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).
+ Summary of Weather Observing Programs Element Reported Wind Visibility Temperature & Dew Point Altimeter Density Altimeter Cloud / Ceiling Precipitation Identification Thunderstorm / Lightning Precipitation Occurrence Rainfall Accumulation Runway Surface Condition Freezing Rain Occurrence Remarks ASOS AWOS-A AWOS-A/V AWOS-1 AWOS-2 AWOS-3 AWOS-3P AWOS-3T AWOS-3P/T AWOS-4 Manual
Automated weather observing system (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. 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. 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. 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.
AWOS systems disseminate weather data in a variety of ways: A computer-generated voice message which is broadcast via radio frequency to pilots in the vicinity of an airport. The message is updated at least once per minute, and this is the only mandatory form of weather reporting for an AWOS. Optionally, a computer-generated voice message, available over a telephone dial-up modem service. The message is updated at least once per minute. Optionally (but frequently done), AWOS messages may be transmitted to the FAA for national dissemination via computer. These messages are currently in METAR format, and typical reporting frequencies are once every 20 minutes. This option is only available for AWOS III or IV systems (see below).
The following AWOS configurations are defined below in terms of what parameters they measure: AWOS A: barometric pressure and altimeter setting (in inches of Mercury). AWOS I: wind speed and wind gusts (in knots), wind direction (from which the wind is blowing) and variable wind direction (in degrees of the compass), temperature and dew point (in degrees Celsius), altimeter setting and density altitude. AWOS II: all AWOS I parameters, plus visibility and variable visibility (in miles). AWOS III: all AWOS II parameters, plus sky condition (in oktas), cloud ceiling height (in feet), and liquid precipitation accumulation (in inches). AWOS III P: all AWOS III parameters, plus precipitation type (rain, snow and sometimes drizzle) identification. AWOS III T: all AWOS III parameters, plus thunderstorm detection (via a cloud-to-ground lightning detector). AWOS III P/T: all AWOS III parameters, plus precipitation type identification and thunderstorm detection. AWOS IV Z: all AWOS III P/T parameters, plus freezing rain detection via a freezing rain sensor (Note: this configuration used to be called AWOS III PTZ). AWOS IV R: all AWOS III P/T parameters, plus runway surface condition. AWOS IV Z/R: all AWOS III P/T parameters, plus freezing rain detection and runway surface condition.
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:
All Weather Inc. (An ADB SAFEGATE Company) DBT Transportation Services LLC (An ADB SAFEGATE Company) Mesotech International, Inc. Optical Scientific Inc.
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).
Observing equipment Automated airport weather stations use a variety of sophisticated equipment to observe the weather.
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 above the ground, wind speed and direction are measured at .
Visibility To determine visibility, automated airport weather stations use one of two sensor types: forward scatter sensors transmissometers
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: M1/4 (less than 1/4 mile), 1/4, 1/2, 3/4, 1, 1-1/4, 1-1/2, 2, 2-1/2, 3, 4, 5, 7, 10 and 10+ (greater than 10 miles) In steps of 50 m when the visibility is less than 800 m; in steps of 100 m when it is 800 m or more, but less than 5 km; in kilometer steps when the visibility is 5 km or more but less than 10 km; and 10 km when visibility is 10 km or more.
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. 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 but less than . If the temperature is below freezing, humidity is high and visibility is 1/2 mile or less, freezing fog is reported.
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 depending on the model. Clouds above that height are not detectable by automated stations at present.
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.