Automated airport weather station

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An ASOS data collection platform 2008-07-01 Elko ASOS viewed from the south cropped.jpg
An ASOS data collection platform

Airport weather stations are automated sensor suites which are designed to serve aviation and meteorological operations, weather forecasting and climatology. Automated airport weather stations have become part of the backbone of weather observing in the United States and Canada and are becoming increasingly more prevalent worldwide due to their efficiency and cost-savings.


System types within the United States

In the United States, there are several varieties of automated weather stations that have somewhat subtle but important differences. These include the automated weather observing system (AWOS) and the automated surface observing system (ASOS).

Automated weather observing system (AWOS)

A commercial AWOS 2008-07-09 Eureka Airport AWOS viewed from the south cropped.jpg
A commercial AWOS

The automated weather observing system (AWOS) units are mostly operated, maintained and controlled by state or local governments and other non-federal entities and are certified under the FAA non-federal AWOS Program. [1] The FAA completed an upgrade of the 230 FAA owned AWOS and former automated weather sensor systems (AWSS) systems to the AWOS-C configuration in 2017. [2] The AWOS-C is the most up-to-date FAA owned AWOS facility and can generate METAR/SPECI formatted aviation weather reports. The AWOS-C is functionally equivalent to the ASOS. [3] FAA owned AWOS-C units in Alaska are typically classified as AWOS-C IIIP units while all other AWOS-C units are typically classified as AWOS III P/T units. [4]

AWOS systems disseminate weather data in a variety of ways:

The following AWOS configurations are defined below in terms of what parameters they measure: [5]

Also, custom configurations such as AWOS AV (AWOS A parameters plus visibility) are possible. Non-certified sensors may be attached to AWOS systems, but weather data derived from those sensors must be clearly identified as "advisory" in any voice messages and may not be included in any METAR observations.

As of May 22, 2022, the following manufacturers provide FAA-certified, non-federal AWOS systems: [6]

Automated surface observing system (ASOS)

The automated surface observing system (ASOS) units are operated and controlled cooperatively in the United States by the NWS, FAA, and DOD. After many years of research and development, the deployment of ASOS units began in 1991 and was completed in 2004.

These systems generally report at hourly intervals, but also report special observations if weather conditions change rapidly and cross aviation operation thresholds. They generally report all the parameters of the AWOS-III, while also having the additional capabilities of reporting temperature and dew point in degrees Fahrenheit, present weather, icing, lightning, sea level pressure and precipitation accumulation.

Besides serving aviation needs, ASOS serves as a primary climatological observing network in the United States, making up the first-order network of climate stations. Because of this, not every ASOS is located at an airport; for example, one of these units is located at Belvedere Castle in Central Park, New York City; another is located at the Blue Hill Observatory near Boston, Massachusetts.

Automated weather sensor system (AWSS)

The FAA has converted all automated weather sensor system (AWSS) units to AWOS III P/T units. There are no AWSS systems remaining in the US National Airspace System (NAS). [2]

Observing equipment

Automated airport weather stations use a variety of sophisticated equipment to observe the weather.

ASOS ice-free wind sensor Ely Airport ASOS Ice Free Wind Sensor.jpg
ASOS ice-free wind sensor

Wind speed and direction

A majority of older automated airport weather stations are equipped with a mechanical wind vane and cup system to measure wind speed and direction. This system is simple in design: the wind spins three horizontally turned cups around the base of the wind vane, providing an estimation of the wind's speed, while the vane on top turns so that the face of the vane offers the least resistance to the wind, causing it to point in the direction the wind is coming from and thus providing the wind direction.

The new generation of sensors use sound waves to measure wind speed and direction. The measurement is based on the time it takes for an ultrasonic pulse to travel from one transducer to another, which varies depending on - among other factors - the wind speed. The transit time is measured in both directions for several (usually two or three) pairs of the transducer heads. Based on those results, the sensor computes wind speed and direction. Compared to mechanical sensors, the ultrasonic sensors offer several advantages such as no moving parts, advanced self-diagnostic capabilities and reduced maintenance requirements.

