Definition
A ground-based weather station that continuously measures and reports surface weather conditions at an airport without human intervention. ASOS sensors sample wind, temperature, dew point, altimeter setting, visibility, cloud height, precipitation, and other elements, and broadcast the results in a standard observation format updated at least once per minute. Reports are transmitted by a discrete VHF radio frequency and a published telephone number, and feed national weather and aviation systems.
Plain English
An automatic weather station at an airport that takes constant readings of the local weather and reports them out loud over a radio frequency and a phone line, so pilots always have current conditions for that field.
Context Anchor
You will hear or read ASOS information when checking airport weather, especially before departure, before arrival, or when setting the altimeter near an airport.
Derivation
Automated means it runs by itself without an observer. Surface refers to weather measured at ground level rather than aloft. Observation system simply means a setup that watches and reports. Together: a self-running ground-level weather watcher.
Why Pilots Care
Gives pilots the accurate altimeter setting and real-time weather needed to set instruments correctly and decide whether conditions are safe for takeoff or landing.
Intuition Check
ASOS is a current surface observation, not a forecast. It reports conditions at or near the airport, not the weather higher up along the whole route.
Example Sentence 1
Before descending into the pattern, the pilot tuned the ASOS frequency to get the wind, visibility, and altimeter setting.
Example Sentence 2
ASOS data is used to create the METAR that appears on the airport information page.