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b. Pilots usually report height values above MSL,
since they determine heights by the altimeter. This is
taken in account when disseminating and otherwise
applying information received from pilots. (“Ceiling”
heights are always above ground level.) In
reports disseminated as PIREPs, height references
are given the same as received from pilots, that is,
above MSL.
c. In area forecasts or inflight advisories, ceilings
are denoted by the contraction “CIG” when used with
sky cover symbols as in “LWRG TO CIG OVC005,”
or the contraction “AGL” after, the forecast cloud
height value. When the cloud base is given in height
above MSL, it is so indicated by the contraction
“MSL” or “ASL” following the height value. The
heights of clouds tops, freezing level, icing, and
turbulence are always given in heights above ASL or
MSL.
7−1−18. Reporting Prevailing Visibility
a. Surface (horizontal) visibility is reported in
METAR reports in terms of statute miles and
increments thereof; e.g., 1/16, 1/8, 3/
16, 1/4, 5/
16, 3/
8, 1/
2,
5/
8, 3/4, 7/8, 1, 1 1/8, etc. (Visibility reported by an
unaugmented automated site is reported differently
than in a manual report, i.e., ASOS: 0, 1/16, 1/
8, 1/4, 1/
2,
3/4, 1, 1 1/4, 1 1/
2, 1 3/
4, 2, 2 1/
2, 3, 4, 5, etc., AWOS: M1/4,
1/4, 1/
2, 3/4, 1, 1 1/4, 1 1/
2, 1 3/
4, 2, 2 1/
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.
b. Prevailing visibility is the greatest visibility
equalled 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.
c. 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.
7−1−19. Estimating Intensity of Rain and
Ice Pellets
a. Rain
1. Light. From scattered drops that, regardless
of duration, do not completely wet an exposed surface
AIM 2/17/05
7−1−40 Meteorology
up to a condition where individual drops are easily
seen.
2. Moderate. Individual drops are not clearly
identifiable; spray is observable just above pavements
and other hard surfaces.
3. Heavy. Rain seemingly falls in sheets;
individual drops are not identifiable; heavy spray to
height of several inches is observed over hard
surfaces.
b. Ice Pellets
1. Light. Scattered pellets that do not completely
cover an exposed surface regardless of
duration. Visibility is not affected.
2. Moderate. Slow accumulation on ground.
Visibility reduced by ice pellets to less than 7 statute
miles.
3. Heavy. Rapid accumulation on ground.
Visibility reduced by ice pellets to less than 3 statute
miles.
7−1−20. Estimating Intensity of Snow or
Drizzle (Based on Visibility)
a. Light. Visibility more than 1/2 statute mile.
b. Moderate. Visibility from more than 1/4 statute
mile to 1/2 statute mile.
c. Heavy. Visibility 1/4 statute mile or less.
7−1−21. Pilot Weather Reports (PIREPs)
a. FAA air traffic facilities are required to solicit
PIREPs when the following conditions are reported
or forecast: ceilings at or below 5,000 feet; visibility
at or below 5 miles (surface or aloft); thunderstorms
and related phenomena; icing of light degree or
greater; turbulence of moderate degree or greater;
wind shear and reported or forecast volcanic ash
clouds.
b. Pilots are urged to cooperate and promptly
volunteer reports of these conditions and other
atmospheric data such as: cloud bases, tops and
layers; flight visibility; precipitation; visibility
restrictions such as haze, smoke and dust; wind at
　
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