NWS and FAA ASOS stations and most of new AWOS installations are currently equipped with ultrasonic wind sensors.

Unlike all other measurements, which are made between 3 and 9 feet (1 and 3 meters) above the ground, wind speed and direction are measured at 30 feet (10 meters).

ASOS visibility sensor 2008-07-09 Ely Airport ASOS Visibility Sensor in Ely, Nevada.jpg
ASOS visibility sensor


To determine visibility, automated airport weather stations use one of two sensor types:

The forward scatter sensor uses a beam of infrared light which is sent from one end of the sensor toward the receiver, but offset from a direct line to the receiver by a certain angle. The amount of light scattered by particles in the air and received by the receiver determines the extinction coefficient. This is then converted to visibility using either Allard's or Koschmieder's law.

In a transmissometer, a beam of visible light is transmitted from its transmitter to receiver head. The extinction coefficient is derived from the amount of light lost in the air.

There also are sensors that, to a certain degree combine a transmissometer with a forward scatter sensor.

Forward scatter sensors are more popular due to their lower price, smaller size and lower maintenance requirements. However, transmissometers are still used at some airports as they are more accurate at low visibilities and are fail-safe, i.e. in case of failure report visibility lower than actual.

Current sensors are capable of reporting visibility in a wide range. For aviation purposes, the reported values are rounded down to the nearest step in one of the following scales:

ASOS present weather sensor 2008-07-09 Ely Airport ASOS Present Weather Sensor in Ely, Nevada.jpg
ASOS present weather sensor

Present weather (falling precipitation)

Automated airport weather stations use a light emitting diode weather identifier (LEDWI) to determine if and what type of precipitation is falling. The LEDWI sensor measures the scintillation pattern of the precipitation falling through the sensor's infrared beam (approximately 50 millimeters in diameter) and determines from a pattern analysis of the particle size and fall velocity whether the precipitation is rain or snow. [9] If precipitation is determined to be falling, but the pattern is not conclusively identified as either rain or snow, unknown precipitation is reported. Automated airport weather stations are not yet able to report hail, ice pellets, and various other intermediate forms of precipitation.

Obscurations to vision

Automated airport weather stations do not have a separate sensor for detecting specific obscurations to vision. Instead, when visibility is reduced below 7 statute miles, the system uses the reported temperature and dew point to determine an obscuration to vision. If relative humidity is low (i.e., there is a large difference between the temperature and dew point), haze is reported. If relative humidity is high (i.e., there is a small difference between the temperature and the dew point), mist or fog is reported, depending on the exact visibility. Fog is reported when visibility is 1/2 mile or less; mist is reported for visibilities greater than 0.5 miles (0.80 km) but less than 7 miles (11 km). If the temperature is below freezing, [10] [11] humidity is high and visibility is 1/2 mile or less, freezing fog is reported. [12]

ASOS CT12K ceilometer 2008-07-09 Ely Airport ASOS Ceilometer in Ely, Nevada.jpg
ASOS CT12K ceilometer

Cloud coverage and ceiling

Automated airport weather stations use an upward-pointing laser beam ceilometer to detect the amount and height of clouds. The laser is pointed upward, and the time required for reflected light to return to the station allows for the calculation of the height of the cloud base. Because of the limited coverage area (the laser can only detect clouds directly overhead), the system computer calculates a time-averaged cloud cover and ceiling, which is reported to external users. To compensate for the danger of rapidly changing sky cover, the averaging is weighted toward the first 10 minutes of the 30-minute averaging period. The range of the ceilometer is up to 25,000 feet (7,600 m) depending on the model. [13] Clouds above that height are not detectable by automated stations at present.

ASOS HO-1088 thermometer 2013-09-19 09 28 07 ASOS HO-1088 thermometer at Eureka Airport, Nevada.JPG
ASOS HO-1088 thermometer

Temperature and dew point

Automated airport weather stations use a temperature/dew point sensor (hygrothermometer) designed for continuous operation which normally remains on at all times, except during maintenance.

