What is the difference between ASOS and AWOS in aviation weather?

ASOS (Automated Surface Observing System) is the nation's primary surface weather network, operated jointly by the NWS, FAA, and DOD, and it issues hourly METARs plus special reports (SPECIs) when conditions change significantly. AWOS (Automated Weather Observing System) is a similar but generally less capable family of systems that issues METARs every 20 minutes and does not generate SPECIs. AWOS comes in multiple tiers (AWOS-A through AWOS-4), each adding sensors such as visibility, cloud height, or thunderstorm detection to the basic altimeter-only model.

Why does an ASOS or AWOS weather report say AUTO and what does it mean?

The designator AUTO in a METAR indicates that the observation was produced entirely by automated instruments and algorithms with no human input or oversight. When a certified weather observer or qualified tower controller augments the automated station — adding observations for phenomena the sensors cannot detect — the AUTO designator is removed from the report, even though the automated sensors are still operating.

How current is the weather data from an ASOS one-minute observation (OMO)?

Despite being updated every minute, ASOS and AWOS OMOs are not instantaneous snapshots. Cloud height and sky cover are time-averaged over the 30-minute period ending at the observation time, while visibility and most other elements are averaged over the most recent 10 minutes. The averaging algorithm is asymmetric: deteriorating conditions (like a dropping ceiling or falling visibility) appear in the OMO much faster than improving conditions, so a sudden improvement may take up to 10 minutes to be fully reflected in the broadcast.

ASOS and AWOS: How Automated Stations Sense the Weather

Every flight begins with a weather check, and for the vast majority of U.S. airports that weather check starts with data from an automated surface observing station. Before the mid-1990s, certified human observers made nearly all surface weather observations. Today, automated and augmented systems generate the overwhelming share of reports. Understanding exactly what these machines measure, how they average and disseminate data, and where their limitations lie is essential knowledge for any pilot — whether planning a local flight under Part 91 or flying instrument approaches under Part 121 or 135.

The FAA Aviation Weather Handbook (FAA-H-8083-28B), Chapter 24, organizes surface observations into three types: manual (certified human observer only), automated (instruments and algorithms, no human input), and augmented (automated system with certified human oversight added). Automated reports always carry the designator AUTO in the body of the METAR unless a qualified observer is present and augmenting the report, in which case AUTO is omitted.

The Two Automated Systems: ASOS and AWOS

The United States relies on two families of automated stations. Although they share a common philosophy — sensors gather data, algorithms process it, and computer-generated voice broadcasts push it to pilots — they differ significantly in origin, capability, and reporting schedule.

Automated Surface Observing System (ASOS)

ASOS is a joint program of the National Weather Service (NWS), the FAA, and the Department of Defense (DOD). It serves as the nation's primary surface weather observing network. ASOS stations observe, format, archive, and transmit observations automatically. They produce standard Aviation Routine Weather Reports (METARs) on an hourly basis and generate Aviation Selected Special Weather Reports (SPECIs) whenever conditions cross pre-selected thresholds — for example, when visibility drops below 3 statute miles. Each ASOS station also broadcasts computer-generated voice weather directly to aircraft via FAA ground-to-air radio frequencies and makes the same data available by telephone.

In addition to the standard hourly or special METARs, ASOS produces One-Minute Observations (OMOs), updated every 60 seconds. Pilots should be aware that the voice broadcast heard on the airport's ASOS frequency is the OMO, which can differ from the METAR posted on the internet or received via FIS-B. The OMO is not instantaneous snapshot data; cloud heights are time-averaged over the 30-minute period ending at observation time, and visibility is time-averaged over the most recent 10 minutes. Critically, the averaging algorithms are deliberately asymmetric: deteriorating conditions appear in the OMO much faster than improving conditions. In practical terms, if dense fog suddenly lifts, the OMO may continue to reflect restricted visibility for up to 10 minutes.

ASOS reports a comprehensive set of weather elements:

Sky condition — cloud amount and height (clear, few, scattered, broken, or overcast) up to 12,000 feet AGL.
Visibility — to at least 10 statute miles.
Present weather — type and intensity for rain (RA), snow (SN), freezing rain (FZRA), and unknown precipitation (UP).
Thunderstorms — on-site (TS) or in the vicinity (VCTS).
Obstructions to vision — fog (FG) when visibility is below 5/8 sm; mist (BR) or haze (HZ) for visibilities from 5/8 sm up to less than 7 sm, depending on the temperature-dewpoint spread. If the spread is 4 °F (approximately 2 °C) or less, BR is reported; a larger spread results in HZ. Freezing fog (FZFG) is reported when the temperature is below 0 °C.
Pressure — sea-level pressure and altimeter setting.
Temperature and dewpoint — ambient and dewpoint temperatures.
Wind — direction, speed, and character (gusts, squalls). An important nuance: wind direction distributed nationally via FSS, the internet, and FIS-B is expressed in true degrees, while the local radio and telephone broadcast uses magnetic degrees.
Precipitation accumulation — measured by a rain gauge.
Selected significant remarks — including variable cloud height, variable visibility, precipitation begin/end times, rapid pressure changes, wind shifts, peak wind, and possibly density altitude.

