What does AUTO mean in a METAR and why does it matter?

AUTO in a METAR indicates the report was produced entirely by automated instruments with no human input or oversight. It matters because automated systems have sensor limitations—they may report unknown precipitation (UP), miss phenomena outside sensor range, or lag in reporting improving conditions due to time-averaging. An augmented report, where a human observer supplements the system, will not contain AUTO.

What is the difference between ASOS and AWOS for pilots?

ASOS is the nation's primary automated weather network, operated jointly by the NWS, FAA, and DOD; it issues hourly METARs plus SPECIs when visibility or other elements cross specific qualifying thresholds. AWOS is a similar but generally less comprehensive system that issues METARs at 20-minute intervals and does not generate SPECIs. AWOS also comes in multiple configurations (AWOS-A through AWOS-4) with different sensor suites, so pilots should check which type is installed to know which elements are actually being reported.

How current is the weather data in an automated METAR?

In an automated observation, sky condition (cloud height and coverage) is time-averaged over the 30-minute period ending at observation time, while all other elements—visibility, wind, temperature, pressure, and present weather—are based on sensor data within 10 minutes of observation time. Critically, the averaging algorithms report deteriorating conditions faster than improving ones, so a METAR may temporarily show worse conditions than currently exist after a rapid improvement.

Surface Weather Observations: Manual, Automated, and Augmented

Every weather forecast, flight briefing, and instrument approach depends on accurate, timely surface weather observations. These observations capture the actual conditions at or near an airport—wind, visibility, sky condition, temperature, pressure, and more—and feed the broader meteorological system that keeps aviation safe. Without reliable surface observations, forecasters would be working blind, and pilots would have no way to verify whether conditions match the forecast.

The FAA Aviation Weather Handbook (FAA-H-8083-28B), Chapter 24, organizes surface weather observations into three categories: manual, automated, and augmented. Understanding the differences among them—and knowing the specific capabilities and limitations of each automated system—is essential for any pilot who reads a METAR, listens to an airport weather broadcast, or evaluates whether reported conditions reflect the full weather picture.

The Three Types of Surface Observations

Manual Observations

A manual surface weather observation is made entirely by a human weather observer who is FAA-certified in weather observing. The observer visually assesses sky cover, visibility, present weather, and other elements, then encodes them into a standardized report. Before the mid-1990s, manual observations were the standard across the United States. Today, they represent only a small fraction of total surface observations, having been largely replaced by automated and augmented systems. Where manual observers still operate, the quality and completeness of the observation is generally the highest available, because a trained human can perceive subtle phenomena—blowing dust, virga, partial obscurations—that sensors may miss.

Automated Observations

Automated observations are produced entirely by instruments and software algorithms, with no human input or oversight. The two principal automated systems in the United States are the Automated Surface Observing System (ASOS) and the Automated Weather Observing System (AWOS). A defining feature of a fully automated report is the code AUTO embedded in the METAR body. When you see AUTO in a report, you know no human reviewed or supplemented those observations. This is operationally significant: automated sensors have known limitations, particularly with certain precipitation types, obstructions to vision, and phenomena outside the sensor's field of view.

Augmented Observations

At select airports, a human weather observer or an FAA-certified tower controller supplements the automated system. These are augmented observations. The human adds or corrects elements that are beyond the automated system's capability or that are operationally significant—for example, identifying tornado funnels, dust whirls, or precipitation types the sensor cannot distinguish. Crucially, augmented reports do not contain the AUTO designator, signaling to the reader that a qualified person reviewed and potentially modified the output. The specific elements the human observer reports vary by airport based on local agreements and capabilities.

Recency of Sensor Data in Automated Stations

One subtle but important concept in automated observations is how the system handles the fact that sensors measure a single point over time, whereas a human observer uses spatial averaging—scanning the sky in all directions simultaneously. Automated stations compensate with time averaging. Specifically:

Sky condition (cloud height and amount) is based on sensor data averaged over the 30-minute period ending at the time of the observation.
All other elements—visibility, wind, temperature, pressure, and present weather—are based on data within 10 minutes or less of observation time.

This time-averaging design has a practical consequence: the algorithms are intentionally asymmetric. Deteriorating conditions are reported quickly, while improving conditions are reported more slowly. If dense fog suddenly lifts, the automated system may continue to report low visibility for up to 10 minutes. Pilots checking weather just after a rapid improvement should be aware that the METAR or broadcast may lag reality.

ASOS: The Nation's Primary Observing Network

ASOS is a joint program of the National Weather Service (NWS), the FAA, and the Department of Defense (DOD), and it serves as the primary surface weather observing network in the United States. ASOS automatically observes, formats, archives, and transmits observations. It disseminates both routine hourly reports and special observations (SPECIs) when conditions cross significant thresholds—for example, when visibility decreases to below one of several specific values (such as 3, 2, 1, or 1/2 statute miles), or increases to one of those values or better when it was previously below. ASOS also broadcasts computer-generated voice weather directly to aircraft via FAA ground-to-air radio, and the same information is available by telephone.

