When you pull up a METAR or tune in a weather broadcast on your radio before departure, there is a good chance the data you are reading was collected by a machine rather than a human observer. Two families of automated weather stations — the Automated Surface Observing System (ASOS) and the Automated Weather Observing System (AWOS) — generate the vast majority of surface weather observations at airports across the United States. Every student pilot needs to understand what these systems measure, how they differ from each other, and — critically — what they cannot detect. That last point is where the real safety value lies.
Before automated stations became common, trained human weather observers staffed most reporting airports around the clock. They could see, feel, and even smell the atmosphere in ways no sensor can replicate. As the national network expanded and costs rose, automated systems filled the gap. Today ASOS serves as the FAA and National Weather Service (NWS) primary surface observation network, while AWOS is a separately administered, FAA-managed family of stations found predominantly at smaller general aviation airports.
How ASOS Works
ASOS is a joint program of the NWS, the FAA, and the Department of Defense. A typical ASOS station includes a suite of sensors mounted on or near a standard instrument shelter and a 10-meter anemometer tower. The system continuously samples the atmosphere and updates its observation every minute, producing a full METAR report once per hour (or as a special observation — SPECI — whenever conditions change significantly). Key sensors include:
- Freezing rain sensor: An oscillating wire that detects ice accretion, allowing ASOS to report freezing rain and freezing drizzle — a capability most AWOS units lack.
- Present weather identifier (PWI): An optical sensor that distinguishes rain from snow and reports precipitation type.
- Ceilometer: A laser-based instrument that shoots a pulsed beam upward and times the backscatter to determine cloud-base height and sky cover in tenths (oktas converted to FAA sky cover categories: FEW, SCT, BKN, OVC).
- Visibility sensor: Measures light extinction over a short path length and extrapolates to a prevailing visibility in statute miles.
- Pressure sensors: Redundant aneroid or digital sensors provide altimeter setting, sea-level pressure, and station pressure.
- Temperature and dewpoint sensors: Housed in a radiation shield to reduce solar heating errors.
- Precipitation accumulation gauge: Measures liquid-equivalent precipitation.
- Lightning sensor (at some sites): Detects nearby lightning activity.
ASOS produces data that feeds directly into the national METAR network, can be augmented or corrected by a human observer at staffed locations, and is disseminated via ATIS, D-ATIS, automated voice broadcasts on a dedicated frequency, and through FAA and NWS data services.
How AWOS Works — and Its Configuration Levels
AWOS is an FAA-administered system deployed primarily at smaller airports that do not qualify for a full ASOS installation. Unlike ASOS, AWOS comes in distinct configuration tiers — often called AWOS-A, AWOS-1, AWOS-2, AWOS-3, and AWOS-3P/PT/T. Each successive level adds more sensors and therefore more reported parameters:
- AWOS-A: Reports altimeter setting only. You will see this at very small airports. The broadcast is just a single number — not a full weather observation.
- AWOS-1: Adds wind direction and speed, temperature, dewpoint, and density altitude to the altimeter setting.
- AWOS-2: Adds visibility to the AWOS-1 suite.
- AWOS-3: Adds a ceilometer, providing sky condition and cloud height — this is the most common general aviation version. An AWOS-3 report looks much like a METAR.
- AWOS-3P: Adds a present weather sensor (precipitation type identification).
- AWOS-3PT: Adds both precipitation and thunderstorm/lightning detection.
- AWOS-3T: Adds thunderstorm/lightning detection without the precipitation identifier.
AWOS observations are updated continuously, with the voice broadcast typically refreshed about once per minute on a discreet VHF frequency, and at many airports you can also call a phone number to hear the automated voice report. Some AWOS stations feed into the METAR network; others do not, and their data may only be available via the local broadcast frequency. Always check whether the airport's AWOS data appears on official weather products before relying on it for IFR planning.
Critical Differences Between ASOS and AWOS
Both systems automate observation, but ASOS is generally the more comprehensive and federally standardized of the two. Here are the most operationally significant differences:
- Thunderstorm reporting: A standard ASOS unit can report lightning activity in the vicinity (TS in the METAR) because it includes a lightning sensor. Most AWOS-3 units do not include a lightning detector and therefore will not report thunderstorms or lightning — even if a convective cell is directly overhead. You could read an AWOS-3 METAR, see no mention of thunderstorms, and still have a severe thunderstorm on the field.
