3.3 Clouds, Visibility, and Fog

Clouds, Ceilings, and Visibility

The Part 107 Weather Minimums (Memorize These)

Under 14 CFR 107.51(c), every operation must meet both a visibility and a cloud-clearance standard, measured from the control station.

A handy memory aid is "3-cubed minus": think 3 SM, 5(00), 2,000. If a question shows visibility under 3 SM or a ceiling that prevents 500 ft of vertical clearance, the answer is no-go.

Cloud Classification

Clouds are named by altitude and form (Latin roots: cirro = high, alto = middle, strato = layered, cumulo = heaped, nimbo = rain).

High clouds (above ~20,000 ft):

• Cirrus (Ci) — thin, wispy ice-crystal streaks
• Cirrocumulus (Cc) — small white ripples
• Cirrostratus (Cs) — thin sheet causing a halo around the sun or moon

Middle clouds (~6,500-20,000 ft):

• Altostratus (As) — gray-blue sheet over much of the sky
• Altocumulus (Ac) — gray-white patches, larger than cirrocumulus

Low clouds (surface-6,500 ft):

• Stratus (St) — uniform gray blanket
• Stratocumulus (Sc) — low, lumpy gray rolls
• Nimbostratus (Ns) — dark, steady-rain layer

Clouds of vertical development:

• Cumulus (Cu) — puffy fair-weather clouds, flat bases
• Towering cumulus (TCu) — tall, building rapidly, a warning sign
• Cumulonimbus (Cb) — the thunderstorm cloud and most dangerous for flight

Cloud Coverage and the Definition of a Ceiling

Coverage is reported in eighths of the sky (oktas):

A ceiling is the lowest BKN or OVC layer. FEW and SCT are never ceilings because they do not cover most of the sky.

Worked example: With FEW010 SCT020 BKN040, the ceiling is 4,000 ft AGL (the first broken layer) — not the lower few or scattered layers.

Visibility Obstructions

Five Types of Fog

Applying the Minimums to a Real Sky

Suppose you launch under a layer reported as BKN012 (broken at 1,200 ft AGL). Part 107 caps you at 400 ft AGL, and you must stay 500 ft below clouds — so the highest you could legally fly is 1,200 − 500 = 700 ft, which is above your 400 ft ceiling anyway. The flight is legal on cloud clearance. Now drop the layer to BKN004 (400 ft). The 500-ft-below rule would force you down to roughly the surface, so the operation is a no-go: you cannot hold both 400 ft of useful altitude and 500 ft of clearance. Working the arithmetic, rather than guessing, is what the exam rewards.

Visibility is judged from the control station, not the aircraft. A reported 3 SM exactly meets the minimum; anything lower (mist at 1.5 SM, fog under 5/8 SM) is an automatic no-go regardless of how clear it looks straight up.

Why Cloud Type Matters Beyond Naming

Memorizing Latin names earns a point or two, but the operational message is what counts. Cumulonimbus equals thunderstorm and everything in Section 3.6 that goes with it. Towering cumulus is the visible warning that the atmosphere is destabilizing and a storm may be an hour away. A lowering nimbostratus or stratus deck means a falling ceiling that can clip your legal clearance fast. Watching cloud trends, not just current coverage, keeps a flight legal as conditions evolve.

Common Cloud, Visibility, and Fog Traps

• FEW or SCT counted as a ceiling. Only BKN and OVC form a ceiling.
• Forgetting the hundreds-of-feet conversion. OVC008 is 800 ft, not 8 ft or 8,000 ft.
• Swapping fog drivers. Radiation fog needs calm; advection fog needs wind to push warm moist air over a cool surface.
• Reading visibility from the aircraft. Part 107 measures the 3 SM minimum from the control station.

The Temperature/Dew-Point Spread

The single most useful fog predictor on the exam is the temperature/dew-point spread. As an air mass cools toward its dew point, relative humidity climbs toward 100 percent; when temperature and dew point converge (a spread near 0 to 2 deg C), the air is saturated and fog or low stratus is likely. A METAR reading 02/01 (1 deg spread) is a fog warning; 24/09 (15 deg spread) is dry, clear air. The exam often gives you a METAR and asks which condition is most likely — pick the saturated, small-spread answer for fog.

Fog Types Compared

For the exam: The two heavily tested types are radiation fog (clear, calm, cool night, burns off) and advection fog (warm moist air over a cool surface, needs wind, can last all day). Note that advection fog needs wind while radiation fog needs calm — a classic contrast question. A small temperature/dew-point spread on the supplied METAR is your tell that fog is imminent.