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14-foot frangible pole. Infrared light is emitted from
the transmitter and received by the receiver. Any
obscuring matter such as rain, snow, dust, fog, haze
or smoke increases the amount of scattered light
reaching the receiver. The resulting measurement
along with inputs from the runway light intensity
monitor and the ambient light sensor are forwarded to
the DPU which calculates the proper RVR value. The
RVR values are displayed locally and remotely on
controller displays.
k. The runway light intensity monitors both the
runway edge and centerline light step settings (steps1
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
CategoryIIIb operations (150 feet RVR). RVR
values are displayed in 100 feet increments and are
reported as follows:
1. 100-feet increments for products below
800feet.
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
150meters.
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.
7-1-16. 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.
2/14/08 AIM
Meteorology 7-1-39
REFERENCEAIM,
Key to Aerodrome Forecast (TAF) and Aviation Routine Weather
Report (METAR), Paragraph 7-1-30, 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. (“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.
7-1-17. 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
7miles, 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
　
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