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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.