JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
VISIBILITY DEFINITIONS
Visibility is important to all aviators since it plays an essential role in takeoffs, approaches, and
landings. Visibility is defined as the ability to see and identify prominent unlighted objects by
day and prominent lighted objects at night, and is expressed in statute miles, hundreds of feet, or
meters. There are several particular methods of reporting visibility, some of which are defined
below.
Flight Visibility – The average forward horizontal distance, measured in statute miles from the
cockpit of an aircraft in flight, at which a pilot can see and identify prominent unlighted objects
by day and prominent lighted objects at night.
Prevailing Visibility – The greatest horizontal visibility, measured in statute miles, equaled or
exceeded throughout at least half the horizon circle, which need not be continuous. Figure 4-11
illustrates how prevailing visibility is determined. The center of the circles depict the observation
point, and the edge of the circles represent a distance of 3 miles, the furthest that prominent
objects may be seen and identified. In the left depiction, the maximum visibility common to half
or more of the horizon circle is 3 miles, so the prevailing visibility is 3 miles. If a bank of fog
were to roll in to the airfield, as in the right depiction, visibility toward the east would be
reduced. However, the observer can still see 3 miles throughout at least 180° of view, so the
prevailing visibility is still 3 miles. Look at the visibility for each of the runways, and notice how
the actual visibility may vary significantly from the prevailing visibility.
Figure 4-11 — Prevailing Visibility Determination
Slant Range Visibility – The distance on final approach when the runway environment is in
sight. This is probably the most vital weather information needed during a final approach in
questionable weather. Unfortunately, slant rage visibility is not often provided because of great
difficulty in estimating or measuring it from the ground. RVR provides the best indication of the
slant range visibility. However, other weather information such as precipitation and prevailing
visibility help indicate slant range visibility.
Runway Visual Range (RVR) – The horizontal distance, expressed in hundreds of feet or meters,
a pilot will see by looking down the runway from the approach end. For take off and landing
under IFR, prevailing visibility is not as important as the visibility within the runway
environment.
Version 3.2/Dec 08 4-17