Storm Station

The Field Book/Vol. VII, The Instruments/FB-INS-002

ASOS

ASOS, the Automated Surface Observing System, is the weather station at most U.S. airports: a set of sensors and a computer that measure the wind, temperature, dew point, pressure, visibility, clouds, precipitation and present weather every minute, and send a report every hour and whenever the weather changes. It is the backbone of the country's surface observations.

Plate FB-INS-002The stationRevision 1, 2026-09-25Status draftAlso called Automated Surface Observing System, airport weather station, METAR, SPECI, automated observation, AO2, ceilometer, forward scatter visibility sensor, sonic anemometer

Elevation of an airport weather station: a wind mast 33 feet (10 metres) tall with a sonic anemometer at its top; a fan aspirated temperature and dew point shield at 5 feet (1.5 metres); and, drawn schematic, a ceilometer pointing a laser straight up to find cloud bases, a forward scatter visibility sensor, a present weather sensor that tells rain from snow, a precipitation gauge in its wind shield, and the shelter that holds the barometers and the computer.1122334455667788AABBCCDDEE© 2026 STORM STATION 247THE FIELD BOOKPLATE FB-INS-002-AELEVATION OF AN AIRPORT STATION'S SENSORS, THE WIND MAST AND THE TEMPERATURE SHIELD AT THEIR HEIGHTS, THE REST SCHEMATICFIG. A AN AIRPORT STATION05101520253035FTWIND: SONIC ANEMOMETER, 33 FT (10 M)TEMPERATURE AND DEW POINT,FAN ASPIRATED SHIELD, 5 FT (1.5 M)DRAWN SCHEMATIC, NOT TO SCALECEILOMETER: A LASERPOINTED STRAIGHT UPVISIBILITY: FORWARD SCATTER,LIGHT SCATTERED BY THE AIRRAIN OR SNOWPRECIPITATION GAUGESHELTER: PRESSURE,THE COMPUTER, THE RADIO1234567TITLEASOS, an airport stationVOL. VII THE INSTRUMENTS · THE STATIONTYPEELEVATIONSCALEMAST AND SHIELD TO SCALE, THE REST NTSREVREV 1 DRAFT SHEET 1 of 3DATE2026-09-25IDFB-INS-002-ADRAWN AS linework on paperSOURCES NWS, Office of the Federal Coordinator for Meteorology, WMO
Fig. A An airport station. Elevation of an airport station's sensors, the wind mast and the temperature shield at their heights, the rest schematic MAST AND SHIELD TO SCALE, THE REST NTSMaximizeThe sheet, SVG, 11 by 17
  1. The wind mast, 33 ft (10 m)
  2. The temperature and dew point shield, 5 ft (1.5 m)
  3. The ceilometer
  4. The visibility sensor
  5. The present weather sensor
  6. The precipitation gauge
  7. The shelter, with the barometers

What it is

ASOS, the Automated Surface Observing System, is the weather station at most U.S. airports. The Weather Service, the Federal Aviation Administration and the Defense Department run it together. It is a set of sensors and a computer. They measure the wind, temperature, dew point, pressure, visibility, cloud, precipitation and present weather, and send a report every hour and whenever the weather changes enough to matter to a pilot. Its reports are the backbone of the country's surface record and of every forecast that starts from it.

Fig. A draws a station. The wind sensor stands on a mast 33 ft (10 m) tall, the standard height for wind, above the slowing of the ground. The temperature and dew point sensor sits in a fan aspirated shield at about 5 ft (1.5 m), Stevenson screen and siting FB-INS-009. Beside them, drawn schematic, are a ceilometer that fires a laser straight up and times the echo from cloud bases, a forward scatter sensor that measures visibility from the light the air scatters, a present weather sensor that tells rain from snow, a precipitation gauge in its wind shield, and a shelter holding the barometers and the computer.

What it is not

It is not a person looking at the sky. The ceilometer sees only the cloud passing over it, and the visibility sensor only a small sample of air. At larger airports trained observers watch the automated report and add what the sensors cannot see.

It is not an instant reading. Every reported value is an average over its own window, and a sudden gust or a thin patch of fog can fall between them.

Lookalikes

A personal weather station
Sited wherever its owner can, and not held to the airport standards; useful in numbers, Personal weather station FB-INS-003.
A human observer
At larger airports trained observers watch the automated report and add what its sensors cannot see, such as a distant thunderstorm or the kind of cloud.