The measurement of temperature is simple compared to the dew point. Operating under the principle that electrical resistance varies with temperature, a platinum wire resistive temperature device measures the ambient air temperature. The current ASOS thermometer is designated the HO-1088, though some older systems still utilize the HO-83.

ASOS DTS-1 dew point sensor 2013-09-19 09 27 41 ASOS DTS-1 dewpoint sensor at Eureka Airport, Nevada.JPG
ASOS DTS-1 dew point sensor

In contrast, the dew point measurement is considerably more complex. The original dew point sensor deployed on ASOS systems utilized a chilled mirror that is cooled to the point where a fine film of condensation forms on the mirror's surface. The temperature of the mirror at this condition is equal to the dew point temperature. The hygrometer measures the dew point by directing a light beam from a small infrared diode to the surface of the mirror at an angle of 45 degrees. Two photo transistors are mounted so they measure a high degree of reflected light when the mirror is clear (direct) and scattered light when the mirror is clouded with visible condensation (indirect). With the formation of condensation on the mirror, the degree of cloudiness of the mirror surface increases with the direct transistor receiving less light and the indirect transistor more light. The output from these photo transistors controls the mirror cooling module which is an electronic heat pump that operates much like a thermocouple in reverse, producing a heating or cooling effect. When the sensor is first activated, the mirror is clear. As the mirror surface temperature is cooled to the dew point temperature, condensations forms on the mirror. The electronics continuously tries to stabilize the signal levels to the power amplifier to maintain the mirror temperature at the dew point. If the dew point of the air changes or if the circuit is disturbed by noise, the loop makes the necessary corrections to restabilize at the dew point and maintaining continuous operation.

Due to problems with the chilled mirror sensor, NWS ASOS sites now use Vaisala's DTS1 sensor, which measures humidity only via capacitance. The sensor is based on a solid state capacitive relative humidity element that incorporates a small heater so that the sensing element is always above the ambient temperature, eliminating the formation of dew or frost. The sensor reports directly in dew point through a calculation based on measured relative humidity and the measured temperature of the heated capacitive element. [14]

Older AWOS systems used a lithium chloride dew point sensor. Current AWOS systems use capacitive relative humidity sensors, from which dew point is calculated. [15]

ASOS acquisition control unit, including the three pressure transducers towards the bottom 2008-07-09 Ely Airport ASOS ACU in Ely, Nevada.jpg
ASOS acquisition control unit, including the three pressure transducers towards the bottom

Barometric pressure and altimeter setting

Data from a barometric pressure sensor are used to calculate QNH altimeter setting. Pilots rely on this value to determine their altitude. To ensure safe separation from terrain and other obstructions, high degree of accuracy and reliability is required from a pressure sensor.

Most aviation weather stations use two (required for an AWOS) or three independent pressure transducers. The transducers may or may not share their associated tubing and external ports (designed to minimize effect of wind/wind gusts). Should the reported pressures differ by more than a preset maximum, the pressure values are discarded and altimeter setting is not reported or is reported as "missing."

Altimeter setting is calculated based on barometric pressure, site elevation, sensor elevation and - optionally - air temperature.

Altimeter setting is reported in inches of mercury (in steps of 0.01 inHg) or whole hectopascals, rounded down.

ASOS heated tipping bucket precipitation gauge 2011-12-14 Tonopah Airport ASOS automatic heated tipping bucket precipitation gauge.JPG
ASOS heated tipping bucket precipitation gauge

Precipitation accumulation

The original precipitation accumulation measuring device used for automated airport weather stations was the heated tipping bucket rain gauge. The upper portion of this device consists of a 1-foot (0.30 m) diameter collector with an open top. The collector, which is heated to melt any frozen precipitation such as snow or hail, funnels water into a two-chamber, pivoting container called a bucket. Precipitation flows through the funnel into one compartment of the bucket until 0.01-inch (0.25 mm) of water (18.5 grams) is accumulated. That amount of weight causes the bucket to tip on its pivots, dumping the collected water and moving the other chamber under the funnel. The tipping motion activates a switch (either a reed switch or a mercury switch), which sends one electrical pulse for each 0.01-inch (0.25 mm) of precipitation collected.

