When must the tower take/report prevailing visibility?

• A. Greater than 6 SM
• B. Equal to 5 SM
• C. Less than 4 SM VIS
• D. Variable

Answer: (C)

Prevailing visibility is the distance over which the majority of the horizon is visible in the weather observations, and it’s a key factor in safety for departures and arrivals. In the tower, you must take and report prevailing visibility when it falls below four statute miles because that level of reduced visibility can meaningfully affect takeoffs, landings, and approach operations, and pilots and the rest of the ATC system need to be aware of it promptly. When visibility is at or above four miles, reporting it as the prevailing condition isn’t required for a tower weather update; other systems and reports (like METAR/ATIS) cover the current conditions. So the trigger for taking/reporting is any value less than four statute miles.