Automated Surface Observing System (ASOS) Automated Weather Observing System (AWOS)
Many airports in the United States are equipped with either ASOS or AWOS which can provide
minute weather via phone and/or radio.
METARs (and information via ATIS) represent the official current weather at the
airport, but pilots can obtain up to the minute weather information via either automated system.
Note that METARs (and other written weather reports) provide wind direction referenced to true north.
AWOS/ASOS received via radio provide wind direction referenced to magnetic north.
The word TEST in the recorded message indicates
that the system is not commissioned and should not be used. TEMPORARILY INOPERATIVE is mentioned
when the system is, uh, inoperative.
AWOS-A only reports altimeter reading only. Any other information is advisory only.
AWOS-1 usually reports altimeter setting, wind data, temperature, dew point, and density altitude.
AWOS-2 provides AWOS-1 plus visibility.
AWOS-3 provides AWOS-2 plus cloud/ceiling data.
ASOS provides AWOS-3 plus precipitation.
ASOS installations have different service levels which may include backups, multiple sensors and
Density altitude is reported when it exceeds the field elevation by more than 1,000 feet.
Reported visibility is derived from a sensor near the touchdown of the primary instrument
runway, and converted using a 10-minute harmonic average. Reported sky condition/ceiling
is derived from data received from the ceilometer over the previous 30 minutes. Reported sky
conditions may differ from observer sky conditions because the ceilometer is limited to observations
over the sensor site.
Radio transmissions are engineered to be receivable to a maximum of 25 NM from the site and a
maximum of 10,000' AGL. They may not be available on the surface at the airport. Local
conditions may limit maximum reception distance and/or altitude. Generally, they consist
of a 20 to 30 second message updated each minute. A computer-generated voice is used to
automate the weather observation, though some systems allow the addition of operator-generated
Information on this page is derived from the Aeronautical Information Manual (AIM).
For further information, see: