Visibility & obscurations

Where is the fog–mist boundary?

Separate the physical phenomenon from the criterion for reporting its aviation code.

Droplets and dry particles

Fog is a near-surface suspension of droplets, or in very cold conditions ice particles, that reduces horizontal visibility below 1,000 m. Mist is a less dense droplet obscuration with visibility at least 1,000 m; in aviation reporting BR is used from 1,000 through 5,000 m. Haze is an obscuration dominated by very small dry particles rather than suspended liquid droplets. For the relevant aviation weather reports, HZ is used when such particles reduce visibility to 5,000 m or less.

The thresholds alone do not identify the material. A visibility of 3,000 m can be associated with mist, haze, smoke, rain or another cause. Fog and mist require a moisture-and-temperature history bringing a sufficient droplet population into the layer: cooling, moisture addition or mixing may contribute. A small temperature–dew-point spread is a useful clue, but no universal spread automatically determines the reported visibility.

Haze commonly builds when fine dust, salt, pollution or other aerosols remain suspended, especially beneath a stable layer with limited dispersion. Humidity can change aerosol size and optical properties, but haze should not be defined solely by a fixed RH number. Fog patches, shallow fog and fog in the vicinity have additional reporting qualifications; their code need not mean that the whole aerodrome's prevailing visibility is below 1,000 m.

Does sub-zero fog necessarily consist of ice crystals?

Use the phase of the suspended water, not temperature alone.

Two cold-weather situations

Freezing fog contains supercooled liquid droplets that can freeze on contact with surfaces. It can produce rime on an aircraft, sensors, ground equipment and the runway environment. The FZ descriptor denotes the supercooled nature of the relevant fog in aviation reporting. A temperature of −5 or −15 °C does not make every fog droplet an ice crystal; liquid can persist well below freezing.

Ice fog consists of tiny suspended ice particles and is associated with very cold air, often below about −30 °C and especially in colder extreme conditions. Local moisture sources such as open water, combustion or exhaust can contribute when the cold air requires very little vapour to reach ice saturation. It can remain concentrated near a source or in a stable low layer. Freezing fog and ice fog can both impair visibility, but their particle phase and formation details differ.

Do not infer the exact observed weather code, icing mechanism or local visibility from temperature alone. An observer assesses the actual phenomenon, while aircraft procedures address the surface condition and atmospheric encounter. A clear patch on the runway or a clean upper cloud view does not establish that the approach path or the whole airframe is free of a cold-fog hazard.

Why can visibility vary sharply within one weather system?

Compare particle concentration and path length rather than relying on a single phenomenon name.

Precipitation can obscure the scene

Drizzle contains many small droplets and can produce a persistent near-surface obscuration, often with low stratus or mist. Rain reduces contrast by scattering and attenuation, with heavier precipitation generally giving poorer visibility, but drop size, concentration and the background conditions matter. Snowflakes can strongly scatter light and obscure the horizon even at a moderate precipitation rate; visibility can fall abruptly in a snow shower or squall.

Fog and mist suspend droplets in the viewing path, whereas haze, smoke and volcanic ash involve different airborne particles. Smoke can spread far from a fire, and ash can remain a serious engine and system hazard even when it does not resemble dense visible smoke. Suspended dust (DU) and sand (SA) can impair horizontal and slant views after transport from a distant source. Visibility is one consequence; the absence of a very low reported value does not remove the other hazards of ash or dust ingestion.

Showers and narrow plumes create local variations: one station may report good visibility while an approaching precipitation curtain or smoke plume obscures a nearby part of the route. Compare the location, movement and extent of the obscuration with the flight path, not just the latest single-number report at the destination.

Why can the runway disappear when you descend towards it?

Distinguish the observation at the ground from the cockpit's actual sight line.

Different definitions, different paths

Ground visibility is the visibility at an aerodrome as reported by an accredited observer or automatic system. Flight visibility is the forward visibility from the cockpit of an aircraft in flight. Slant visibility refers to an oblique line of sight, while vertical visibility describes how far one can see vertically through an obscuring layer. A reported vertical visibility in an obscured sky is not a measured normal cloud-base height.

From above a shallow fog or haze layer, the vertical path through particles can be short enough to see ground features almost directly below. During descent and at a shallow approach viewing angle, the line of sight can pass through a much longer distance within the same layer. Contrast can then deteriorate and previously visible features can disappear. Within the layer, forward visibility may be much worse than the earlier downward view suggested.

For a horizontally uniform layer of thickness d, a straight ray at angle θ above the horizontal passes through approximately d/sinθ of that layer when it crosses the whole depth. A 200 m layer gives a 200 m vertical path but about 2,295 m at 5°. This geometric example does not predict a pilot's actual visual range: particle optical properties, target contrast, illumination and whether the ray crosses the whole layer all matter. It explains why the paths cannot be equated.

How can visibility collapse when nothing is falling from cloud?

Distinguish freshly falling precipitation from wind-lifted particles.

Drifting and blowing

Strong wind can lift loose snow, dust or sand from the surface without any new precipitation. In the aviation descriptors, low drifting (DR) refers to material raised below about 2 m above ground, while blowing (BL) raises it to 2 m or more above ground. BLSN can reduce visibility severely across a runway or route even with a blue sky overhead. Low drifting material can obscure low visual cues and create strong local contrasts even when the observer's eye-level visibility is less affected.

The same distinction applies to dust and sand: DRDU, DRSA, BLDU and BLSA describe the material and how it is lifted. Dust storms (DS) and sandstorms (SS) can produce deep, moving regions of concentrated particles and abrupt visibility changes. Gust fronts and thunderstorm outflows can raise a dust wall, while strong synoptic winds can maintain a much wider event.

The source, particle availability, wind and stability determine the concentration and vertical reach. Dust or snow can remain suspended and advect downwind after the strongest surface lifting has passed. A weather report with no rain or snow code therefore does not establish a clear visual route. Assess runway conditions and other particle hazards as well as atmospheric visibility.

Why does the same air look different towards a low Sun?

Include lighting and the windshield in the interpretation.

Illumination changes contrast

Looking towards a low bright Sun can increase glare and scattered light in haze, fog, thin cloud and precipitation, reducing the contrast of terrain and traffic. Looking away from it can give a much clearer view through the same nominal atmospheric layer. The reported visibility is therefore not a promise of identical cockpit detection performance towards every light source and background.

A bright top surface of haze, fog or cloud can reflect and scatter sunlight towards the aircraft. The resulting glare can mask ground detail and weaken horizon cues, particularly over snow-covered or featureless terrain. Diffuse lighting and a white surface can also remove shadows and texture, contributing to whiteout-like visual difficulty even where some meteorological visibility is measurable. The geometric and illumination context matters.

Ice, frost, water or contamination on the windshield further reduce the pilot's useful view by scattering, distortion or obscuration. Windshield icing can therefore worsen the cockpit sight line independently of a ground visibility report. Use the aircraft's approved windshield and icing systems and procedures; a visibility value measured outside the aircraft cannot certify the condition of its windows.

Can you apply the ideas?

Four original questions. Decide from the facts, then review the explanation.