Automated Weather Observation System (AWOS/ASOS) and Related Airport Weather Equipment

Automated Weather Observation System (AWOS)

An Automated Weather Observation System (AWOS) is a fully automated suite of ground-based meteorological sensors and processors installed at airports. AWOS units continuously monitor, measure, and report vital surface weather parameters—such as wind speed and direction, temperature, dew point, barometric pressure, visibility, cloud ceiling, and precipitation. Advanced AWOS configurations can also detect thunderstorms, freezing rain, and runway surface conditions.

AWOS installations operate 24/7, ensuring constant, reliable data flow for pilots, air traffic controllers, and airport authorities. Data is disseminated via VHF radio, telephone, and digital networks, typically with updates every minute. Pilots use AWOS data for pre-flight briefings, in-flight weather checks, and real-time approach/departure decisions. The systems automatically encode observations in the METAR or SPECI formats, supporting standardized communication within global aviation networks.

AWOS units are modular, with capability levels ranging from basic (AWOS-A: altimeter setting only) to comprehensive (AWOS-III P/T: full meteorological suite). Sensors are calibrated to international standards for accuracy and reliability, and periodic maintenance ensures data integrity. AWOS data also feeds into national and international weather databases for forecasting and research.

Automated Surface Observing System (ASOS)

The Automated Surface Observing System (ASOS) is the United States’ primary network of automated surface weather stations, jointly managed by the National Weather Service, Federal Aviation Administration, and Department of Defense. ASOS provides the most comprehensive, high-frequency surface weather data, serving both aviation and public meteorology.

ASOS stations are strategically located at major airports and climatological sites. They deliver detailed, routine, and special weather observations, including advanced precipitation identification, intensity monitoring, and full climate data integration. ASOS data is broadcast by VHF radio, telephone, and digital networks, and encoded in METAR/SYNOP formats for use in aviation and general weather forecasting.

ASOS features a broad sensor set: laser ceilometers for cloud height, visibility sensors, advanced precipitation gauges, present weather detectors, and precision barometric pressure sensors. Automated calibration, redundancy, and remote monitoring maximize reliability and data availability.

METAR (Meteorological Aerodrome Report)

A METAR is an internationally standardized coded weather report for airports and aerodromes, maintained by ICAO. METARs are generated at least once per hour (or more frequently as SPECI updates) and are the global standard for aviation weather reporting.

Each METAR includes the station identifier, timestamp, wind data, visibility, current weather, cloud cover, temperature, dew point, pressure, and remarks. Automated stations indicate “AUTO” in reports. METARs are essential for pilots, controllers, meteorologists, and dispatchers for flight planning and operational safety.

ATIS (Automated Terminal Information Service)

The Automated Terminal Information Service (ATIS) is a continuous automated broadcast of current, non-control information for pilots at busy towered airports. ATIS typically includes the latest weather, active runways, approaches, NOTAMs, and other operational details.

ATIS reduces controller workload, standardizes pilot briefing, and enhances safety. Pilots must listen to ATIS before contacting ATC. Updates occur at least hourly or when significant changes arise. Digital ATIS (D-ATIS) is increasingly common, delivering information via data link for equipped aircraft.

Ceilometer

A ceilometer is an automated optical instrument that measures the height of the cloud base above ground level by emitting a laser or light beam and detecting its reflection from the cloud base. Ceilometers are integral to AWOS, ASOS, and staffed stations, providing real-time cloud ceiling data for METAR/SPECI reports. Modern ceilometers can detect multiple cloud layers and are calibrated for all-weather operation.

Transmissometer

A transmissometer measures atmospheric transparency over a runway, determining the Runway Visual Range (RVR)—the distance pilots can see runway markings or lights. Used primarily at airports with ILS Category II/III operations, transmissometers are essential for safe landings and takeoffs in low visibility.

Present Weather Detector (PWD)

A Present Weather Detector (PWD) identifies and reports the type and intensity of precipitation or weather phenomena at the airport, such as rain, snow, drizzle, fog, or hail. PWDs use optical or acoustic methods and are standard in advanced AWOS/ASOS installations, automatically integrating their findings into METAR/SPECI reports.

Runway Surface Condition Sensor

A Runway Surface Condition Sensor monitors runway temperature, moisture, and the presence of ice, snow, or water. These sensors inform airport operators, ATC, and pilots about real-time runway conditions, supporting safe aircraft operations, especially in winter or adverse weather.

Ultrasonic Anemometer

An Ultrasonic Anemometer uses ultrasonic sound waves to measure wind speed and direction, offering high reliability with no moving parts. Widely adopted in AWOS, ASOS, and other airport weather stations, these sensors provide accurate, real-time wind data for aviation safety.

Barometric Pressure Sensor

A Barometric Pressure Sensor measures atmospheric pressure, enabling calculation of altimeter settings vital for aircraft altitude reference. Accurate barometric pressure is foundational for AWOS, ASOS, and all aviation weather reports.

Density Altitude

Density Altitude is the pressure altitude corrected for temperature, representing the altitude in the standard atmosphere with the same air density as the observed conditions. High density altitude reduces aircraft performance. AWOS/ASOS systems commonly calculate and broadcast this value.

Freezing Rain Sensor

A Freezing Rain Sensor detects freezing rain at the surface via ice accretion on a probe or detection surface. Included in advanced AWOS/ASOS systems, these sensors trigger automated METAR/SPECI updates and operational alerts for runway and aircraft safety.

Thunderstorm Detector

A Thunderstorm Detector senses thunderstorm activity by detecting lightning discharges or changes in atmospheric electric fields. Integrated into higher-level AWOS/ASOS stations, they provide critical alerts for airport operations during convective weather.

Visibility Sensor

A Visibility Sensor uses optical scattering or transmissometer techniques to measure horizontal visibility, directly supporting METAR/SPECI reporting and operational decisions during fog, precipitation, or haze.

Data Processing Unit (DPU)

A Data Processing Unit (DPU) is the central controller in AWOS/ASOS installations, collecting, processing, and validating sensor inputs to generate standardized weather reports. The DPU ensures data integrity and compliance with aviation reporting standards.

Airport weather observation systems are the backbone of safe, efficient aviation. By integrating advanced sensors, automated reporting, and standardized communication, AWOS, ASOS, and their components deliver the actionable, real-time data that keeps global air travel moving—no matter the weather.