moisture is added until the air is saturated (PHAK Chapter 12).
- Radiation fog wants a clear night, light or calm wind, and a small temperature–dewpoint spread; advection and upslope fog need wind.
- A ceiling is the lowest broken or overcast layer, or the vertical visibility into an obscuration, in feet AGL — scattered and few are not ceilings.
- Haze and smoke commonly sit under inversions and wreck inflight visibility; volcanic ash is avoided entirely (AIM / Aviation Weather Handbook).
- Flight visibility is what you see from the cockpit; prevailing visibility is the METAR value; RVR is an instrument runway measurement in feet used mainly for IFR minima.
ACS PA.I.C.K3j is fog and mist. PA.I.C.K3l is other obstructions to visibility — smoke, haze, volcanic ash, and their relatives. The weather-theory chapter already gave you dewpoint-spread arithmetic. This section is the hazard: which fog forms tonight, whether the METAR has a ceiling, and which particle cloud you never enter.
Fog is a cloud on the runway
Fog is a cloud based at the surface. Air near the ground reaches saturation, and water vapor condenses on condensation nuclei. Mist is the same family at higher visibility (METAR BR versus FG). PHAK classifies fog by how the air reached saturation, not by how scary it looks from the FBO window.
| Type | How it forms | Wind | Where PAR sees it | Operational note |
|---|---|---|---|---|
| Radiation | Ground loses heat by terrestrial radiation on a clear night; air in contact with the ground cools to the dew point | Little or no wind (too much wind mixes the cooling away) | Valleys and low spots after a clear, nearly calm night; small dewpoint spread | Often burns off after sunrise as the ground warms. If it is less than 20 feet thick, PHAK calls it ground fog. |
| Advection | Warm, moist air moves over a colder surface | Wind is required. PHAK: winds up to about 15 knots allow it to form and persist | Coasts (sea breeze over cooler land or a cold current) | Can form day or night, can be deep, and can persist. Rising sun does not reliably erase it. |
| Upslope | Moist, stable air is forced up sloping terrain and cools adiabatically | Wind is required | Plains rising toward the Rockies; any long slope | May last for days. Warming at the bottom does not automatically clear the slope. |
| Steam (sea smoke) | Cold, dry air moves over much warmer water; evaporating water saturates the cold air | Often some breeze over the water | Lakes and coastal water in cold weather | Unstable and can be turbulent. Looks like the lake is boiling. |
| Precipitation-induced (frontal) | Rain or drizzle falls into cooler air and evaporates, saturating that air | Whatever the front brings | Common with warm fronts | Can be widespread and stubborn. The “in-flight hazard most commonly associated with warm fronts” on many tests. |
| Ice fog | In extreme cold, water vapor deposits as ice crystals | Light, like radiation fog | Arctic and very cold mid-latitude outbreaks | PHAK: same setup as radiation fog except the temperature is much below freezing, usually −25 °F or colder. Aircraft exhaust can manufacture more of it on the ramp. |
Radiation fog is the one you can often forecast from a METAR before you go to bed: clear sky, light wind, temperature–dewpoint spread already small and shrinking. Favorable topography (a valley) helps. After sunrise, mixing and heating usually lift or burn it. If you launch into a ½-mile radiation fog layer “because it always burns off by 9,” you are gambling with a legal VFR departure that is not yet legal.
Advection fog is the one students try to treat like radiation fog. It needs wind. A 10-knot sea breeze over a cold coastal runway can flip VFR to below minimums in minutes, in daylight. Do not wait for “calm and clear” as your only fog watch.
Ceiling is a defined word
PHAK: a ceiling is the lowest layer reported as broken or overcast, or the vertical visibility into an obscuration such as fog or haze. Heights are AGL in the METAR sky group.
- Few about one- to two-eighths, scattered about three- to four-eighths. Not a ceiling.
- Broken five- to seven-eighths. Ceiling.
- Overcast the entire sky. Ceiling.
- Vertical visibility (
VV) into fog or haze is a ceiling.
