Visibility is the ability to see and identify prominent objects by day and night as determined by atmospheric conditions. It can be reported in numerous measurements, such as hundreds of feet, statue miles, or meters. Visibility is not only our ability to see, but a minimum requirement for any kind of flight. There is a minimum visibility requirement for IFR approaches, VFR flight, VFR approaches, SFVR flight, and pilot training. It is also used by pilots when determining the proper approaches to fly into an airport, if an airport has enough visibility to land, or to go to an alternate if one is needed. With respect to visibility, there are five main types: flight visibility, ground visibility, prevailing visibility, runway visible value, and runway visible range.
Flight visibility is the average forward distance that can be seen from the cockpit in a horizontal distance when trying to identify prominent lighted or unlighted objects. These objects can be other planes, landmarks, mountains, buildings, radio towers, lighting towers, or any other type of object that can be clearly seen from a distance while in the air on a standard atmospheric day with unrestricted visibility.
Ground visibility is the highest distance that can be seen from the ground as reported by the National Weather Service or any other accredited reporting agency. When identifying objects from the ground, a set of known distances for different landmarks is normally used as to properly assess the visible distance from a predetermined point.
Prevailing visibility is the greatest distance that can be visibly seen in a horizontal direction throughout at least half of the horizon circle, and which does not need to be seen continuously. Prevailing visibility is measured by controllers in a tower by using a series of different landmarks like buildings, towers, or other geographical features that have predetermined distances figured out. The visibility is usually reported in the local METER. In situations where the prevailing visibility is less than 4 miles, the tower controllers are required to compare it to the ground visibility reading and use the less of the two for flight operations.
A runway visibility value is determined on a particular runway by using a device called a transmissometer. It is reported in miles or fractions of miles. It gives a continuous reading and is used instead of prevailing visibility when determining the minimums for that particular runway.
Runway visible range is a value that is derived using instruments and standard calibrations that represent the visible range a pilot can see down the runway during takeoff or landing. It is based on what the pilot sees, referencing either the intensity of the runway lights or other visual targets, choosing the greater of the two. It is measured by using horizontal and not slant range distance. Runway visual range will be used instead of prevailing visibility or runway visual value for determining visibility minimums for the airport. There are 3 main types of runway visible range:
Touchdown Runway Visible Range: visibility readout from the touchdown end of the runway.
Mid-Runway Visible Range: visibility readout from the middle of the runway.
Rollout Runway Visible Range: visibility readout from the nearest part of the rollout end of the runway.