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 observations (ASOS/AWOS) are not instantaneous but are generated from continuous sensor data sampled over varying time intervals (e.g., 1-30 minutes), then processed by algorithms to provide a representative, smoothed view of weather conditions.
These automated systems can lag behind rapidly changing weather due to their sampling periods and have specific limitations, such as only reporting weather directly over the sensors (not the broader vicinity), clouds below 12,000 feet (ASOS), or certain precipitation types and phenomena.
Pilots should be aware of operational nuances like the "lockout period" (when ASOS temporarily pauses formal special reports before hourly METARs) and that automated visibility reports can occasionally appear more optimistic than human perception, emphasizing the need to carefully evaluate all weather information.

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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.