An "RVR" in a METAR report measures what?

The term "RVR" in a METAR report stands for "Runway Visual Range." This is a crucial measurement that reflects the distance over which a pilot can see the runway surface and perceive the runway's lighting and markings. RVR is particularly significant during poor visibility conditions, such as fog or heavy precipitation, as it informs pilots about their ability to land safely.

Runway Visual Range is measured using instruments located near the runway, which take into account the visibility conditions and can change based on weather conditions. This measurement is essential for airport operations, especially for those using instrument landing systems (ILS) for landings.

The other options, while they may sound similar, do not correctly represent what "RVR" means in the context of aviation reporting. Radar visibility refers to radar-derived visibility calculations, which are not specifically tied to runway conditions. Rainfall velocity rates and relative visibility range are not standard aviation terms and do not apply in this context. Thus, "Runway Visual Range" captures the true essence of what RVR signifies in a METAR report.