background image





(1) Automated “REMARKS.”

[a] Density Altitude.
[b] Variable Visibility.
[c] Variable Wind Direction.

(2) Manual Input “REMARKS.”

[a] Sky Condition.
[b] Visibility.
[c] Weather and Obstructions to Vision.
[d] Temperature.
[e] Dew Point.
[f] Wind; and
[g] 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 Sensor System

(AWSS). The ASOS/AWSS is the primary surface

weather observing system of the U.S. (See Key to

Decode an ASOS/AWSS (METAR) Observation,

FIG 7−1−7 and FIG 7−1−8.) 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. AWSS is a follow−on program that

provides identical data as ASOS. ASOS/AWSS is

designed to support aviation operations and weather

forecast activities. The ASOS/AWSS 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/AWSS transmissions on a discrete

VHF radio frequency are engineered to be receivable

to a maximum of 25 NM from the ASOS/AWSS site

and a maximum altitude of 10,000 feet AGL. At many

locations, ASOS/AWSS 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 accumula-

tion, 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.

(a) The ASOS/AWSS at each airport location

consists of four main components:

(1) Individual weather sensors.
(2) Data collection and processing units.
(3) Peripherals and displays.

(b) The ASOS/AWSS 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/AWSS will contain the

following basic set of sensors:

(a) Cloud height indicator (one or possibly


(b) Visibility sensor (one or possibly three).
(c) Precipitation identification sensor.
(d) Freezing rain sensor (at select sites).
(e) Pressure sensors (two sensors at small

airports; three sensors at large airports).

(f) Ambient temperature/Dew point tempera-

ture sensor.

(g) Anemometer (wind direction and speed


(h) Rainfall accumulation sensor.
(i) Automated Lightning Detection and Re-

porting System (ALDARS) (excluding Alaska and

Pacific Island sites).