Section 2, Chapter 5 - Prevailing Visibility

Question 1

How is prevailing visibility defined?

Answer 1

Prevailing visibility is the visibility value that is reached or exceeded within at least half the horizon circle or at least half of the aerodrome surface. These areas can be contiguous or non-contiguous sectors.

Question 2

What are the guidelines for reporting prevailing visibility at an observing station?

Answer 2

• Always report the prevailing visibility.
• Report the lowest visibility if it is less than the prevailing visibility and less than 1500 meters or 50% of the prevailing visibility.
• Indicate the direction of lower visibility using compass points.
• If the lowest visibility is in multiple directions, report the most operationally significant one.
• If visibility is fluctuating rapidly and prevailing visibility cannot be determined, report only the lowest visibility without indicating direction.

Question 3

How should visibility reports be transmitted to pilots, and how are ATC procedures affected by visibility values?

Answer 3

Transmitting Visibility Reports:
* Report visibility without using the term “prevailing visibility.”
* Example: “METAR EGxx 1250 1800 0800NE” should be transmitted as “Weather at one two five zero, visibility one thousand eight hundred metres, eight hundred metres to the North East.”

ATC Procedures:
* Implement specific ATC procedures based on visibility conditions.
* When two visibility values are present, use the lower value to determine whether to implement procedures.