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based on FAA/NWS accepted aviation weather
reports or products. If products are used which do not
meet this criteria, they should be so identified. The
operator must determine the applicability of such
products to their particular flight operations.
(b) In the case of a weather product which is
the result of the application of a process which alters
the form, function or content of the base FAA/NWS
accepted weather product(s), that process, and any
limitations to the application of the resultant product,
should be described in the vendor's user guidance
material.
2. An example would be a NEXRAD radar
composite/mosaic map, which has been modified by
changing the scaling resolution. The methodology of
assigning reflectivity values to the resultant image
components should be described in the vendor's
guidance material to ensure that the user can
accurately interpret the displayed data.
AIM 2/14/08
7-1-24 Meteorology
7-1-12. Weather Observing Programs
a. Manual Observations. With only a few
exceptions, these reports are from airport locations
staffed by FAA or NWS personnel who manually
observe, perform calculations, and enter these
observations into the (WMSCR) communication
system. The format and coding of these observations
are contained in paragraph 7-1-30, Key to Aviation
Routine Weather Report (METAR) and Aerodrome
Forecasts (TAF).
b. Automated Weather Observing System
(AWOS).
1. Automated weather reporting systems are
increasingly being installed at airports. These
systems consist of various sensors, a processor, a
computer‐generated voice subsystem, and a transmitter
to broadcast local, minute‐by‐minute weather data
directly to the pilot.
NOTEWhen
the barometric pressure exceeds 31.00 inches Hg.,
see paragraph 7-2-2, Procedures, for the altimeter setting
procedures.
2. The AWOS observations will include the
prefix “AUTO” to indicate that the data are derived
from an automated system. Some AWOS locations
will be augmented by certified observers who will
provide weather and obstruction to vision information
in the remarks of the report when the reported
visibility is less than 7 miles. These sites, along with
the hours of augmentation, are to be published in the
A/FD. Augmentation is identified in the observation
as “OBSERVER WEATHER.” The AWOS wind
speed, direction and gusts, temperature, dew point,
and altimeter setting are exactly the same as for
manual observations. The AWOS will also report
density altitude when it exceeds the field elevation by
more than 1,000 feet. The reported visibility is
derived from a sensor near the touchdown of the
primary instrument runway. The visibility sensor
output is converted to a visibility value using a
10-minute harmonic average. The reported sky
condition/ceiling is derived from the ceilometer
located next to the visibility sensor. The AWOS
algorithm integrates the last 30 minutes of ceilometer
data to derive cloud layers and heights. This output
may also differ from the observer sky condition in
that the AWOS is totally dependent upon the cloud
advection over the sensor site.
3. These real‐time systems are operationally
classified into four basic levels:
(a) AWOS-A only reports altimeter setting;
NOTEAny
other information is advisory only.
(b) AWOS-l usually reports altimeter setting,
wind data, temperature, dew point, and density
altitude;
(c) AWOS-2 provides the information provided
by AWOS-l plus visibility; and
(d) AWOS-3 provides the information provided
by AWOS-2 plus cloud/ceiling data.
4. The information is transmitted over a discrete
VHF radio frequency or the voice portion of a local
NAVAID. AWOS transmissions on a discrete VHF
radio frequency are engineered to be receivable to a
maximum of 25 NM from the AWOS site and a
maximum altitude of 10,000 feet AGL. At many
locations, AWOS signals may be received on the
surface of the airport, but local conditions may limit
the maximum AWOS reception distance and/or
altitude. The system transmits a 20 to 30 second
weather message updated each minute. Pilots should
monitor the designated frequency for the automated
weather broadcast. A description of the broadcast is
contained in subparagraph c. There is no two‐way
communication capability. Most AWOS sites also
have a dial‐up capability so that the minute‐by‐minute
weather messages can be accessed via telephone.
5. AWOS information (system level, frequency,
phone number, etc.) concerning specific locations is
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