ASOS all-weather precipitation accumulation gauge (AWPAG) 2013-09-19 09 27 07 ASOS AWPAG precipitation gauge at Eureka Airport, Nevada.JPG
ASOS all-weather precipitation accumulation gauge (AWPAG)

Because of problems the heated tipping bucket has with properly measuring frozen precipitation (particularly snow), the all-weather precipitation accumulation gauge (AWPAG) was developed. This sensor is essentially a weighing gauge where precipitation continuously accumulates within the collector, and as the weight increases, precipitation is recorded. Only select NWS ASOS units have been equipped with the AWPAG. [16]

ASOS freezing rain sensor Ely Airport ASOS Freezing Rain Sensor.jpg
ASOS freezing rain sensor

Icing (freezing rain)

Automated airport weather stations report freezing rain via the resonant frequency of a vibrating rod. The resonant frequency decreases with increasing accretion (additional mass) of ice, hoarfrost, freezing fog, freezing drizzle, rime, or wet snow.

To report freezing rain, the system combines the sensor output from the freezing rain sensor with data from the LEDWI. The LEDWI must provide a positive indication of unknown precipitation or rain before the system can transmit a report of freezing rain. If the LEDWI reports either no precipitation or snow, the system will ignore the input from the freezing rain sensor. The sensor is designed to detect and report icing from all weather conditions.

ASOS thunderstorm sensor 2008-07-09 Ely Airport ASOS Thunderstorm Sensor in Ely, Nevada.jpg
ASOS thunderstorm sensor

Lightning (thunderstorms)

Many automated airport weather stations within the United States use the National Lightning Detection Network (NLDN) to detect lightning via the automatic lightning detection and reporting system (ALDARS). The NLDN uses 106 sensors nationwide to triangulate lightning strikes. Data from the detection grid is fed into ALDARS, which in turn sends messages to each automated airport station informing it of the proximity of any lightning strikes. Lightning strikes within 5 miles (8.0 km) of the station result in a report of a thunderstorm at the station (TS). Lightning strikes more than 5 miles (8.0 km) but less than 10 miles (16 km) from the station result in a report of a thunderstorm in the vicinity of the station (VCTS). Lightning more than 10 miles (16 km) but less than 30 miles (48 km) from the station results only in a remark of distant lightning (LTG DSNT). [17]

However, some stations now have their own lightning sensor to actually measure lightning strikes at the site rather than requiring an external service. This thunderstorm sensor works by detecting both the flash of light and momentary change in the electric field produced by lightning. When both of these are detected within a few milliseconds of each other, the station registers a possible lightning strike. When a second possible lightning strike is detected within 15 minutes of the first, the station records a thunderstorm. [18]

Data dissemination

Data dissemination is usually via an automated VHF airband radio frequency (108-137 MHz) at each airport, broadcasting the automated weather observation. This is often via the automatic terminal information service (ATIS). Most automated weather stations also have discrete phone numbers to retrieve real-time observations over the phone or through a modem.

In the United States, the AWOS/ASOS data acquisition system (ADAS), a computer system run by the FAA, polls the systems remotely, accessing the observations and disseminating them worldwide electronically in METAR format.

Limitations requiring human augmentation

At present, automated airport weather stations are unable to report a variety of meteorological conditions. These include:

Because many of these can pose dangers to aircraft and all of these are of interest to the meteorological community, most of the busier airports also have part-time or full-time human observers who augment, or provide additional information to, the automated airport weather station's observations. Research is on-going to allow the automated stations to detect many of these phenomena.

Automated stations can also suffer from mechanical breakdown, requiring repair or replacement. This can be either due to physical damage (either natural or human caused), mechanical wear, or severe icing during winter weather. During system outages, human observers are often required to supplement missing or non-representative observations from the automated station. Research is also ongoing to produce more robust systems which are less vulnerable to natural damage, mechanical wear and icing.

