Definition
A method used by automated weather observing systems to calculate reported visibility by averaging visibility sensor readings collected over the previous 10 minutes using a harmonic mean rather than a simple arithmetic mean. The harmonic average gives greater weight to lower visibility values, so the reported figure leans toward the more restrictive readings within the 10-minute window.
Plain English
It is the way an automated weather station figures out the visibility number it reports. Instead of just adding up the last 10 minutes of readings and dividing, it uses a math method that pulls the result toward the worst readings. So if visibility has been bouncing around, the reported value reflects the lower, more cautious side of what the sensor saw.
Context Anchor
Seen in discussions of automated weather observing systems, especially how AWOS or ASOS equipment reports visibility.
Derivation
"Harmonic" here comes from mathematics, not music. A harmonic mean is calculated by averaging the reciprocals of the values, then taking the reciprocal of that result. This kind of average naturally weights small numbers more heavily than large ones, which is why it is useful for visibility -- a brief drop to low visibility matters more to a pilot than a brief jump to high visibility.
Why Pilots Care
Gives pilots a reliable wind speed figure for performance calculations instead of reacting to brief spikes or lulls.
Analogy
If you drive part of a trip very slowly, your overall trip average drops a lot, even if you drive faster later. A harmonic average works in a similar spirit: the lower values matter more.
Grounding Statement
For automated visibility, this kind of average helps the report show a real short-term reduction instead of hiding it inside a simple 10-minute average.
Intuition Check
Do not read “harmonic” as something related to sound or music here. Also, do not assume this is a normal average; it is an average that is pulled more strongly by low readings.
Example Sentence 1
The visibility you hear on the AWOS broadcast is a 10-minute harmonic average, not the instantaneous reading from the sensor.
Example Sentence 2
Before takeoff the pilot checked that the 10-minute harmonic average remained below the aircraft crosswind limit.