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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, FIG 7


8 and FIG 7


9.) 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 temperature, wind, and precipitation accumulation, both the
automated system and the observer use a fixed location and time-averaging technique. The quantitative
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 three).


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 temperature sensor.


Anemometer (wind direction and speed sensor).


Rainfall accumulation sensor.


Automated Lightning Detection and Reporting System (ALDARS) (excluding Alaska and Pacific

Island sites).

3. The ASOS/AWOS data outlets include:


Those necessary for on-site airport users.