There are three general types of surface weather observations:

Manual surface weather observations are made by a human weather observer who is certified by the FAA. While manual observations were the primary type prior to the mid-1990s, automated and augmented observations make up the vast majority of today’s surface observations in the United States.

Automated observations are derived from instruments and algorithms without human input or oversight. In the United States, there are two main kinds of automated observing systems: ASOS and AWOS. Automated observations contain “AUTO” in the report unless they are augmented by a human weather observer.

At select airports in the United States, the automated observing system will have input and oversight by human weather observers or tower controllers certified in weather observing. These are referred to as augmented observations. Human observers report weather elements that are beyond the capabilities of the automated system and/or are deemed operationally significant. The weather elements observed and reported by the human observer vary, depending on the airport. “AUTO” is not used in augmented reports.

For those elements that the human observer evaluates using spatial averaging techniques (e.g., sky cover and visibility), the automated station substitutes time averaging of sensor data. Therefore, in an automated observation, sky condition is an evaluation of sensor data gathered during the 30-minute period ending at the actual time of the observation. All other elements are based on sensor data that is within 10 minutes or less of the actual time of the observation.

The ASOS program is a joint effort of the NWS, the FAA, and the DOD. ASOS serves as the nation’s primary surface weather observing network.

ASOS detects significant changes, disseminating hourly and special observations. Additionally, ASOS routinely and automatically provides computer-generated voice observations directly to aircraft in the vicinity of airports, using FAA ground-to-air radio. These messages are also available via a telephone. ASOS observes, formats, archives, and transmits observations automatically. ASOS transmits a special report when conditions exceed preselected weather element thresholds (e.g., the visibility decreases to less than three miles).

All ASOS locations prepare and disseminate METARs/SPECIs in accordance with the format shown in Section 24.4.3.

In addition to the ASOS METARs/SPECIs are ASOS OMOs, which are updated once a minute. OMOs can be in various formats and are sometimes encoded in the METAR format.

ASOS broadcasts can be different from the METAR seen on the internet or FIS-B, since the broadcast is the OMO ASOS data.

The OMOs are not “instant weather”; rather, the clouds and visibility are time averaged (30 minutes for clouds, 10 minutes for visibility). The averaging algorithms are designed to report deteriorating conditions much quicker than improving conditions.