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 U.S.'s primary surface weather observing network and supports forecast activities, aviation operations, and the needs of the meteorological, hydrological, and climatological research communities.

Currently, more than 900 ASOS stations operate across the United States and its territories, and each one works non-stop, providing observations every minute of every hour of every day. In general, ASOS stations are located at airports. Stations include sensors to measure wind speed and direction, dew point, air temperature, and station pressure. The vast majority also measure precipitation type and amount, visibility, and cloud height and thickness. ASOS data are archived in NCEI’s Global Surface Hourly database.

ASOS: Observing Climate, Providing Continuity

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. 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, NCEI 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, NCEI archives one- and five-minute data, hourly, summary of the day, summary of the month, as well as special observations when significant short-term changes in weather occur. NCEI provides access to all these observations. Bulk downloads are available for larger volumes of data. There is generally a one- to two-day lag in availability of ASOS data. 

ASOS and Human Observations

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. However, systems located at many airports allow staff to augment the ASOS data with their observations. Human observers can supplement ASOS stations if a sensor is damaged or requires replacement.

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