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Are Surface Observations Based on Instantaneous Measurements?

All observations, whether automated or taken by human observers, should be used with care.

Scott Dennstaedt, Ph.D

Wednesday, October 02, 2024 at 02:25 PM ET

Verified

Edited By: tccjeremy

At many airports listening to the one-minute weather will provide the latest altimeter setting, winds, visibility, and sky conditions prior to departure. [Courtesy: Scott Dennstaedt]

Key Takeaways:

Automated surface observations (ASOS/AWOS) are not instantaneous but process continuous sensor data over specific periods (e.g., 1-30 minutes) using algorithms to provide a smoothed, representative view of weather within the terminal area.
These automated systems inherently lag behind rapidly changing weather conditions, meaning reports for sudden shifts in elements like sky cover or visibility will be delayed compared to the actual event.
Automated observations have specific limitations, including not reporting weather outside the immediate sensor array (vicinity weather), clouds above 12,000 feet (ASOS), multiple precipitation types, or certain phenomena like tornadoes or virga, which may require human augmentation.
Pilots must understand these nuances, including "lockout periods" for formal reports, and exercise caution, as automated observations, while objective, may not always precisely match human perception or capture all critical local weather details.

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Are surface observations based on measurements that are done instantaneously?

Answer: Whether in the form of a METAR or by the ground-to-air radio broadcasts, every pilot uses surface observations to make many routine operational decisions during any particular flight. As we listen to the broadcast prior to taxi, it provides us with an altimeter setting and will likely determine the runway we use for departure.

When approaching an airport under instrument flight rules (IFR), it will help us determine if we’ll be flying a visual approach or need to execute a standard instrument approach procedure. And when Mother Nature is at her worst, it will let us know when we should skip the airport altogether and fly to our alternate destination.

Scott Dennstaedt, Ph.D

Scott resides in Charlotte, North Carolina, and flies regularly throughout the Mid-Atlantic and Southeast U.S. He is a CFI and former NWS meteorologist. Scott is the author of "The Skew-T log (p) and Me: A Primer for Pilots" and the founder of EZWxBrief.