Visibility & fog

050 MeteorologyChapter 4

Visibility categories

Condition	Range
Fog (FG)	under 1000 m
Mist (BR)	1000 m – 5000 m
Haze (HZ)	under 5000 m (solid particles)

Key fact

Worst visibility: blowing snow (1–50 m), tropical downpour, heavy snow. Drizzle: 500–3000 m. Moderate rain: 3–10 km. Prevailing visibility: greatest value within at least half the horizon circle at aerodrome.

Radiation fog

Key fact

Conditions: over land, clear sky (1–2 oktas), high RH, light wind 2–8 kt (up to 5 kt). Ground radiates heat → surface cools to dew point. Average depth ~500 ft (to 1500 ft). Valley fog from cold air drainage. Most likely early morning (sunrise + 30 min). Dissipates: sun (insolation), wind above 10 kt → lifts to St/Sc. Associated with high pressure.

Advection fog

Key fact

Warm, moist air over ** colder surface**. Wind up to 15 kt land / 20 kt sea to advect air mass. Forms over land in winter (cold surface), sea in summer (cold sea current e.g. Labrador). May form offshore initially. Clears by air mass change or wind increase → St/Sc.

Other fog types

Type	Mechanism
Steam/arctic sea smoke	Cold air over warm water; no wind
Frontal	Evaporation of warm rain into cold moist layer ahead of warm front (~150 nm belt)
Orographic/hill	Upslope cooling to dew point; cap cloud
Freezing fog	Supercooled droplets freeze on contact
Ice fog	Ice crystals in extremely cold arctic air

Key fact

Shallow fog (MIFG): depth 2 m land / 10 m water — good visibility above layer. Valley/mixing fog: cold katabatic air meets moist valley floor.

Haze & obscuration

Key fact

Haze: solid particles (smoke, dust); worse at sunset (low sun angle). Low-level inversions trap dust/smoke — no vertical exchange. Volcanic ash: engine/airframe damage primary hazard.

Flight visibility & RVR

Key fact

Flight visibility: subjective from cockpit. Oblique/slant visibility in fog improves with steeper approach (stay above layer longer). Into sun = worst; into moon = best in haze/fog layer.

Key fact

RVR: runway-direction visibility from threshold; measured at ~2.5 m AGL (transmissometer). Reported when prevailing vis under 1500 m; discontinued above 2000 m. Trends: U up, D down, N no change. METAR RVR = touchdown zone value. RVR often greater than MET visibility.

Exam trap

Advection fog can appear suddenly day or night and persist until air mass changes. Radiation fog only on land, typically after sunrise dissipation period. Do not confuse BR (mist) with FG (fog) threshold at 1000 m.