Visibility tells you how far atmospheric conditions allow you to see and identify an object. A forecast value, a human observation and an airport runway report describe different things. The distinction matters when fog is shallow, precipitation is moving through or hills break up the view.
The number is also not one unit. Kilometres, metres and statute miles appear in different products.
What Visibility Means
A visual observation is based on recognisable targets at known distances. By day, that might be a building, ridge or mast; by night, observers use suitable lights. At a staffed weather station, prevailing visibility is the greatest distance reached or exceeded across at least half of the horizon circle. The sectors do not have to be continuous, so one direction may be much less clear than the reported value.
Automatic stations commonly estimate Meteorological Optical Range (MOR). The World Meteorological Organization definition is the path over which a standard light beam falls to 5 per cent of its original luminous flux. A forward-scatter sensor infers this from light scattered through a small volume of air; a transmissometer measures how much light is lost along a fixed path. Both turn an optical measurement into a distance, whereas an observer judges whether a real target can be seen and identified.
- Human observation: targets seen around a station; the result depends on direction, target and lighting.
- Instrument or MOR: light extinction in sampled air; a sensor covers one small area or path.
- Model forecast: expected near-surface visibility in a grid cell; small fog banks and terrain may be unresolved.
Kilometres, Metres and Miles
Public weather forecasts often use kilometres. Many meteorological and aviation reports outside the United States use metres. US aviation observations report statute miles, not nautical miles. Nautical miles are a navigation unit. They are not the unit printed after SM on a METAR.
One statute mile is about 1.61 kilometres (1,609 metres). One kilometre is about 0.62 statute miles. The US fog threshold of 5/8 statute mile is about 1 kilometre, which is why it matches the WMO fog distance in practice.
METAR Reports
A METAR is an aviation observation with its own coding rules. In the United States, prevailing visibility is reported in statute miles. 10SM means 10 statute miles or more, about 16 kilometres. Some US forecasts use P6SM for more than 6 statute miles.
Most other countries report metres as a four-digit group. 9999 means 10 kilometres or more, not a measured 9,999 metres. 1200 is 1,200 metres. 0500 is 500 metres. CAVOK replaces the visibility group when several conditions are met together, including visibility of at least 10 kilometres; it is not used in US domestic METAR format. METARs and TAFs Explained covers the surrounding codes.
Units and local conventions matter, so a bare number should never be separated from the report that contains it.
What Reduces Visibility
Visibility falls when droplets, ice crystals or dry particles scatter or absorb light. Fog is made of tiny water droplets near the surface, or ice crystals in ice fog. Under the WMO convention, it is called fog when those particles reduce horizontal visibility below 1 kilometre. Mist is the lighter form in which visibility does not fall below 1 kilometre.
There is no universal 2-kilometre upper limit for mist. Some national forecasts label mist across a range of several kilometres. Those are local reporting practices, not a second WMO fog threshold. In US observations, fog is generally used below 5/8 statute mile, and mist from that distance up to 7 statute miles when the obstruction is water droplets.
Haze comes from very small dry particles such as smoke, dust, sea salt and pollution. Humidity can make some particles take up water and scatter more light, so a hazy view may worsen even before fog forms. Heavy rain, falling or blowing snow and sea spray can also hide contrast very quickly. A value below 1 kilometre therefore does not prove that fog is the cause. Check the weather symbol and precipitation forecast as well as the distance. Humidity and Dew Point Explained and What Is Precipitation? give the moisture and fallout context.
Fog can be patchy, shallow and strongly shaped by the ground. Valleys collect cool air, a light onshore flow can bring moist air across a cold surface, and a bank may sit below a nearby hilltop. For the formation process and named fog types, see Fog Explained.
Observation and the Airpult Forecast
Airpult shows a near-surface visibility estimate from configured numerical weather models, not a live measurement at your exact position. A model divides the atmosphere into grid cells and calculates how conditions develop at set time steps. The value attached to a location represents the model’s surroundings, which may not match the elevation or fine terrain at the pin.
This is particularly relevant in mountains, along coasts and beside lakes or rivers. Forecast models smooth terrain to fit their grid, while a narrow valley, pass or shoreline can control where low cloud and fog settle. Around Sion Airport, for example, use the forecast to follow the expected trend but remember that conditions on the valley floor and surrounding slopes need not change together.
Shallow fog, a smoke plume or a heavy shower may cover only part of a grid cell. Timing can also shift between model updates. Airpult normalises values from several data sources, and some source-processing paths have an upper ceiling near 16.09 kilometres. That distance is 10 statute miles, a common reporting cap. A value that repeatedly reaches a ceiling means “at least this far” for practical reading; it is not a measurement accurate to the last ten metres.
The trend is usually more useful than false precision. Compare adjacent hours: a fall from 15 kilometres to 3 kilometres alongside rising humidity and light winds carries more information than one isolated value. Look at neighbouring places too, especially when your route crosses a valley, ridge or coast. Recent station observations and local warnings can reveal a bank that the forecast grid misses.
Day, Night and Terrain
The displayed distance describes the atmosphere, not your eyes, headlights or camera. A dark object is harder to pick out at night, while a bright light may remain visible through haze farther away. Looking towards a low sun can also reduce usable contrast. Hills, buildings and trees block the line of sight even in clear air, so “visibility 10 km” never promises an unobstructed ten-kilometre view.
Low cloud is a separate issue. Horizontal visibility at the surface may be good while a summit is hidden in cloud, or a valley may be foggy beneath clear higher ground. When an obscuration hides the sky, a METAR may report vertical visibility instead of a cloud ceiling. That vertical figure is not the same as the horizontal distance. Cloud Cover Explained covers layers and ceilings.
Walkers, climbers and photographers should check cloud, precipitation, daylight and wind alongside visibility. If recognising landmarks is part of navigation, carry the appropriate map and navigation equipment rather than relying on a distant view remaining available.
Driving and Aviation
Road users and pilots both care about how far they can see, but they use different products, units and legal limits. A consumer forecast is a planning aid for both, not a substitute for the view ahead or for official aviation information.
Driving
There is no worldwide visibility category that determines a safe road speed. Follow local warnings and road-lighting laws, slow down enough to stop within the distance you can see, leave more space and use the demister and wipers. Low-beam headlights improve conspicuity; high beams reflect from fog droplets and can worsen glare. The US National Weather Service guidance also advises delaying the journey or leaving the road for a safe stopping place when visibility is near zero.
The often-quoted 100-metre figure belongs to particular road codes, including the UK rules for fog lights. It is not a universal meteorological threshold and says nothing about surface grip, traffic, darkness or sudden patches. Use the forecast to anticipate a problem, then respond to what is actually visible through the windscreen.
Aviation
Airpult is not an aviation briefing. Flight decisions require official METARs, TAFs, NOTAMs, published procedures and current air traffic information. Runway visual range (RVR) is an instrument-derived, runway-specific estimate of how far a pilot on the centreline can see runway markings or lights. It differs from prevailing visibility and from a consumer model forecast. Operational minima depend on the procedure, aircraft, crew authorisation and working ground and airborne equipment; a forecast visibility value cannot replace any of them.
Visibility and Air Quality
Smoke and aerosols can spoil a view, but visibility alone cannot identify a pollutant or its health effect. Clear-looking air can still contain harmful pollution, while mist can reduce visibility without implying a high particle concentration. Read AQI: How the Air Quality Index Works and official smoke or health advice separately.
Check the Local Forecast
Use Airpult Explore to find a place and compare its hourly visibility with fog, cloud and precipitation. Before driving, flying or heading onto exposed ground, add current observations, transport information and official warnings for the area.
