Definition
A network of automated weather stations installed at airports across the United States that continuously measure and report current surface weather conditions, including wind, visibility, sky condition, temperature, dew point, altimeter setting, and precipitation. ASOS transmits this information through a recorded voice broadcast on a dedicated radio frequency or telephone line, and feeds data into the national aviation weather system.
Plain English
An automatic weather station at an airport that constantly checks what the weather is doing right now and reports it out loud over a radio frequency or by phone, so pilots can listen in and know what to expect.
Context Anchor
Pilots encounter ASOS when checking airport weather before departure, before landing, or while flying near an airport without a live weather observer.
Derivation
The name describes itself plainly: 'automated' (runs without a human), 'surface' (weather measured at ground level, not aloft), 'observing system' (a system that observes and reports). The key word is 'automated' -- replacing what used to be a human weather observer at the airport.
Why Pilots Care
Pilots rely on ASOS for current weather information when planning flights or approaching an airport, especially at locations without a control tower or human weather observers.
Grounding Statement
The system automatically detects changes in wind, clouds, and temperature right where the runway is.
Intuition Check
“Surface” does not mean only the runway pavement; it means weather near the ground at that airport. “Automated” does not mean flawless; pilots still compare the report with what they see and with other weather information.
Example Sentence 1
Before starting the engine, the student tuned 118.025 to listen to the ASOS broadcast and noted the wind, visibility, and altimeter setting.
Example Sentence 2
ASOS data is used to generate the METAR for the airport.