The final module turns from the weather itself to the machine that reports it, and starts with the two numbers a crew acts on more often than any others: the wind and the visibility. Both come in several variants that look interchangeable and are not, and this lesson pins down exactly what each one measures, in what units, and which of them belongs to which decision.
The wind in the report, the wind from the tower
The METAR's wind and the tower's wind are cousins, not twins:
The METAR's wind — a 10-minute mean, with the direction in degrees true, from the aerodrome's standard anemometer: a climatological, planning number. A gust group is appended when the maximum wind in those 10 minutes exceeds the mean by 10 kt or more.
The tower's wind — for takeoff and landing, a different instrument reading for a different purpose: a 2-minute mean, with the direction in degrees magnetic (magnetic so it can be set directly against the runway heading), from sensors representative of the runway in use.
Same sky, two winds: true and averaged for the record, magnetic and fresh for the landing.
What visibility means
Visibility has an exact definition, and it comes in two halves; the greater of the two counts: the greatest distance at which a black object of suitable size near the ground can be seen and recognised against a bright background, and the greatest distance at which lights of about 1,000 candelas can be seen and identified against an unlit background. Day and night in one definition: the reason a visibility number means the same thing at noon and at midnight.
Measuring it
Two ways to put a number on the air's transparency:
Human observers — using landmarks at known distances by day and lights of known intensity by night.
Instruments — transmissometers and forward-scatter meters, which measure how much a light beam is attenuated or scattered by the air and convert that into a visibility.
The instruments never blink; the observer sees the whole horizon. The reporting system uses both.
Prevailing and ground visibility
Two defined variants carry the observation into the reports:
Ground visibility — the visibility at an aerodrome as reported by an accredited observer or automatic systems.
Prevailing visibility — the METAR's headline value: the greatest visibility reached or exceeded within at least half the horizon circle, or at least half the aerodrome surface; the qualifying areas need not be contiguous.
One fog bank in an otherwise clear circle does not drag the prevailing value down; it appears instead as a lowest-visibility group with its direction.