Definition
A network of automated weather sensors located at airports that continuously measures and reports surface weather conditions, including wind, visibility, sky condition, temperature, dew point, altimeter setting, and precipitation. ASOS transmits its observations every minute and broadcasts them via a discrete VHF radio frequency and/or telephone line, and the data is also encoded into routine aviation weather reports (METARs).
Plain English
A set of weather instruments at an airport that takes weather readings on its own and reports them out loud over the radio, by phone, and in written weather reports — updated every minute.
Context Anchor
Pilots encounter ASOS reports when checking airport weather before flight, while approaching an airport, or when listening to weather information by radio or phone.
Derivation
The name describes what it does: 'Automated' (runs by itself, no human observer), 'Surface' (measures conditions at ground level, not aloft), 'Observing' (taking weather observations), 'Station' (a fixed location). Worth noting because pilots sometimes assume ASOS includes forecasts — it does not. It only reports what is happening right now at that airport.
Why Pilots Care
Gives pilots the real-time surface weather they need for takeoff, landing, and go/no-go decisions when no human observer is present.
Intuition Check
Do not assume “automated” means less important or unofficial. In this context, it means the weather is measured and reported by installed airport equipment instead of by a person looking outside.
Example Sentence 1
Ten miles out, the pilot tuned the ASOS frequency and copied the wind, visibility, and altimeter setting before starting the approach.
Example Sentence 2
ASOS data is used to update the airport METAR when the tower is closed for the night.