5/19/22
AIM
through 5). Centerline light step settings are used for
CAT IIIb operations. Edge Light step settings are
used for CAT I, II, and IIIa operations.
l.
New Generation RVRs can measure and display
RVR values down to the lowest limits of
Category IIIb operations (150 feet RVR). RVR
values are displayed in 100 feet increments and are
reported as follows:
1.
100
−
feet increments for products below
800 feet.
2.
200
−
feet increments for products between
800 feet and 3,000 feet.
3.
500
−
feet increments for products between
3,000 feet and 6,500 feet.
4.
25
−
meter increments for products below
150 meters.
5.
50
−
meter increments for products between
150 meters and 800 meters.
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
−
14. 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.
REFERENCE
−
AIM, Paragraph 7
−
1
−
28 , Key to Aerodrome Forecast (TAF) and Aviation
Routine Weather Report (METAR), defines “broken,” “overcast,” and
“obscuration.”
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
MSL.
7
−
1
−
15. Reporting Prevailing Visibility
a.
Surface (horizontal) visibility is reported in
METAR reports in terms of statute miles and
3/
5/
3/
1/
increments thereof; e.g.,
1
/
16
,
1
/
8
,
16
,
1
/
4
,
16
,
8
,
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/AWOS:
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:
M
1
/
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
thereof.
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
−
16. 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
Meteorology
7
−
1
−
39