Beyond standard METARs, ASOS also produces One-Minute Observations (OMOs), updated every 60 seconds. OMOs can appear in METAR format and are what pilots hear on the ASOS radio broadcast. Importantly, the broadcast OMO data may differ from the METAR found on the internet or via FIS-B, because the broadcast reflects the most current one-minute data. Even so, OMOs are not instantaneous snapshots; the same 30-minute cloud and 10-minute visibility averaging applies.

ASOS reports a comprehensive set of weather elements, including:

Sky condition: cloud height and coverage (clear, few, scattered, broken, overcast) up to 12,000 feet AGL.
Visibility: reported to at least 10 statute miles.
Present weather: 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 statute mile; freezing fog (FZFG) when temperature is below 0°C; mist (BR) or haze (HZ) for visibilities from 5/8 to less than 7 statute miles, depending on the temperature-dewpoint spread (BR if the spread is 4°F/~2°C or less; HZ if greater).
Pressure: sea-level pressure and altimeter setting.
Temperature and dewpoint.
Wind: direction, speed, and character (gusts, squalls). Note: wind direction is reported in true degrees for national network distribution (internet, FIS-B, FSS) but in magnetic degrees for local broadcasts (radio, telephone).
Precipitation accumulation.
Significant remarks: variable cloud height, variable visibility, precipitation begin/end times, rapid pressure changes, wind shifts, peak wind, and potentially density altitude.

AWOS: A Scalable Automated Alternative

AWOS is similar in concept to ASOS but generally does not report all the same elements and may have fewer backup sensors or a lower maintenance response level. AWOS provides pilots with the weather information needed to conduct Part 91 operations and, depending on operations specifications, others as well. AWOS can be located at airports, heliports, and even offshore platforms and drill ships. Units are either Federal (FAA-owned and maintained) or non-Federal (owned by the site operator).

A key operational difference from ASOS: AWOS generates a METAR only at 20-minute intervals and does not issue SPECIs. It does provide OMOs by phone or radio, subject to the same time-averaging rules described above.

AWOS comes in six configurations with progressively more sensors:

AWOS-A: Altimeter setting only.
AWOS-AV: Altimeter plus visibility.
AWOS-1: Wind (speed, direction, gusts), temperature, dewpoint, altimeter, density altitude.
AWOS-2: All AWOS-1 parameters plus visibility.
AWOS-3: All AWOS-2 parameters plus precipitation accumulation and cloud height. Optional sensors for precipitation type/intensity (AWOS-3P) and/or thunderstorm/lightning (AWOS-3PT).
AWOS-4: All AWOS-3PT parameters plus freezing rain detection.

When planning operations at unfamiliar airports, checking which AWOS type is installed tells you exactly which elements will be reported and which won't. An AWOS-1, for instance, provides no cloud height or visibility—critical information gaps for instrument operations.

Why Observation Type Matters Operationally

For instrument approaches, alternate airport planning, and dispatch decisions, understanding the source of a weather report is not academic—it is a safety issue. An automated report with AUTO may miss a nearby thunderstorm that is beyond the sensor's lightning-detection range if the station lacks that sensor. It may also report unknown precipitation (UP) when it cannot determine precipitation type, leaving the pilot uncertain about icing or runway contamination risk. An augmented report, by contrast, benefits from a human who can look out the window and report what the sensors cannot see.

Pilots should also remember the asymmetric averaging behavior when weather is changing rapidly. During a fast-moving frontal passage, the METAR may not yet reflect improved conditions even though the actual sky has cleared. Conversely, a sudden deterioration—a fast-moving shower reducing visibility—will be captured and reported more quickly. When conditions are dynamic, cross-check METAR data with pilot reports (PIREPs), ATC observations, and ATIS to build a complete picture.

Key Numbers and Rules

Sky condition in automated observations: time-averaged over the last 30 minutes.
All other automated elements: sensor data within 10 minutes of observation time.
ASOS issues METARs hourly plus SPECIs when visibility crosses specific threshold values (e.g., below 3, 2, 1, or 1/2 sm), among other qualifying conditions.
AWOS issues METARs at 20-minute intervals; no SPECIs.
Fog (FG) reported when visibility below 5/8 sm; FZFG when temperature below 0°C.
BR vs. HZ distinction: temp-dewpoint spread ≤4°F (~2°C) = mist (BR); greater spread = haze (HZ), for visibilities 5/8 to less than 7 sm.
Wind direction on radio/telephone broadcasts: magnetic. On internet/FIS-B/FSS: true.
Automated report identifier: AUTO present. Augmented report: no AUTO.
ASOS sky condition reported up to 12,000 feet AGL.

Common Test Traps

AUTO means no human involvement at all. A report without AUTO could be manual or augmented—not necessarily augmented. Know the distinction and remember that augmented specifically means a human supplemented an automated system.
AWOS does not issue SPECIs. Exam questions may imply that all automated systems issue special reports when conditions change; only ASOS does this automatically.
Wind direction on radio is magnetic, on network distribution is true. Students often assume all automated wind reports use the same reference. The difference can matter when comparing an ATIS broadcast to a METAR.
The 30-minute averaging applies only to sky condition. Visibility uses 10-minute averaging. Do not apply the 30-minute rule to all elements.
OMO broadcasts may differ from the internet METAR. The ASOS radio broadcast reflects one-minute data; the published METAR is the official hourly or special report. These can legitimately disagree, especially in rapidly changing conditions.