- Freezing precipitation: ASOS includes a freezing rain sensor and can report freezing rain (FZRA) and freezing drizzle (FZDZ). Basic AWOS units cannot detect freezing precipitation; they may report plain rain even when that precipitation is freezing on contact with surfaces.
- Precipitation type nuance: ASOS present-weather identifiers can distinguish rain, drizzle, snow, snow grains, and ice pellets. Lower-tier AWOS units may only know that precipitation is falling, not what kind.
- Update frequency: Both ASOS and AWOS update continuously (about once per minute) and can issue METARs and SPECI reports when conditions change significantly. AWOS is not limited to a 20-minute broadcast cycle.
- Human augmentation: At staffed ASOS sites (or NWS offices), a trained observer can add remarks, correct sensor errors, and issue special observations the automation would miss — such as a tornado on the field. AWOS has no such oversight at most locations.
- Network integration: ASOS is the official NWS observation network. AWOS feeds may or may not appear in official NWS products, depending on the station's data-sharing agreements.
What Neither System Can Detect
Both ASOS and AWOS share important blind spots that every pilot must internalize. The ceilometer on either system samples only a narrow column of sky directly overhead. If a broken layer exists in only one portion of the sky, the system may report fewer clouds than a human observer watching the full horizon would report. Conversely, it might report more clouds if thin wisps pass overhead while the broader sky is clear. The reported sky condition represents a statistical assessment of clouds passing overhead over a 30-minute window — not a panoramic picture.
Neither system reliably detects tornadoes, waterspouts, funnel clouds, blowing dust, haze layers at a distance, or volcanic ash. Neither can report runway surface conditions (wet, icy, snow-covered), and neither has the situational awareness to notice a rapidly developing microburst just off the departure end of the runway. Pilot reports (PIREPs) are invaluable precisely because they fill these gaps with real human observations from the air.
Key Numbers and Rules
- ASOS issues a METAR every 60 minutes, plus SPECIs for significant changes; AWOS updates continuously, approximately once per minute, and can also issue SPECIs.
- AWOS-A reports altimeter setting only — it is not a full weather observation.
- AWOS-3 is the minimum AWOS tier that reports sky condition and cloud height.
- Only AWOS-3PT and AWOS-3T tiers include thunderstorm/lightning detection; a standard AWOS-3 does not.
- ASOS visibility sensors measure down to 0 statute miles and report in increments down to 1/16 SM at low values.
- Ceilometers on both systems detect cloud bases up to approximately 12,000 feet AGL; clouds above that altitude may not be reported.
- ASOS is operated jointly by the NWS, FAA, and DoD; AWOS is operated by the FAA and airport authorities.
Common Test Traps
- Assuming AWOS reports thunderstorms: The FAA knowledge test frequently tests whether students know that a basic AWOS-3 will NOT report a thunderstorm even if one is occurring at the airport. Only AWOS-3T and AWOS-3PT tiers include lightning detection.
- Confusing AWOS-A with a full observation: AWOS-A provides only the altimeter setting. If a question asks what weather information is available from an AWOS-A station, the answer is just one parameter — not visibility, clouds, or wind.
- Trusting a clear METAR from an automated station near convection: A METAR that shows no TS remark from an AWOS station does not mean thunderstorms are absent. Students sometimes treat the absence of a TS report as confirmation of clear skies — a dangerous assumption.
- Overestimating ceilometer accuracy: The reported sky cover is a statistical sample of a narrow overhead column over 30 minutes, not a full-sky assessment. In rapidly changing conditions or patchy cloud cover, it can differ significantly from what a pilot would actually encounter.
- Forgetting that AWOS data may not feed official NWS products: Some AWOS stations only broadcast locally. Students planning an IFR flight should verify whether the destination airport's automated observation appears in official METAR feeds and weather planning tools, rather than assuming it does.
Understanding the capabilities and limitations of ASOS and AWOS is not just exam knowledge — it is preflight discipline. Use automated observations as a starting point, cross-check them with PIREPs, radar, and pilot weather briefings, and remember that no automated sensor can fully replace a trained human eye watching the full sky. When conditions are marginal or convective activity is possible, give these systems the skepticism they deserve and gather every available source of weather information before you go.