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AIM

10/12/17

7

−1−25

Meteorology

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;

NOTE

Any other information is advisory only.

(b) AWOS

−AV reports altimeter and visibili-

ty;

NOTE

Any other information is advisory only.

(c) AWOS

−l usually reports altimeter setting,

wind data, temperature, dew point, and density
altitude;

(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-
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 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

the phrase “AUTOMATED WEATHER OBSERVA-
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
announced.

EXAMPLE

“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.