Which condition triggers a SPECI for visibility?

• A. When prevailing visibility is below the aerodrome's highest alternate minimum visibility or 5000m, whichever is greater
• B. Wind speed changes by 20 knots
• C. Thunderstorms (Ts) etc.
• D. Cloud base is 1500 ft or higher

Answer: (A)

The key idea here is when a SPECI for visibility must be issued. A SPECI is sent to flag a significant change in weather conditions that directly affects aviation operations, and for visibility the rule is that it’s triggered when prevailing visibility falls below the aerodrome’s highest alternate minimum visibility, or 5000 meters, whichever is greater. This ensures pilots and operators are warned if visibility drops to a level that would increase the difficulty of planning alternates or executing approaches, using the most conservative minimum that could be required.

That’s why this choice is the right one: the threshold ties the report to the operational planning minimums and a 5 km baseline, adjusted up if the aerodrome requires higher visibility for alternates. The other options don’t define when a SPECI for visibility must be issued—wind changes by 20 knots, thunderstorms, or a cloud base of 1500 ft or higher may influence conditions, but they do not establish the specific visibility threshold that triggers a SPECI for visibility.