AWOS, ASOS and ATIS are three forms services which pilots use to retrieve a report of observed weather for the terminal area of an airport. The Aeronautical Information Manual serves as a valuable resource for understanding what these services provide. Knowing their capabilities, limitations and what's expected of the pilots of aircraft is important for safe operations.
Automated Weather Observing System (AWOS) and Automated Surface Observation System (ASOS) are similar in purpose. AWOS and ASOS stations come in a variety of configurations with varying levels of capability. AWOS stations are generally maintained by the Federal Aviation Administration (FAA), while ASOS stations are generally mainted by the National Weather Service (NWS).
Retrieving the observation made by an AWOS or ASOS station can be performed in a number of ways. This includes tuning a radio frequency, making a phone call, copying and Automated Terminal Information System (ATIS) broadcast, retrieving coded weather from an ADS-B receiver or having it relayed by air traffic control (ATC). Pilots of aircraft should always use these services to become aware of the observed conditions at airports of intended use and should advise ATC of this receipt of weather when using their services. The Aeronautical Information Manual covers procedures for retrieving weather and advising ATC in 5-4-4.
AWOS and ASOS stations are usually the source of the weather found encoded in a METAR report.