Automated Surface Observing Systems (ASOS)

Haines Alaska ASOS, Photograph by Tom Burgdorf When you hear the high temperature of the day from your local TV weathercaster, that information came from an Automated Surface Observing Systems (ASOS) machine from your local airport. ASOS is installed at more than 900 airports across the country, where they make observations.

Federally funded, ASOS is a joint program of the National Weather Service, the Federal Aviation Administration, and the Department of Defense. The ASOS systems serve as the nation's primary surface weather observing network.

The many sensors that comprise ASOS detect different weather elements and can update the official weather observation up to 12 times each hour. ASOS works non-stop, 24 hours a day, every day of the year.

ASOS's constant stream of data benefits the forecast and research communities and promotes more accurate forecasts of all kinds. ASOS sensors also perform well at night, a difficult time for human observers to make accurate observations.

Getting more information on the atmosphere, more frequently and from more locations is the key to improving forecasts and warnings. Thus, ASOS information helps the NWS increase the accuracy and timeliness of its forecasts and warnings--the overriding goal of the National Weather Service.

The primary concern of the aviation community is safety, and weather conditions often threaten that safety. A basic strength of ASOS is that critical aviation weather parameters are measured where they are needed most: airport runway touchdown zones.

ASOS detects significant changes, transmitting hourly and special observations via the various networks. Additionally, ASOS routinely and automatically provides computer-generated voice observations directly to aircraft in the vicinity of airports, using FAA ground-to-air radio. These messages are also available via a telephone dial-in port. ASOS observes, formats, archives and transmits observations automatically. ASOS transmits a special report when conditions exceed preselected weather element thresholds, e.g., the visibility decreases to less than 3 miles.

Typical ASOS unitASOS Reports the following basic weather elements:
  • Sky conditions such as cloud height and cloud amount up to 12,000 feet,
  • Surface visibility up to at least 10 statute miles,
  • Basic present weather information such as the type and intensity for rain, snow, and freezing rain,
  • Obstructions to vision like fog, haze, and/or dust,
  • Sea-level pressure and altimeter settings,
  • Air and dew point temperatures,
  • Wind direction, speed and character (gusts, squalls),
  • Precipitation accumulation, and
  • Selected significant remarks including- variable cloud height, variable visibility, precipitation beginning/ending times, rapid pressure changes, pressure change tendency, wind shift, peak wind.

However, like all technology, there are limitations as to what the equipment can do. The main limitation is its ability to see around the horizon. It's eyes only see directly overhead. Should there be a storm front moving in with darkening conditions, ASOS will not detect it until the storm begins to move over the sensors. Likewise, ASOS cannot see patchy fog that is not located directly at the station location.

Therefore, weather around the airport that has not encountered the sensors will not be measured. The system is not designed to report clouds above 12,000 feet, virga, tornadoes, funnel clouds, ice crystals, snow pellets, ice pellets, drizzle, freezing drizzle, blowing obstructions such as snow, dust, or sand, snow fall and snow depth. Many of these elements will be provided by other sources. New sensors are being added to measure some of these weather elements.

As a result, many of the ASOS stations, with staffed air traffic control towers, are monitored and human observers can edit or augment the automated observations.

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