AIM
3/21/24
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.
NOTE
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When the barometric pressure exceeds 31.00 inches Hg., see AIM, Para 7
3, Altimeter Errors.
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 Chart Supplement. 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 nine basic levels:
(a) AWOS
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A
only reports altimeter setting;
NOTE
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Any other information is advisory only.
(b) AWOS
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AV
reports altimeter and visibility;
NOTE
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Any other information is advisory only.
(c) AWOS
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l
usually reports altimeter setting, wind data, temperature, dew point, and density altitude;
(d) AWOS
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2
provides the information provided by AWOS
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l plus visibility; and
(e) AWOS
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3
provides the information provided by AWOS
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2 plus cloud/ceiling data.
(f) AWOS
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3P
provides reports the same as the AWOS 3 system, plus a precipitation identification
sensor.
(g) AWOS
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3PT
reports the same as the AWOS 3P System, plus thunderstorm/lightning reporting
capability.
(h) AWOS
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3T
reports the same as AWOS 3 system and includes a thunderstorm/lightning reporting
capability.
(i) AWOS
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4
reports the same as the AWOS 3 system, plus precipitation occurrence, type and
accumulation, freezing rain, thunderstorm, and runway surface sensors.
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
published, as the systems become operational, in the Chart Supplement, and where applicable, on published
7
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1
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28
Meteorology