Definition
An instrument system used at airports to measure the transparency of the atmosphere along a fixed horizontal path, providing the data used to calculate Runway Visual Range (RVR). It works by projecting a light beam from a transmitter to a receiver a known distance away and measuring how much of that light reaches the receiver.
Plain English
A device beside the runway that shines a light beam across a fixed distance and measures how much of the light gets through. The clearer the air, the more light arrives. That reading is turned into a visibility value pilots see as RVR.
Context Anchor
Seen in airport weather reporting, especially at airports that report runway visual range during low-visibility conditions.
Derivation
From 'transmission' (Latin transmittere, to send across) plus '-meter' (Greek metron, a measure). Literally, an instrument that measures how much light is sent across the air -- which is exactly what it does.
Why Pilots Care
RVR readings from transmissometers directly affect landing and takeoff minimums, helping pilots decide whether conditions are safe for operations.
Example Sentence 1
The tower reported the RVR as 2,400 feet, based on the transmissometer at the touchdown zone.
Example Sentence 2
During the preflight briefing the pilot noted that the transmissometer values had improved enough to meet the published landing minimums.