What factor is essential when determining RVR values?

When determining Runway Visual Range (RVR) values, visibility at different points on the runway is essential. RVR is a measurement that indicates the distance over which a pilot can see the runway surface markings and lights, and adequate visibility is critical for safe takeoff and landing operations. This visibility can vary significantly along the length of the runway due to factors such as weather conditions, obstructions, and the specific location of the RVR measuring equipment. Therefore, it is important to consider visibility at different points on the runway to accurately assess the conditions that pilots will encounter.

Other factors mentioned, such as cloud cover, runway lights, and runway surface conditions, can influence overall visibility and contribute to the flight's safety, but they do not directly determine the RVR values themselves. RVR is specifically focused on how far a pilot can see and is quantified in terms of distance along the runway, making the visibility at various points along it the most critical factor for accurate RVR assessment.