Under Cooperative Weather Watch, what condition triggers reporting changes in tower prevailing visibility to the local EU?

• A. When tower prevailing visibility is at or above 6 miles
• B. When tower prevailing visibility is less than 4 miles and differs from the surface prevailing visibility
• C. When surface visibility is worse than tower visibility
• D. When METAR is not current

Answer: (B)

In Cooperative Weather Watch, the reporting trigger is when the tower’s prevailing visibility falls below four miles and is different from the surface prevailing visibility. This combination flags a notable, localized change in conditions near the tower that isn’t reflected at the surface level, and it’s important to alert the local En Route Facility (EU) so en route weather updates remain accurate for flight planning and safety.

The four-mile threshold ensures the change is significant enough to impact operations, while the difference from surface visibility signals a microclimate discrepancy that the EU needs to know about. If the tower visibility is not below four miles, or if it matches the surface visibility, there isn’t the same urgency to report this specific change. METAR currency isn’t the factor here; this rule targets the relationship between tower and surface conditions.