Automated Surface Observing System:

The Automated Surface Observing Systems (ASOS) program is a joint effort of the National Weather Service (NWS), the Federal Aviation Administration (FAA), and the Department of Defense (DOD). The ASOS serves as the nation's primary surface weather observing network. ASOS is designed to support aviation operations at both large and small airports and support the needs of the meteorological, hydrological, and climatological research communities. As a primary support component for surface weather observations at approximately one thousand locations. ASOS advises air traffic management of the current weather conditions in support of the national airspace, safety of the flying public and Navy, Marine Corps, and Army operational airfields. The surface weather observation is a crucial component of day to day operations at these locations. The absence of ASOS observations at a location can delay flight operations, prevent air traffic from landing or taking off and cause second and third order effects that prevent passengers and cargo from reaching their final destinations on schedule, and precipitate economic consequences. ASOS works non-stop, updating observations every minute, 24 hours a day, every day of the year.
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 zone(s).

ASOS detects significant meteorological changes, disseminating hourly and special observations as weather criteria thresholds are met. Additionally, ASOS routinely and automatically provides computer-generated voice observations to aircraft in the vicinity using FAA ground-to-air radio frequencies. These messages are also available via a telephone dial-in line. ASOS observes, formats, archives and transmits observations automatically, transmitting a special report when conditions exceed weather element criteria outlined in FAA JO 7900.5E.

ASOS Reports the following basic weather elements:

Wind: direction, speed and character (gusts, squalls).

Prevailing Visibility and obstructions to vision from zero to 10 miles.

Present weather information: type and intensity for rain, snow, and freezing rain.

Sky condition: cloud height and amount (clear, scattered, broken, overcast) up to 12,000 feet.

Ambient air temperature, dew point temperature.

Pressure: sea-level pressure, altimeter setting.

Precipitation accumulation, both liquid and frozen water equivalent.

Select significant remarks appended to the end of the observation that include variable cloud height, variable visibility, precipitation beginning/ending times, rapid pressure, changes, pressure change tendency, wind shift, and peak wind.