Since the early days of aviation, the taking and dissemination of surface aviation observations has been an essential function for safe and efficient flight. The cost of personnel and material limits the availability of stations and it has been increasingly practical to automate many observing functions. The need for additional weather sources spurred production of several automated systems, one of which is the AWOS.
AWOS (Automated surface Weather Observing System) is a highly sophisticated data sensing, processing and dissemination system.
The differences between reports from a certified observer and the AWOS are negligible for "Objective" elements. In other words, the observer and the AWOS unit will produce the same results for wind, temperature, dew point, precipitation accumulation and pressure.
For "Subjective" elements such as visibility and sky condition, the AWOS uses a fixed location and time averaging technique while certified observers use a fixed time and spatial averaging technique. Although the techniques are different, the manual and automated techniques yield remarkably similar results within the limits of their respective capabilities. This would include:
- SKY CONDITION: A stationary laser beam ceilometer integrates 30 minutes of data. It reports ceiling of "M" Measured, "W" Indefinite Ceiling, "X" Sky Obscured or "-X" Sky Partially Obscured. The Indefinite ceiling is inferred from visibility. The maximum it will report is 12,000 feet, however, only the first three layers are detected and reported.
- VISIBILITY: It does not report prevailing visibility, but instead reports a ten minute average of what it detects between two sensors. This is why it will not report the fog until it actually reaches the sensor. It takes darkness into account by using equations for day and night. The reported visibility values are <1/4 mile up to 10 miles and will report "V" for variables.
Because of the averages used by the AWOS, it takes a little longer for the reported ceiling or visibility to "go up" or "go down" than would a manual observation.
Updated weather reports are produced once each minute, however, observations are only transmitted every 20 minutes to the Flight Service and NWS. Other ways of receiving the information are over a radio frequency or by phone.