What is defined as the greatest visibility, equaled or exceeded throughout at least half the horizon circle?

The term that refers to the greatest visibility that is equaled or exceeded throughout at least half of the horizon circle is known as prevailing visibility. This definition is important in aviation and meteorology because it helps pilots and air traffic controllers understand the atmospheric conditions best suited for safe operations.

Prevailing visibility is a critical parameter because it reflects the most common visibility conditions for a given location, essential for flight planning and safety assessments. Being informed about the prevailing visibility allows pilots to make informed decisions about takeoffs, landings, and in-flight operations, especially in conditions where visibility can significantly change, such as during fog or rain.

In contrast, minimum visibility relates to the smallest visibility observed, average visibility provides a mathematical mean, and standard visibility does not specifically define visibility in the context of the horizon circle. Understanding these distinctions can help you better interpret aviation weather reports and ensure all safety precautions are taken.