In which condition is clearances particularly necessary?

Clearances are particularly necessary in conditions of reduced visibility because such circumstances can significantly impact a pilot's ability to see and interpret their surroundings, including other aircraft, the runway, and navigational aids. When visibility is compromised—whether due to fog, rain, or other atmospheric conditions—clearances from air traffic control (ATC) help to ensure that aircraft are sequenced properly and are safely separated from one another. This is crucial to prevent collisions and to maintain an effective flow of traffic, as pilots may not be able to judge distances or identify other aircraft visually as accurately as they would in clear conditions.

In contrast, other options like low traffic volume or favorable weather do not necessarily require the same level of vigilance in communication and coordination because, with fewer aircraft, the risk for conflicts diminishes. Similarly, high visibility conditions allow pilots to navigate and observe their environment more easily, reducing the necessity for stringent clearances compared to reduced visibility.