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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.

ASOS installations have different service levels which may include backups, multiple sensors and manual observers.

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 voice message.

Information on this page is derived from the Aeronautical Information Manual (AIM). For further information, see:

More information is available on the map's Forms and Controls, on the map's icons, and on the data sources.

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