If the prevailing visibility varies rapidly during the observation, the prevailing visibility is reported as:

When prevailing visibility varies rapidly during an observation, the chosen method to report it is to calculate the average of all observed values. This approach provides a more comprehensive assessment of visibility conditions over the period of observation, rather than focusing on a specific set of extreme data points or the highest or lowest individual readings.

By averaging all values observed, the report reflects a more realistic and representative measure of visibility that pilots and other aviation stakeholders can rely on. They need to understand the general visibility trends rather than just the extremes, which can fluctuate significantly in a short time frame.

Other methods, such as using the extremes (highest or lowest) or simply averaging the extremes, could lead to partially informative reports that don't account for the full range of visibility experienced during the observation, creating potential misunderstandings in operational contexts.