Ask FLYING Pilot Proficiency

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 Observing Systems (ASOS/AWOS) provide crucial weather data for aviation but are not instantaneous; they continuously sample and process environmental data over specific time intervals (e.g., 1-30 minutes) using algorithms to create a representative observation for the airport's terminal area.
Due to these sampling periods, automated observations can lag rapidly changing weather conditions and have an hourly "lockout period" during which formal METARs/SPECIs are paused, though continuous real-time data is still accessible.
Pilots must be aware of the inherent limitations of automated observations, such as not reporting weather in the immediate airport vicinity, clouds above 12,000 feet (with some exceptions), multiple precipitation types simultaneously, or specific phenomena like virga or blowing obstructions, requiring careful evaluation.

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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.