What is the primary purpose of a transmissometer within the RAWS framework?

• A. Measures the prevailing visibility along a defined path to determine RVR.
• B. Measures precipitation rate.
• C. Measures humidity.
• D. Measures wind speed.

Answer: (A)

A transmissometer is used to quantify how much light travels along a fixed optical path, and that measurement is directly used to determine visibility along that path. In the RAWS framework, the amount of light that makes it from the emitter to the receiver decreases as fog, haze, rain, or other aerosols increase, so the device reports a transmissivity value that is converted into the prevailing visibility for that specific path. This path-based visibility is what defines Runway Visual Range (RVR), giving pilots and controllers a precise, runway-focused visibility measure. The other instruments serve different purposes: precipitation sensors measure how much rain or other precipitation is falling, humidity sensors gauge moisture in the air, and wind sensors (anemometers) measure wind speed.