See also

Related Research Articles

Rain gauge Metal instrument for measurement of rainfall and other pre-cipitations

A rain gauge is an instrument used by meteorologists and hydrologists to gather and measure the amount of liquid precipitation over an area in a predefined area, over a period of time. It is used for determining the depth of precipitation that occurs over a unit area and thus measuring rainfall amount.

Diamond dust Ground-level cloud of ice crystals

Diamond dust is a ground-level cloud composed of tiny ice crystals. This meteorological phenomenon is also referred to simply as ice crystals and is reported in the METAR code as IC. Diamond dust generally forms under otherwise clear or nearly clear skies, so it is sometimes referred to as clear-sky precipitation. Diamond dust is most commonly observed in Antarctica and the Arctic, but can occur anywhere with a temperature well below freezing. In the polar regions of Earth, diamond dust may persist for several days without interruption.

Weather station Facility for atmospheric research and prediction

A weather station is a facility, either on land or sea, with instruments and equipment for measuring atmospheric conditions to provide information for weather forecasts and to study the weather and climate. The measurements taken include temperature, atmospheric pressure, humidity, wind speed, wind direction, and precipitation amounts. Wind measurements are taken with as few other obstructions as possible, while temperature and humidity measurements are kept free from direct solar radiation, or insolation. Manual observations are taken at least once daily, while automated measurements are taken at least once an hour. Weather conditions out at sea are taken by ships and buoys, which measure slightly different meteorological quantities such as sea surface temperature (SST), wave height, and wave period. Drifting weather buoys outnumber their moored versions by a significant amount.

METAR is a format for reporting weather information. A METAR weather report is predominantly used by aircraft pilots, and by meteorologists, who use aggregated METAR information to assist in weather forecasting.

Automatic weather station Meteorological instrument

An automatic weather station (AWS) is an automated version of the traditional weather station, either to save human labour or to enable measurements from remote areas. An AWS will typically consist of a weather-proof enclosure containing the data logger, rechargeable battery, telemetry (optional) and the meteorological sensors with an attached solar panel or wind turbine and mounted upon a mast. The specific configuration may vary due to the purpose of the system. The system may report in near real time via the Argos System and the Global Telecommunications System, or save the data for later recovery.

Index of meteorology articles Wikipedia index

This is a list of meteorology topics. The terms relate to meteorology, the interdisciplinary scientific study of the atmosphere that focuses on weather processes and forecasting.

A pilot report or PIREP is a report of actual flight or ground conditions encountered by an aircraft. Reports commonly include information about atmospheric conditions or airport conditions. This information is usually relayed by radio to the nearest ground station, but other options also exist in some regions. The message would then be encoded and relayed to other weather offices and air traffic service units.

SYNOP is a numerical code used for reporting weather observations made by manned and automated weather stations. SYNOP reports are typically sent every six hours by Deutscher Wetterdienst on shortwave and low frequency using RTTY. A report consists of groups of numbers describing general weather information, such as the temperature, barometric pressure and visibility at a weather station. It can be decoded by open-source software such as seaTTY, metaf2xml or Fldigi.

Precipitation types

In meteorology, the different types of precipitation often include the character, formation, or phase of the precipitation which is falling to ground level. There are three distinct ways that precipitation can occur. Convective precipitation is generally more intense, and of shorter duration, than stratiform precipitation. Orographic precipitation occurs when moist air is forced upwards over rising terrain and condenses on the slope, such as a mountain.

Station model Type of meteorological illustration

In meteorology, station models are symbolic illustrations showing the weather occurring at a given reporting station. Meteorologists created the station model to fit a number of weather elements into a small space on weather maps. This allows map users to analyze patterns in atmospheric pressure, temperature, wind speed and direction, cloud cover, precipitation, and other parameters. The most common station plots depict surface weather observations although upper air plots at various mandatory levels are also frequently depicted.