The machine

Averaging

Fig. B draws the windows. The reported wind is the mean of the last 2 minutes; its gust is the highest 5 second wind of the last 10. The temperature and dew point are 5 minute means. The visibility is drawn from the last 10 minutes of the sensor's readings. The sky is built from the last 30 minutes of ceilometer echoes, the most recent 10 counted twice, because the ceilometer sees one point and the wind must carry the sky over it. Clouds are reported up to 12,000 ft (3.7 km); higher cloud needs an observer or a satellite.

The report

Each hour, near 53 minutes past, the station sends a routine report, a METAR, and between them special reports when the weather crosses a threshold such as a falling ceiling or thickening fog. Fig. C decodes one, an example written for the sheet rather than a real report. It carries the wind from 270° at 15 knots, 17 mph, gusting 25 knots, 29 mph, and the temperature to the whole degree Celsius in its body. In its remarks it gives the sea level pressure, 1,013.1 mb, and the temperature and dew point to a tenth: 71.1 °F (21.7 °C) and 46.9 °F (8.3 °C), 42 per cent humidity, Humidity variables FB-WAT-002.

Checking itself

The station compares its sensors before it reports. Its barometers are checked against each other, and a sensor that fails its checks drops out of the report rather than sending a bad value. A report missing an element is a report telling the truth.

Time strip of the 30 minutes before an observation, one bar for each element: the wind from the last 2 minutes; the gust from the last 10; the temperature and dew point from the last 5; the visibility from the last 10; and the sky from the last 30, the most recent 10 counted twice.1122334455667788AABBCCDDEE© 2026 STORM STATION 247THE FIELD BOOKPLATE FB-INS-002-BTHE MINUTES EACH REPORTED ELEMENT IS DRAWN FROM, ON ONE STRIP ENDING AT THE OBSERVATIONFIG. B WHAT EACH REPORT AVERAGESTHE OBSERVATION−5 MIN−10 MIN−15 MIN−20 MIN−25 MIN−30 MINWINDTHE 2 MINUTE MEAN DIRECTION AND SPEEDGUSTTHE HIGHEST 5 SECOND WIND IN THE LAST 10 MINUTESTEMPERATURE AND DEW POINTTHE 5 MINUTE MEANVISIBILITYTHE LAST 10 MINUTES OF SENSOR READINGSSKYTHE LAST 30 MINUTES OF CEILOMETER HITS, THE LAST 10 COUNTED TWICEEACH BAR: THE MINUTES A REPORTED VALUE IS DRAWN FROM; SKY HATCHED WHERE COUNTED TWICE12345TITLEASOS, what each report averagesVOL. VII THE INSTRUMENTS · THE STATIONTYPETIME STRIPSCALETIME TO SCALEREVREV 1 DRAFT SHEET 2 of 3DATE2026-09-25IDFB-INS-002-BDRAWN AS linework on paperSOURCES NWS, Office of the Federal Coordinator for Meteorology, WMO
Fig. B What each report averages. The minutes each reported element is drawn from, on one strip ending at the observation TIME TO SCALEMaximizeThe sheet, SVG, 11 by 17
  1. Wind, 2 minutes
  2. Gust, 10 minutes
  3. Temperature and dew point, 5 minutes
  4. Visibility, 10 minutes
  5. Sky, 30 minutes
An hourly airport report, written as an example and not from a real station, decoded field by field: the day and time in UTC; the wind from 270 degrees at 15 knots, 17 miles an hour, gusting 25 knots, 29 miles an hour; the visibility, 10 miles or more; few clouds at 4,500 feet and scattered at 25,000 feet; the temperature and dew point to the whole degree; the altimeter setting, 29.92 inches of mercury; the station type; the sea level pressure, 1,013.1 millibars; and the temperature and dew point to a tenth, 71.1 and 46.9 degrees Fahrenheit, 42 per cent humidity.1122334455667788AABBCCDDEE© 2026 STORM STATION 247THE FIELD BOOKPLATE FB-INS-002-CONE HOURLY REPORT DECODED FIELD BY FIELD; AN EXAMPLE WRITTEN FOR THIS SHEET, NOT A REAL REPORTFIG. C READING A REPORTMETAR KXXX 251753Z 27015G25KT 10SM FEW045 SCT250 22/08 A2992 RMK AO2 SLP131 T02170083AN EXAMPLE WRITTEN FOR THIS SHEET, NOT A REAL REPORT; KXXX STANDS FOR ANY STATION251753ZDAY 25, 17:53 UTC27015G25KTWIND FROM 270° AT 15 KT (17 MPH), GUSTS 25 KT (29 MPH)10SMVISIBILITY 10 MILES OR MOREFEW045 SCT250FEW AT 4,500 FT, SCATTERED AT 25,000 FT22/08TEMPERATURE 72 °F (22 °C), DEW POINT 46 °F (8 °C), WHOLE DEGREES CELSIUSA2992ALTIMETER 29.92 inHg (1013.2 MB)AO2AN AUTOMATED STATION THAT TELLS RAIN FROM SNOWSLP131SEA LEVEL PRESSURE 1013.1 MB (29.92 inHg)T02170083TO A TENTH: 71.1 °F (21.7 °C), DEW POINT 46.9 °F (8.3 °C), 42% HUMIDITY1234TITLEASOS, reading a reportVOL. VII THE INSTRUMENTS · THE STATIONTYPEDOCUMENTSCALENOT TO SCALEREVREV 1 DRAFT SHEET 3 of 3DATE2026-09-25IDFB-INS-002-CDRAWN AS linework on paperSOURCES NWS, Office of the Federal Coordinator for Meteorology, WMO
Fig. C Reading a report. One hourly report decoded field by field; an example written for this sheet, not a real report NOT TO SCALEMaximizeThe sheet, SVG, 11 by 17
  1. The wind and its gust
  2. The temperature and dew point, whole degrees
  3. The sea level pressure
  4. The temperature and dew point, to a tenth

