Definition
A network of automated weather stations located at airports and other sites that continuously measure surface weather conditions — including wind, visibility, cloud height, temperature, dew point, and altimeter setting — and broadcast the results in standard format for use by pilots, controllers, and forecasters.
Plain English
A system of unmanned weather stations on the ground that constantly measure local weather and report it automatically to pilots and air traffic control.
Context Anchor
Pilots encounter this term in preflight weather briefings, airport weather reports, and automated airport weather broadcasts by radio or phone.
Derivation
‘Automated’ because no human observer is required, ‘Surface’ because the readings describe conditions at ground level (not aloft), and ‘Observing System’ because it gathers and reports observations rather than forecasts.
Why Pilots Care
Supplies current weather information at airports without a human observer, supporting safe go/no-go decisions and in-flight weather awareness.
Intuition Check
Do not read this as just any automatic weather device. In aviation, an Automated Surface Observing System is a specific airport weather reporting system focused on conditions at the airport surface.
Example Sentence 1
Before departure, the pilot tuned in the ASOS frequency to get the current wind and altimeter setting.
Example Sentence 2
Automated Surface Observing System data is transmitted continuously and used to create the hourly METAR for the airport.