Definition
An automated weather station, typically located at an airport, that continuously measures and reports current surface weather conditions including wind, visibility, cloud height, temperature, dew point, altimeter setting, and precipitation. ASOS transmits its observations by computer-generated voice over a dedicated radio frequency and by telephone, and feeds data into the national aviation weather system once per minute.
Plain English
A robotic weather station at an airport that takes constant readings of the local weather and broadcasts them in a computer voice, so pilots can hear up-to-the-minute conditions before landing or takeoff.
Context Anchor
Pilots encounter ASOS when checking airport weather before departure, before landing, during instrument planning, or by listening to the airport’s weather broadcast.
Derivation
Surface here means weather measured at ground level, as opposed to upper-air weather. The system is automated because it runs without a human observer, replacing the older practice of staffed weather offices reading instruments by hand.
Why Pilots Care
Gives pilots immediate access to accurate, up-to-date airport weather without needing a human observer, supporting safe go/no-go decisions and in-flight planning.
Grounding Statement
If you are approaching an airport, ASOS is one of the places you can get the latest local weather before deciding how to continue.
Intuition Check
Do not assume “automated” means a person has checked every report before you hear it. In ASOS, sensors collect the weather data and the system reports it automatically.
Example Sentence 1
About fifteen miles out, she tuned the ASOS frequency and listened to the wind, visibility, and altimeter setting before starting her descent.
Example Sentence 2
When the ASOS indicated a 400-foot ceiling and light rain, the flight instructor adjusted the approach briefing accordingly.