Aeronautical Information Manual (AIM), page 467

Index   466 -- Page 467 -- 468


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-A only reports altimeter setting;
Any other information is advisory only.

(b) AWOS-AV reports altimeter and
Any other information is advisory only.

(c) AWOS-l usually reports altimeter setting,
wind data, temperature, dew point, and density

(d) AWOS-2 provides the information pro-

vided by AWOS-l plus visibility; and

(e) AWOS-3 provides the information pro-
vided by AWOS-2 plus cloud/ceiling data.

(f) AWOS- 3P provides reports the same as
the AWOS 3 system, plus a precipitation identifica-
tion sensor.

(g) AWOS- 3PT reports the same as the
AWOS 3P System, plus thunderstorm/lightning
reporting capability.

(h) AWOS- 3T reports the same as AWOS 3
system and includes a thunderstorm/lightning
reporting capability.

(i) AWOS- 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-min-
ute 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 U.S., and where applicable, on
published Instrument Approach Procedures. Selected
individual systems may be incorporated into
nationwide data collection and dissemination net-
works in the future.

c. 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
explanations and examples of the exceptions.

1. Location and Time. The location/name and
TION," followed by the time are announced.

(a) If the airport's specific location is
included in the airport's name, the airport's name is
"Bremerton National Airport automated weather observa-
tion, one four five six zulu;"
"Ravenswood Jackson County Airport automated weather
observation, one four five six zulu."

(b) If the airport's specific location is not
included in the airport's name, the location is
announced followed by the airport's name.

Meteorology 7-1-23

Page 467 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   466 -- Page 467 -- 468