Overview of Calculations Used to Measure Weather Elements Within the ASOS and AWOS: Not as Simple as You Think

The Automated Surface Observing System (ASOS) system detects and measures atmospheric conditions occurring at designated airports across the United States. The program is a joint effort between the National Weather Service, the Federal Aviation Administration (FAA), and the Department of Defense. The Automated Weather Observing System (AWOS) is a similar system to ASOS but for use beyond the federal government. AWOS was designed to provide basic weather observation data, while the more sophisticated ASOS was developed to support weather forecast activities for aviation operations. Systems not owned and operated by the FAA are termed “Non-Federal” systems.

Almost 2500 ASOS and AWOS systems are currently in operation throughout the US, including Alaska, Hawaii, and territories. Both ASOS and AWOS observe, format, archive, and transmit observations automatically in the form of an Aviation Routine Weather Report (METAR) or special (SPECI). Basic weather elements measured and reported by ASOS include sky condition, visibility, precipitation identification, obstructions to vision, pressure, temperature and dew point, wind direction and speed, and precipitation accumulation. AWOS measures many of these same elements, depending on the classification level of the system.

While the types of weather elements measured by ASOS and AWOS are known by most users, including meteorologists, how those elements are measured and reported are less understood. For example, temperature is not reported as an instantaneous measurement by the sensor but instead is calculated using an average of temperature readings taken over a certain length of time. This presentation will describe the sometimes complex processes within ASOS and AWOS used to calculate the values of these weather elements, including dew point, sky condition, visibility, and pressure.