Visibility Explained: What the Distance Means

What Is Visibility?

Visibility is the greatest distance at which you can see and identify objects clearly. In meteorology, it’s defined more precisely as the Meteorological Optical Range (MOR): the distance over which a beam of light is reduced to 5% of its original intensity by particles and droplets in the air.

When a forecast says “visibility 2 km,” it means that’s roughly how far you can see before the atmosphere obscures things. It’s measured at roughly eye level and reported for a specific location; conditions can vary even a short distance away.

What Affects Visibility?

Anything suspended in the air that scatters or absorbs light will reduce visibility:

• Fog: The most common cause of very low visibility. Tiny water droplets suspended near the surface can reduce visibility to anywhere from a few hundred metres down to near zero in the densest cases.
• Mist: A thinner veil of water droplets that reduces visibility but keeps it above the 1 km fog threshold.
• Rain: Light rain has a modest effect. Heavy rain and thunderstorms can reduce visibility to 1–3 km, and extreme downpours can push it below 1 km.
• Snow: Falling snow scatters light significantly. Blizzards, combined with blowing snow, can reduce visibility to near zero.
• Haze: Fine dry particles like dust, smoke, or pollution suspended in the air. Common in cities and during wildfires.
• Humidity: Even without fog, high humidity causes airborne particles to swell, scattering more light and reducing visibility.

Visibility Categories

The internationally agreed threshold between fog and mist is 1 km. Below that, it’s officially fog. Above 1 km but below about 2 km with water droplets present, it’s mist. Beyond that, forecasts typically describe visibility as poor, moderate, or good, though the exact boundaries vary between meteorological services and regions.

As a rough guide: below 4 km is generally considered poor, above 10 km is good, and anything in between is moderate.

How Fog Forms

Fog forms when air near the surface cools to its dew point and water vapour condenses into tiny suspended droplets. The most common types:

• Radiation fog: Forms on clear, calm nights as the ground loses heat. Common in valleys and low-lying areas, and typically burns off after sunrise.
• Advection fog: Warm, moist air moves over a colder surface, like warm air drifting over cold ocean water. Can persist day and night and cover large areas.
• Upslope fog: Moist air is pushed uphill by terrain and cools until it reaches saturation. Common in mountainous areas.
• Steam fog: Cold air moves over much warmer water, causing rapid evaporation and condensation. Often seen over lakes and rivers in autumn.

How Visibility Is Measured

Modern weather stations use optical sensors to measure visibility automatically. The two main types:

• Transmissometers: Send a beam of light across a fixed distance to a detector. The amount of light that arrives tells you how much the atmosphere is absorbing and scattering. Highly accurate at short range and widely used at airports for runway visual range.
• Forward scatter sensors: Emit a beam of light while a detector, angled off to the side, measures how much light is scattered forward by particles in the air. Work well across a wide range of visibilities and are more compact to install.

Historically, trained observers estimated visibility by identifying known landmarks at known distances, for example a church spire at 3 km and a hilltop at 10 km. Some staffed weather stations still use this method as a supplement.

Why Visibility Matters

• Driving: Visibility is generally considered seriously reduced when you can’t see beyond about 100 metres. You should use headlights, slow down, and increase your following distance. In dense fog, low beam headlights are safer because high beams reflect off the droplets and create glare.
• Aviation: Pilots rely on precise visibility readings to determine whether they can land visually or need instrument approaches. Airports have strict minimum visibility requirements for take-off and landing.
• Shipping: In restricted visibility, vessels must proceed at a safe speed and use radar and sound signals. Fog at sea is particularly dangerous because there are no landmarks for reference.
• Air quality: Persistently poor visibility on dry days often signals high levels of fine particulate matter in the air.

How Airpult Shows Visibility

On Airpult, visibility is shown on the forecast page alongside other conditions. Use the explore page to search for any location and check its visibility forecast.