Meteorological Optical Range (MOR), or more simply ‘met vis’ is the greatest horizontal distance at which a dark object can be recognized by an observer with normal eyesight, or at which lights of specified candlepower can be seen by night. Ground visibility is the visibility of an aerodrome as reported by an accredited observer. In effect, visibility is a measure of atmospheric clarity, or obscurity. This can be caused by water droplets – cloud, fog, rain, or solid particles – sand, dust or smoke, or by a mixture of the two – smog (fog and smoke). Ice, in the form of crystals, hail or snow will also reduce visibility.
Poor visibility is usually associated with stable conditions, anticyclones, cols, inversions and light winds. Visibility is generally better upwind of towns and industrial areas. The presence of hygroscopic nucleii means that condensation is likely to take place at relative humidities of less than 100%, giving rise to the formation of mist and fog. The various types of reduction in visibility are:
• Mist. There is mist if the visibility is 1000 m or more and the relative humidity is greater than 95% with very small water droplets. The upper limit for reporting mist is usually 5000 m, this
is discussed under METARs.
• Fog. There is fog if the visibility is less than 1000 m and the obscuring agent is water droplets. Relative Humidity (RH) will be near 100%.
• 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 m, it is shown on synoptic charts as . Again, haze is not usually reported when the visibility is more than 5000 m.