Automated Weather Observing System (AWOS)

AWOS is conceptually similar to ASOS but is generally less capable and may lack the same level of redundant sensors and maintenance response. Despite this, AWOS provides pilots the weather information necessary to conduct Part 91 flight operations and others, depending on their operations specifications. Like ASOS, AWOS broadcasts computer-generated voice weather via FAA ground-to-air radio and by telephone.

A key operational difference is the reporting cycle: AWOS generates a METAR every 20 minutes and does not issue SPECIs. AWOS also provides OMOs by radio and telephone, using the same 30-minute cloud and 10-minute visibility time-averaging as ASOS, with the same asymmetric algorithm favoring rapid reporting of deteriorating conditions. AWOS may be located on airports, at heliports (ground-based or rooftop), and on offshore platforms and drill ships. Units are either Federal (owned, operated, and maintained by the FAA) or non-Federal (owned, operated, and maintained by the site owner).

There are six AWOS configurations, each adding capability to the previous level:

AWOS-A — altimeter setting only.
AWOS-AV — altimeter plus a visibility sensor.
AWOS-1 — wind speed, direction, and gusts; temperature; dewpoint; altimeter; and density altitude.
AWOS-2 — all AWOS-1 parameters plus visibility.
AWOS-3 — all AWOS-2 parameters plus precipitation accumulation (rain gauge) and cloud height. Optional sensors can be added: precipitation type/intensity upgrades it to AWOS-3P; adding thunderstorm/lightning detection as well creates AWOS-3PT.
AWOS-4 — all AWOS-3PT parameters plus freezing rain detection.

Why These Systems Matter to Pilots

Automated observations are the foundation on which forecasts, flight plans, and approach decisions are built. A pilot shooting an ILS to minimums needs to trust that the reported ceiling and visibility reflect actual conditions at the airport surface. Understanding the time-averaging behind these numbers — especially the 30-minute cloud average — helps explain why a rapidly deteriorating situation might show up quickly in the OMO while a rapidly improving situation lags behind reality. This asymmetry exists by design to protect pilots from being lured into deteriorating conditions by a stale improving trend.

The distinction between ASOS and AWOS also matters for instrument approaches. Certain approach procedures require a full ASOS or augmented observation to be used for minimums; an AWOS-1 or AWOS-2 station that cannot report cloud ceilings may not satisfy approach requirements at a given facility. Pilots should check approach plate notes and NOTAMs regarding the reporting capability of the station serving a given airport.

Augmented observations provide an additional safety layer at busy airports. When a certified observer or tower controller is present, they can report weather phenomena beyond sensor capability — such as tornadoes, waterspouts, or precipitation types the sensors cannot distinguish. The absence of AUTO in the METAR header is your signal that a trained human has reviewed and supplemented the automated data.

Key Numbers and Rules

Sky condition: time-averaged over the 30 minutes ending at observation time.
Visibility and all other elements: based on sensor data within 10 minutes or less of observation time.
ASOS issues METARs hourly plus SPECIs when thresholds are crossed (e.g., visibility below 3 sm).
AWOS issues METARs every 20 minutes; no SPECIs.
ASOS sky condition reported up to 12,000 feet AGL.
FG reported when visibility is less than 5/8 sm; FZFG when temperature is below 0 °C.
BR vs. HZ threshold: temperature-dewpoint spread of 4 °F (≈2 °C) or less = BR; greater spread = HZ.
Wind direction: true degrees on national distribution (internet, FIS-B, FSS); magnetic degrees on local radio and telephone.
Reports without human oversight carry AUTO; augmented reports do not.

Common Test Traps

AUTO vs. augmented confusion. Examinees often think any automated data means AUTO appears. In fact, if a certified observer augments the station, AUTO is removed from the report even though the underlying sensors are still automated.
AWOS issues SPECIs. This is false — only ASOS issues SPECIs. AWOS reports on a fixed 20-minute cycle regardless of weather changes.
OMO as instant weather. The one-minute update does not mean one-minute-old sensor data for clouds. Clouds are still averaged over 30 minutes, making the OMO potentially sluggish to reflect a sudden ceiling improvement.
Wind direction true vs. magnetic. On the written exam, a question about a wind direction read from an ATIS or telephone AWOS broadcast expects a magnetic value; a METAR wind transmitted via FIS-B or the internet is in true degrees. Mixing these up affects your crosswind calculation.
AWOS-3 vs. AWOS-3P vs. AWOS-3PT. An AWOS-3 alone does not report precipitation type or thunderstorms. The suffix letters tell you exactly which optional sensors are installed. Assuming an AWOS-3 will detect thunderstorms is a dangerous error.