AIM ¶ 7-1-15 — Prevailing Visibility Reporting
AIM 7-1-15 explains how prevailing visibility is determined and reported in METARs, including tower vs. surface observations and obscuration reporting.
In Plain English
Prevailing visibility is the greatest horizontal visibility equaled or exceeded throughout at least half of the horizon circle — and that half doesn't have to be contiguous. It's reported in METARs in statute miles (e.g., 1/4, 1/2, 1, 1 1/4, 2, 3, 5, 10).
Key points to remember:
- Visibility is measured by identifying preselected, prominent objects at known distances from the usual observation point.
- When visibility drops below 7 miles, the report must identify the obscuring phenomenon — fog, haze, smoke, mist, or combinations.
- Sectors with significantly different visibility (e.g., 2 SM in one quadrant vs. 3 SM elsewhere) are noted in the remarks section.
- Manual, ASOS, and AWOS sites use slightly different reportable increments.
- When prevailing visibility is less than 4 miles, certified tower controllers also take an observation. The lower of the surface or tower value is used as the official prevailing visibility for aircraft operations.
This matters operationally because that reported number drives whether you can legally depart, shoot an approach, or operate VFR in a given airspace.
AIM Source Text
FAA AIM ¶ 7-1-157-1-15. 7-1-15. Reporting Prevailing Visibility
Surface (horizontal) visibility is reported in METAR reports in terms of statute miles and increments thereof; e.g.,/ 16 , / 8 , 16 ,/ 4 , 16 , 8 , 2 , 8 ,/ 4 , / 8 , 1, 1 / 8 , etc. (Visibility reported by an unaugmented automated site is reported differently than in a manual report, i.e., ASOS/AWOS: 0, / 16 , 8 ,/ 4 , 2 , / 4 , 1, 1 / 4, 1 2, 1 4, 2, 2 2, 3, 4, 5, etc., AWOS: M/ 4 , / 4 , 2 , / 4 , 1, 1 / 4 , 1 2, 1 4, 2, 2 2, 3, 4, 5, etc.) Visibility is determined through the ability to see and identify preselected and prominent objects at a known distance from the usual point of observation. Visibilities which are determined to be less than 7 miles, identify the obscuring atmospheric condition; e.g., fog, haze, smoke, etc., or combinations thereof. Prevailing visibility is the greatest visibility equaled or exceeded throughout at least one half of the horizon circle, not necessarily contiguous. Segments of the horizon circle which may have a significantly different visibility may be reported in the remarks section of the weather report; i.e., the southeastern quadrant of the horizon circle may be determined to be 2 miles in mist while the remaining quadrants are determined to be 3 miles in mist. When the prevailing visibility at the usual point of observation, or at the tower level, is less than 4 miles, certificated tower personnel will take visibility observations in addition to those taken at the usual point of observation. The lower of these two values will be used as the prevailing visibility for aircraft operations.
Oral Exam Questions a DPE Might Ask
Q1How is prevailing visibility defined?
Per AIM 7-1-15, prevailing visibility is the greatest visibility equaled or exceeded throughout at least one half of the horizon circle, which does not need to be contiguous.
Q2When tower visibility and surface visibility differ, which value is used?
Per AIM 7-1-15, when prevailing visibility at the usual point of observation or tower level is less than 4 miles, certified tower personnel also take an observation, and the lower of the two values is used as the prevailing visibility for aircraft operations.
Q3When must an obscuring phenomenon be identified in a visibility report?
Per AIM 7-1-15, whenever visibility is determined to be less than 7 miles, the obscuring atmospheric condition — such as fog, haze, smoke, or combinations thereof — must be identified.
Related Paragraphs in AIM Chapter 7