Visibility — Page 269, Lesson 255

I want to walk you through the topic of visibility, which is a fundamental concept in aviation meteorology. Let's start with the core definition. The primary measure we use is called Meteorological Optical Range, or MOR. More simply, we call it 'met vis'. It is defined as the greatest horizontal distance at which a dark object can be recognized by an observer with normal eyesight during the day, or at which lights of a specified candlepower can be seen at night. So it's a measure of how far you can see a distinct object or a light source horizontally. There's also a specific term for the aerodrome environment: ground visibility. This is the visibility of an aerodrome as reported by an accredited observer on the field. In essence, visibility is a measure of atmospheric clarity — or, conversely, atmospheric obscurity. What causes that obscurity? It can be caused by water droplets — such as in cloud, fog, or rain. It can be caused by solid particles — like sand, dust, or smoke. Or it can be a mixture of the two, which we call smog — a blend of fog and smoke. Ice in the form of crystals, hail, or snow will also reduce visibility. Now, poor visibility is not random. It is usually associated with stable conditions, anticyclones (which are high-pressure systems), cols (the region between two high- and two low-pressure systems), inversions (where temperature increases with altitude), and light winds. These are the meteorological setups that trap particles and moisture near the surface. Visibility is generally better upwind of towns and industrial areas. Why? Because those areas are sources of pollution. The presence of hygroscopic nuclei — particles that attract water vapour — means that condensation can take place at relative humidities of less than 100%. This gives rise to the formation of mist and fog. Let's now look at the specific types of visibility reduction, defined by precise criteria. First, Mist. There is mist if the visibility is 1000 metres or more and the relative humidity is greater than 95%, with very small water droplets as the obscuring agent. The upper limit for reporting mist is usually 5000 metres — this is discussed further under METARs, the aviation weather reports. Second, Fog. There is fog if the visibility is less than 1000 metres and the obscuring agent is water droplets. The relative humidity will be near 100% — the air is saturated. Third, Haze. There is haze if the visibility is reduced by extremely small solid particles — sand, dust, or smoke. If the visibility is reduced below 1000 metres, it is shown on synoptic charts with a special symbol. Again, haze is not usually reported when the visibility is more than 5000 metres. Now let's move to a specific type of fog: Radiation Fog. Radiation fog is caused by the radiation of the earth's heat at night, combined with the conductive cooling of the air in contact with the ground to below the dew point. Let me explain that. During a clear night, the ground loses heat by radiating it into space. The ground becomes cold. The air that is in direct contact with that cold ground is then cooled by conduction — it loses heat to the ground. If that cooling brings the air temperature down to the dew point, condensation occurs and fog forms. There is an important distinction based on wind speed. If there is a light wind, then fog will form. If conditions are calm — no wind at all — the result will be the formation of dew, not fog. The air doesn't mix, so moisture condenses on the ground itself. There are three necessary conditions for radiation fog to form. First, Clear sky — to increase the rate of terrestrial radiation. No cloud cover means the ground's heat can escape freely into space. Second, High relative humidity — so that only a little cooling will be enough to cause saturation and condensation. The air is already nearly saturated. Third, Light wind — of 2 to 8 knots. This wind is just enough to mix the layers of air, causing turbulence. That turbulence keeps the droplets in suspension so they don't just settle out, and it brings warmer air from above down into contact with the cold ground, which thickens the fog layer. So to summarise: radiation fog forms on clear nights with high humidity and a light wind of 2 to 8 knots. Calm conditions give you dew instead.