Picture Climate: What’s ASOS?

Image of an Automated Surface Observing System Station

An Automated Surface Observing System Station
Credit: NOAA

The Automated Surface Observing System (ASOS) program is a joint effort between the National Weather Service, the Federal Aviation Administration, and the Department of Defense. ASOS serves as the nation's primary surface weather observing network and supports forecast activities, aviation operations, and the needs of the meteorological, hydrological, and climatological research communities.

There are currently more than 900 ASOS stations across the United States and its territories, and each one works non-stop, providing observations every minute of every hour of every day. Each station includes sensors to measure wind speed and direction, dew point, air temperature, precipitation type and amount, visibility, cloud height and thickness, and station pressure.

In general, ASOS stations are located at airports. However, there are a few exceptions where scaled-down versions have been installed at locations significant to the community. Examples include Central Park in New York City and the Baltimore Science Center in Baltimore, Maryland.

Currently, ASOS stations are limited by their ability to only detect weather that is directly above them. For instance, if a storm was moving in and the skies were darkening nearby, the ASOS station wouldn’t detect the storm until it moved over the sensors. Likewise, ASOS stations cannot detect shallow or patchy fog, tornadoes, hail and ice pellets, multiple forms of precipitation falling at the same time, in-cloud and cloud-to-cloud lighting, clouds not directly above them, clouds more than 12,000 feet above ground level, and cloud type. However, systems located at many airports allow staff to augment the ASOS data with their observations of these weather phenomena. Human observers can also serve as temporary replacements for ASOS stations if a sensor is damaged or requires replacement.

ASOS observations also provide continuity for studying climate. Since the late 1940s, most climate observing sites, representing city locations, were located at airports where staff took meteorological measurements. Throughout the years, many of these offices have moved, and more than 500 additional observation locations have been added at smaller airports. In order to maintain a long history of observations, ASOS data are used in conjunction with human observations to continue climate records.

When the climate records from a human observer and an automated system are merged, there may be an artificial shift in the averages at the time of the change. Fortunately, NCDC and the National Weather Service maintain records indicating when these changes took place. Using that information, the record can be normalized for use in long-term climate comparisons.

To ensure continued preservation of the vast amount of data observed by ASOS stations, NCDC archives one- and five-minute ASOS data and provides access to them through an interactive user interface or through FTP access for large-volume downloads. There is generally a month or two lag in availability of ASOS data.

For more information on these unique observation systems, visit the National Weather Service Automated Surface Observing System page.