What visibility means
Visibility is the horizontal distance at which an object can be recognised. It is a measure of atmospheric clarity, not merely a weather report detail: it controls what can be seen during taxi, approach, landing and departure.
Which visibility is being discussed?
Meteorological optical range, or MOR, is the greatest horizontal distance at which a black object of suitable size can be recognised against the horizon in daylight. At night the equivalent idea uses specified lights. Ground visibility is the visibility observed at the aerodrome. Both are distinct from the oblique view a pilot has from the cockpit.
Mist, fog, haze and smog
| Phenomenon | What is in the air? | Visibility and cue |
|---|---|---|
| Fog | Water droplets, or ice crystals in very cold air. | Below 1000 m; relative humidity is nearly 100 percent. |
| Mist | Water droplets. | 1000 m to 5000 m; high humidity, often above 95 percent. |
| Haze | Very small dust, smoke or other dry particles. | Usually reported at 5000 m or less; the air looks milky. |
| Smog | A mixture of fog with smoke haze. | Favoured by stagnant air and a strong inversion. |
Poor visibility is common where air is stable: in a high pressure system, a col, an inversion or a light wind. The lack of mixing keeps moisture and pollution near the surface. Visibility is often worst when looking toward the sun because forward scattering makes the background brighter and contrast poorer.
What reduces visibility
Anything that fills the line of sight with drops, crystals or particles reduces contrast. The weather need not be foggy: precipitation, smoke, dust, sand, sea spray, volcanic ash and industrial pollution can all obscure the air.
Precipitation and suspended particles
Drizzle is particularly effective because a huge number of small drops lie in the line of sight. Oxford’s planning values show the scale: moderate rain commonly gives 3 to 10 km, heavy rain can lower visibility below 1000 m, moderate snow about 1000 m and heavy snow only 50 to 200 m. These are guides, not promises.
Dust haze is favoured by strong winds over desert or semi arid land and may extend several kilometres vertically. Smoke haze is more characteristic of calm or light winds beneath a strong inversion. Sandstorms can reduce visibility abruptly; sea spray is most relevant close to a rough sea and strong onshore wind.
Why stability matters
A slack pressure gradient means little wind and little vertical mixing. Under an inversion, contaminants cannot rise through the stable layer, so their concentration increases near the surface. This is why highs and cols can be poor visibility environments even when the weather feels otherwise quiet.
Visibility versus cloud base
Cloud base and visibility often deteriorate together in wet weather, but they are separate quantities. A low stratus base does not itself define fog. Fog is cloud at ground level, while a low cloud base above the field may still leave a useful horizontal view beneath it.
| Reducer | Most likely setting | Useful distinction |
|---|---|---|
| Smoke haze | Light wind, stable inversion, built up area. | Dry particles rather than a saturated cloud. |
| Dust or sand | Strong wind, dry ground, desert or semi arid land. | May be deep and can occur in clear sky above. |
| Heavy rain or snow | Active precipitation shaft. | Can reduce range rapidly without fog. |
| Sea spray | Strong wind over a rough sea. | Most concentrated near the surface and coast. |
How fog forms
Every fog mechanism brings air to saturation, but the route matters. Some cool the air in place, some move warm moist air over a colder surface, and some lift stable air until the cloud base descends to the ground.
Radiation fog
Radiation fog forms over land on a clear night when the ground loses heat by radiation. It needs enough moisture, a clear sky and a light wind, commonly about 2 to 8 kt. The light wind spreads the cooling through a shallow layer; a stronger wind mixes in drier air and prevents the fog. It is frequent in valleys and northern India during December to February, often first collecting in low ground.
Advection fog
Advection fog forms when warm moist air moves over a colder land or sea surface and is cooled below its dew point. Unlike radiation fog, it can form by day or night and can persist over a wide area. It is common over land in winter or early spring and over the sea in late spring or early summer. A change of air mass or a stronger wind can disperse it.
Other fogs to recognise
Hill fog is stratus or stratocumulus with a base below hill summits, formed as stable moist air is forced up a slope. Frontal fog commonly occurs ahead of a warm front or an occlusion when precipitation lowers the cloud base to the surface; it may occupy a broad belt. Steam fog appears when cold stable air passes over warmer water. Freezing fog has supercooled droplets; ice fog is associated with exceptionally low temperatures.
Measuring visibility and RVR
By day an observer can assess ground visibility against known objects. Instrument systems supply an objective reading when the range is too short for that method or when a runway specific value is needed.
Runway visual range
Runway visual range, RVR, is the range over which a pilot at about 15 ft above the touchdown zone can see runway markings or lights along the runway. It is reported when visibility is below 1500 m. RVR is measured, not forecast, and is generally greater than MOR because runway lights are designed to be seen.
A transmissometer sends a known light beam to a photoelectric receiver and derives the atmospheric transmission. A forward scatter meter measures light scattered by particles. An instrumented RVR system may report at the touchdown, midpoint and stop end to show how the runway varies from one end to the other.
What a number cannot tell you alone
RVR is not the same as cloud base, slant visibility or the general view from an aircraft in flight. It is an operational runway measurement under stated geometry. It is therefore the value used for many low visibility approach and take off decisions.
| Measure | What it represents | How it is obtained |
|---|---|---|
| MOR | Atmospheric horizontal clarity. | Object contrast by day or specified lights at night. |
| Ground visibility | Visibility observed at the aerodrome. | Observer or automatic system. |
| RVR | Runway view from a pilot near touchdown height. | Runway specific instrumented value. |
| Slant visibility | View along an oblique line from an aircraft. | Changes with altitude, sun angle and layers. |
The practical habit is to translate every report into the feature that affects the flight: what is obscuring the view, how quickly could it change, and which reported quantity applies to the runway decision.