Ingredients

  • Sensors for each element, sited on the airfield to the standards of the Federal Meteorological Handbook
  • A computer that averages each element over its own window and writes the report
  • Quality checks that compare sensors with each other and with limits before a value is sent

Scales

time
sensors read every minute or faster; a routine report each hour, and special reports between
horizontal
a point, at the airport; the sky and visibility sensors see a few miles at most
vertical
from the ground to 12,000 ft (3.7 km), the highest cloud its ceilometer reports
orlanski
micro

Signatures

surface
a report every hour near 53 minutes past; and special reports between when the weather changes

The numbers

QuantityValue, and the kind of number it is
Wind sensor height33 ft (10 m) above the ground, the standard exposureStandard, Automated Surface Observing System (ASOS) User's Guide
Temperature sensor heightAbout 5 ft (1.5 m), in a fan aspirated shieldTypical, Automated Surface Observing System (ASOS) User's Guide
Reported windThe 2 minute mean direction and speed; the gust, the highest 5 second wind in the last 10 minutesStandard, Automated Surface Observing System (ASOS) User's Guide
Reported temperature and dew pointThe 5 minute mean, to the whole degree Celsius in the report's body and to a tenth in its remarksStandard, Automated Surface Observing System (ASOS) User's Guide
Reported skyCloud layers to 12,000 ft (3.7 km) from the last 30 minutes of ceilometer readings, the last 10 minutes counted twiceStandard, Automated Surface Observing System (ASOS) User's Guide
Reported visibilityUp to 10 miles (16 km), from the last 10 minutes of sensor readingsStandard, Automated Surface Observing System (ASOS) User's Guide
Report scheduleA routine report (METAR) each hour, and a special report (SPECI) when the weather crosses set thresholdsStandard, Federal Meteorological Handbook No. 1

How the station sees it

This is the station. Its reports reach this site each hour, and every airport station's page shows its latest reading and its record. Its pressure is worked down to sea level for the map, Hydrostatic balance FB-ENG-003, and its visibility names fog when it falls below 5/8 mile, Fog types FB-WAT-020.

How it is warned

ASOS does not warn. Its readings confirm warnings and verify them afterwards: the gust that met a warning's threshold, the visibility under a fog advisory, the inch of rain in an hour. Forecasters watch its minute by minute data as a storm arrives.

See also

Sources

  1. National Weather Service. Automated Surface Observing System (ASOS) User's Guide.
  2. Office of the Federal Coordinator for Meteorology. Federal Meteorological Handbook No. 1, Surface Weather Observations and Reports.
  3. World Meteorological Organization. Guide to Instruments and Methods of Observation (WMO-No. 8).

Definition after the Automated Surface Observing System (ASOS) User's Guide. Plate FB-INS-002, revision 1, 2026-09-25. The number is permanent; cite it.