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AIM 

6/17/21 

provide weather and obstruction to vision informa-
tion 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 U.S. 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

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. 

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Meteorology