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Automated “REMARKS.”


Density Altitude.


Variable Visibility.


Variable Wind Direction.


Manual Input “REMARKS.”


Sky Condition.




Weather and Obstructions to Vision.




Dew Point.


Wind; and


Altimeter Setting.


“Remarks ... density altitude, two thousand five hundred ...
visibility variable between one and two ... wind direction
variable between two four zero and three one zero ...
observer ceiling estimated two thousand broken ...
observer temperature two, dew point minus five.”

d. Automated Surface Observing System

(ASOS)/Automated Weather Observing System

The ASOS/AWOS is the primary surface

weather observing system of the U.S. (See Key to
Decode an ASOS/AWOS (METAR) Observation,


7 and FIG 7


8.) The program to install

and operate these systems throughout the U.S. is a
joint effort of the NWS, the FAA and the Department
of Defense. ASOS/AWOS is designed to support
aviation operations and weather forecast activities.
The ASOS/AWOS will provide continuous minute-
by-minute observations and perform the basic
observing functions necessary to generate an aviation
routine weather report (METAR) and other aviation
weather information. The information may be
transmitted over a discrete VHF radio frequency or
the voice portion of a local NAVAID. ASOS/AWOS
transmissions on a discrete VHF radio frequency are
engineered to be receivable to a maximum of 25 NM
from the ASOS/AWOS site and a maximum altitude
of 10,000 feet AGL. At many locations, ASOS/
AWOS signals may be received on the surface of the
airport, but local conditions may limit the maximum
reception distance and/or altitude. While the
automated system and the human may differ in their
methods of data collection and interpretation, both
produce an observation quite similar in form and

content. For the “objective” elements such as
pressure, ambient temperature, dew point tempera-
ture, wind, and precipitation accumulation, both the
automated system and the observer use a fixed
location and time-averaging technique. The quantita-
tive differences between the observer and the
automated observation of these elements are
negligible. For the “subjective” elements, however,
observers use a fixed time, spatial averaging
technique to describe the visual elements (sky
condition, visibility and present weather), while the
automated systems use a fixed location, time
averaging technique. Although this is a fundamental
change, the manual and automated techniques yield
remarkably similar results within the limits of their
respective capabilities.

1. System Description.


The ASOS/AWOS at each airport location

consists of four main components:


Individual weather sensors.


Data collection and processing units.


Peripherals and displays.


The ASOS/AWOS sensors perform the

basic function of data acquisition. They continuously
sample and measure the ambient environment, derive
raw sensor data and make them available to the
collection and processing units.

2. Every ASOS/AWOS will contain the

following basic set of sensors:


Cloud height indicator (one or possibly



Visibility sensor (one or possibly three).


Precipitation identification sensor.


Freezing rain sensor (at select sites).


Pressure sensors (two sensors at small

airports; three sensors at large airports).


Ambient temperature/Dew point tempera-

ture sensor.


Anemometer (wind direction and speed



Rainfall accumulation sensor.


Automated Lightning Detection and Re-

porting System (ALDARS) (excluding Alaska and
Pacific Island sites).