A METAR with SCT012 BKN080 has an 8,000-foot ceiling, not 1,200. The 1,200-foot scattered layer is still a visual and scud-running problem; it is not the legal ceiling. Chapter 6 already used this definition for theory. Here it is a hazard: a 700-foot overcast is a VFR-into-IMC machine, and a 3,000-foot broken layer with 2 miles in fog is still a legal-weather problem under 91.155.
Do not invent a “PAR ceiling is 3,000 broken” rule. The ceiling is whatever the observation says. VFR weather minima live in 14 CFR 91.155 and the airspace chapter.
Visibility words the FAA actually uses
Visibility in PHAK is the greatest horizontal distance at which prominent objects can be seen with the unaided eye. The regulations tighten the vocabulary (14 CFR 1.1):
- Flight visibility is the average forward horizontal distance, from the cockpit, at which prominent unlighted objects may be seen and identified by day, and prominent lighted objects by night. That is the number 91.155 cares about in the air.
- Ground visibility is the visibility from a point on the surface. Tower or ATIS visibility is this family.
- Prevailing visibility is the METAR value: the greatest visibility equaled or exceeded throughout at least half the horizon circle (not necessarily continuous). If visibility is not uniform, the METAR may add a directional remark.
- Runway visual range (RVR) is an instrument measurement of how far a pilot can see down that runway, reported in feet. AIM ties certain SPECI triggers to RVR (for example, the designated RVR falling below or rising through 2,400 feet). RVR is an IFR approach-minima tool. It is not a VFR substitute for “I have 3 miles.” If the exam mentions RVR, treat it as runway-specific instrument visibility, not as prevailing visibility in statute miles.
When visibility is less than 7 miles, the METAR names the obstruction: fog, mist, haze, smoke, precipitation, blowing snow, and so on.
Haze, smoke, and volcanic ash
Haze is fine dry particles. It loves a low-level inversion: smooth air, trapped gunk, a brown or bluish layer, and a sun that looks washed out or reddish. Inflight visibility can be much worse looking into the sun than the METAR’s 5 miles suggested. Slant-range visibility on approach through haze is how VFR pilots lose the runway after they leave the pattern.
Smoke from wildfires can travel hundreds of miles. It is an obstruction to visibility and an air-quality problem; it can also sit under the same inversion. If you cannot see the next landmark at VFR minima, you do not “stay below the smoke deck” as a procedure unless that path is actually legal and you can see what you need to see.
Volcanic ash is in a different category. It is pulverized rock and silica (glass). AIM and the Aviation Weather Handbook: avoid ash clouds entirely. Ash can sandblast the windshield opaque, clog pitot-static ports, contaminate oil and filters, and — in turbine engines — melt and then freeze on turbine vanes until the engine flames out. A piston trainer is not “too small to care.” There is no published “light ash is acceptable below 3,000 AGL” value. A SIGMET is issued for volcanic ash. If you smell sulfur, see a tan or gray haze that does not behave like ordinary smoke, or get an ash advisory, turn around and land somewhere that is not in the plume. Do not take off or land through ash falling on the airport.
Dust and sandstorms that drop surface visibility below 3 miles can earn a (non-convective) SIGMET. They are visibility and abrasion hazards, not fog.
Scenario: Maya’s 4 °F spread at dawn
Maya’s 0515 METAR: sky clear, wind calm, temperature 48 °F, dew point 44 °F, visibility 7 miles. Spread = 4 °F and falling. The field sits in a river valley. She is looking at a textbook radiation-fog watch, not a “clear and a million” launch. By 0630 the visibility can be ½ mile in FG with VV002 — that vertical visibility is the ceiling. Advection fog is the wrong name because there is no wind and no warm-moist-over-cold-surface story. If a warm front is dripping rain into that cool valley instead, she switches names to precipitation-induced fog and does not expect sunrise to fix it. Either way she does not depart VFR into a cloud that happens to be sitting on the grass.
Which set of conditions is most favorable for radiation fog?
A METAR reports SCT008 BKN025 OVC090. What is the ceiling, and why?
Which statement about volcanic ash, haze, and visibility reports matches AIM and PHAK?