Aeronautical Information Manual (AIM), page 482

Index   481 -- Page 482 -- 483


6. 100-meter increments for products between
800 meters and 1,200 meters.

7. 200-meter increments for products between

1,200 meters and 2,000 meters.

7-1-15. Reporting of Cloud Heights

a. Ceiling, by definition in the CFRs and as used
in aviation weather reports and forecasts, is the height
above ground (or water) level of the lowest layer of
clouds or obscuring phenomenon that is reported as
"broken," "overcast," or "obscuration," e.g., an
aerodrome forecast (TAF) which reads "BKN030"
refers to height above ground level. An area forecast
which reads "BKN030" indicates that the height is
above mean sea level.

AIM, Paragraph 7-1-29 , Key to Aerodrome Forecast (TAF) and Aviation
Routine Weather Report (METAR), defines "broken," "overcast," and

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. ("Ceil-
ing" 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

7-1-16. 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/ , 3/ , 7/ , 1, 1 1/ , etc. (Visibility reported by an
8 4 8 8

unaugmented automated site is reported differently

than in a manual report, i.e., ASOS/AWSS: 0, 1/16, 1/8,
1/ , 1/ , 3/ , 1, 1 1/ 1 1/ 1 3/ 2, 2 1/ 3, 4, 5, etc., AWOS:
4 2 4 4, 2, 4, 2,
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 condi-

tion; e.g., fog, haze, smoke, etc., or combinations

b. 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.

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.

7-1-17. 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
up to a condition where individual drops are easily
2. Moderate. Individual drops are not clearly
identifiable; spray is observable just above pave-
ments 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
b. Ice Pellets
1. Light. Scattered pellets that do not com-
pletely 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


3. Heavy. Rapid accumulation on ground.
Visibility reduced by ice pellets to less than 3 statute

7-1-38 Meteorology

Page 482 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   481 -- Page 482 -- 483