Meteorological instrumentation

Meteorological instruments, including meteorological sensors, are the equipment used to find the state of the atmosphere at a given time. Each science has its own unique sets of laboratory equipment. Meteorology, however, is a science which does not use much laboratory equipment but relies more on on-site observation and remote sensing equipment. In science, an observation, or observable, is an abstract idea that can be measured and for which data can be taken. Rain was one of the first quantities to be measured historically. Two other accurately measured weather-related variables are wind and humidity. Many attempts had been made prior to the 15th century to construct adequate equipment to measure atmospheric variables.

Convective storm detection is the meteorological observation, and short-term prediction, of deep moist convection (DMC). DMC describes atmospheric conditions producing single or clusters of large vertical extension clouds ranging from cumulus congestus to cumulonimbus, the latter producing thunderstorms associated with lightning and thunder. Those two types of clouds can produce severe weather at the surface and aloft.

Outline of meteorology Overview of and topical guide to meteorology

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Surface weather observation Fundamental data used for weather forecasts

Surface weather observations are the fundamental data used for safety as well as climatological reasons to forecast weather and issue warnings worldwide. They can be taken manually, by a weather observer, by computer through the use of automated weather stations, or in a hybrid scheme using weather observers to augment the otherwise automated weather station. The ICAO defines the International Standard Atmosphere (ISA), which is the model of the standard variation of pressure, temperature, density, and viscosity with altitude in the Earth's atmosphere, and is used to reduce a station pressure to sea level pressure. Airport observations can be transmitted worldwide through the use of the METAR observing code. Personal weather stations taking automated observations can transmit their data to the United States mesonet through the Citizen Weather Observer Program (CWOP), the UK Met Office through their Weather Observations Website (WOW), or internationally through the Weather Underground Internet site. A thirty-year average of a location's weather observations is traditionally used to determine the station's climate. In the US a network of Cooperative Observers make a daily record of summary weather and sometimes water level information. is a privately owned website for pilots and aviation enthusiasts. The site publishes aeronautical and airport information released by the Federal Aviation Administration (FAA) such as runway distances, airfield traffic patterns, airport frequencies, airport operations, facilities and services, chart location, navigational coordinates and locations, radio aids, ownership information and other pertinent information that all pilots need when traveling into or out of an airport or around the United States National Airspace System (NAS). The same information is published in the Airport/Facility Directory (A/FD), updated every 56 days.

Road Weather Information System Network of weather stations installed along roads

A Road Weather Information System (RWIS) comprises automatic weather stations (AWS) in the field, a communication system for data transfer, and central systems to collect field data from numerous ESS. These stations measure real-time atmospheric parameters, pavement conditions, water level conditions, visibility, and sometimes other variables. Central RWIS hardware and software are used to process observations from ESS to develop nowcasts or forecasts, and to display or disseminate road weather information in a format that can be easily interpreted by a manager. RWIS data are used by road operators and maintainers to support decision making. Real-time RWIS data is also used by Automated Warning Systems (AWS). The spatial and temporal resolution of a station network can be that of a mesonet or sometimes a constituent network in a network of station networks comprising a mesonet. The data is often considered proprietary although it is typically ingested into the major numerical weather prediction models.

Glossary of meteorology List of definitions of terms and concepts commonly used in meteorology

This glossary of meteorology is a list of terms and concepts relevant to meteorology and atmospheric science, their sub-disciplines, and related fields.

Present weather sensor

The present weather sensor (PWS) is a component of an automatic weather station that detects the presence of hydrometeors and determines their type and intensity. It works on a principle similar to a bistatic radar, noting the passage of droplets, or flakes, between a transmitter and a sensor. These instruments in automatic weather stations are used to simulate the observation taken by a human observer. They allow rapid reporting of any change in the type and intensity of precipitation, but include interpretation limitations.

Observer (meteorological)

A meteorological observer, or weather observer, is a person authorized by a weather authority to make or record meteorological observations. They are technicians who are responsible for the accurate observation, rapid measurement, timely collection, recording, and timely submission of meteorological parameters and information and various atmospheric phenomena to the Meteorological Center. Surface, upper air, radar, and satellite are all forms of weather observations.


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