Definition
A network of automated weather stations, typically located at airports, that continuously measures and reports surface weather conditions including wind, visibility, sky condition, temperature, dew point, altimeter setting, and precipitation. ASOS broadcasts this information by radio on a discrete frequency and by telephone, and feeds the data into the national aviation weather system for use in routine and special weather reports.
Plain English
An automatic weather station at an airport that measures the current weather and reports it out loud over a radio frequency or by phone, updated minute by minute.
Context Anchor
Seen on chart legends, airport information, and weather sources when automated local weather reporting is available at or near an airport.
Derivation
Surface refers to weather measured at ground level, as opposed to upper-air or radar-based observations. Observing emphasizes that the system reports what is actually happening right now, not a forecast. Automated means no human observer is required for routine operation.
Why Pilots Care
Gives pilots immediate access to current surface weather for safe takeoff, landing, and flight planning decisions.
Intuition Check
ASOS is not a forecast and not a person on the radio. It is automated equipment reporting the weather it is measuring at that location.
Example Sentence 1
Before descending into the airport, the pilot tuned in the ASOS frequency and copied down the current wind, visibility, and altimeter setting.
Example Sentence 2
Many smaller airports use ASOS to provide continuous weather updates when no human observer is on duty.