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SAFETY
AUTOMATED WEATHER:
A few things you may not know
by Dr. Scott C. Dennstaedt
Surface observations are one of those data points that pilots often take for granted. The truth is that they often play a pivotal role in many routine decisions. They are not just used by pilots; surface observations also provide air traffic controllers and meteorologists with a reasonable depiction of the weather conditions at an airport. Even with something as ubiquitous as a surface observation, there are some nuances to consider.
Whether in the form of a coded or decoded METAR or
by the ground-to-air radio broadcasts, pilots use surface observations to make many operational decisions during any particular flight. When listening to the broadcast before taxi, it provides the altimeter setting and will likely determine
the best runway to use for departure. When approaching an airport, observations help to determine if a visual approach is possible or a standard instrument approach procedure will be required. And when Mother Nature is at her worst, these observations will help determine when to skip the airport altogether and fly to an alternate destination.
Pilots at all experience levels should be familiar with
the two primary automated observing systems deployed at many airports throughout the United States. This includes the Automated Surface Observing System (ASOS) and the Automated Weather Observing System (AWOS). These automated systems consist of electronic sensors that measure the environment and then process the data to construct an observation once every minute.
It’s All About Sampling the Atmosphere
While many high-impact airports throughout the U.S. still rely on a trained weather observer to build the routine or special observation (SPECI), automated systems supply them with uniform and objective data for the observation. However, automated systems measure only the weather that passes directly through the sensor array, so they cannot report what’s happening outside the airport’s runway com- plex. Trained weather observers can undoubtedly augment the observation to add these details when they are opera- tionally significant.
At airports without a trained observer, pilots must ultimately rely on the “raw” automated observation. This observation isn’t as raw as you might think. To provide
a representative observation, the automated hardware must continuously collect the sensor’s real-time data over a period of time. The automated system applies an algo- rithm to the collected data to extrapolate the weather to cover a wider area.
When the weather is sampled over a specified period, it will tend to “smooth out” the conditions, but this is designed to account for the normal meteorological variations we
see in the weather. Each of the various weather elements shown in the table below identifies the required sample times for its algorithms and summarizes where the data are considered valid.
40 MMOPA MAGAZINE JULY / AUGUST 2021