What is a visibility sensor (transmissometer) and how does it relate to RAWS?

• A. Measures prevailing visibility along a defined path to determine Runway Visual Range (RVR) and inform takeoff/landing decisions.
• B. Measures air temperature.
• C. Measures wind direction.
• D. Measures cloud base.

Answer: (A)

A transmissometer provides the prevailing visibility along a fixed optical path, and that measurement is used to determine Runway Visual Range (RVR) to guide takeoff and landing decisions. In practice, a light beam is transmitted along a defined distance to a receiver; the amount of light reaching the receiver is attenuated by atmospheric conditions, and this attenuation is converted into a visibility value for the runway segment.

In RAWS, this visibility data is a key part of the airfield weather subsystem. It feeds RVR readings used by pilots, air traffic control, and operations to assess whether instrument approaches are required, what minima apply, and whether runway operations can continue safely. The transmissometer’s output directly supports the determination of RVR, which is essential for safe airfield and runway management.

The other measurements—temperature, wind direction, and cloud base—are tracked by different sensors, so the function described matches the transmissometer’s purpose.