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8. Weather Observing Programs
8.1 Manual Observations. Aviation Routine
Weather Reports (METAR) are taken at more than
600 locations in the U.S. With only a few exceptions,
these stations are located at airport sites and most are
staffed by FAA or NWS personnel who manually
observe, perform calculations, and enter the
observation into the distribution system. The format
and coding of these observations are contained in
FIG GEN 3.5-23.
8.2 Automated Weather Observing System
(AWOS)
8.2.1 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 Section ENR 1.7, Altimeter Setting Procedures.
8.2.2 The AWOS observations will include the
prefix “AUTO” to indicate that the data are derived
30 AUG 07
AIP
United States of America
GEN 3.5-24
15 MAR 07
Nineteenth Edition Federal Aviation Administration
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 3 miles. These sites, along with
the hours of augmentation, are published in the
Airport/Facility Directory. 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.
8.2.3 Referred to as AWOS, these real-time systems
are operationally classified into four basic levels:
8.2.3.1 AWOS-A: only reports altimeter setting.
NOTEAny
other information is advisory only.
8.2.3.2 AWOS-l: usually reports altimeter setting,
wind data, temperature, dew point, and density
altitude.
8.2.3.3 AWOS-2 provides the information provided
by AWOS-l, plus visibility.
8.2.3.4 AWOS-3 provides the information provided
by AWOS-2, plus cloud/ceiling data.
8.2.4 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 paragraph 8.3, Automated Weather
Observing System (AWOS) Broadcasts. 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.
8.2.5 AWOS information (system level, frequency,
phone number) concerning specific locations is
published, as the systems become operational, in the
Airport/Facility Directory and, where applicable, on
published Instrument Approach Procedure (IAP)
charts. Selected individual systems may be incorporated
into nationwide data collection and dissemination
networks in the future.
8.3 Automated Weather Observing System
(AWOS) Broadcasts. Computer-generated voice is
used in AWOS to automate the broadcast of the
minute-by-minute weather observations. In addition,
some systems are configured to permit the
addition of an operator-generated voice message;
e.g., weather remarks, following the automated
parameters. The phraseology used generally follows
that used for other weather broadcasts. Following are
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