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SAFETY
though automated systems are objective and maintain a cer- tain uniformity among all sites, it does not mean they match what a pilot sees out the windscreen.
ASOS may occasionally report cloud decks lower than what is actually encountered. Sometimes precipitation, lower cloud fragments, or fog triggers these lower values. Pilots have said that these “lower” reported values often indicated the height below which they had to fly before gaining enough forward visibility to see an airport and land. The key les-
son here is to evaluate all reports closely before dismissing them as inaccurate.
Even though the visibility sensor is designed to objectively represent the visibility of the atmosphere over a wide range
of weather conditions, day or night, it occasionally reports
a visibility more optimistic than what a human perceives. During the day, the human eye can be overwhelmed by bright light reflected in clouds, light precipitation, fog, or haze. Many pilots will resort to wearing sunglasses to obtain some relief from the glare.
The ASOS visibility sensor is not as sensitive to this condi- tion and sometimes reports a visibility approximately twice as high as what an individual may determine. Be alert for these bright conditions and expect a more optimistic value from the automated system.
Automated Lightning Reports
Some ASOS sites do have a single-site lightning sensor. If there isn’t a lightning sensor at the site, it is still possible for the ASOS to report lightning. For FAA-sponsored ASOS sites without a lightning sensor, lightning data is made available to the ASOS through the Automated Lightning Detection and Ranging System (ALDARS), which is a ground-based light- ning detection system. ALDARS is not co-resident with the sensor and sends the data to the ASOS. Here’s how it works.
An ASOS will format a METAR or SPECI for lightning in one of three ways: TS, VCTS (thunderstorms in the vicinity), or lightning in the distance.
1. If the cloud-to-ground lightning strike is within five miles
of the ASOS, the ASOS will make a special (SPECI) obser- vation and carry “TS” (for thunderstorm) in the body of
the special observation in the present weather field. “TS” will continue to be carried in the present weather field in subsequent METAR observations until no cloud-to-ground lightning strikes are observed for a 15-minute period within five miles of the sensor. At that time, the ASOS will make
a SPECI observation and end the thunder (removes the TS
from the observation).
2. If the cloud-to-ground lightning strike is between five
miles and ten miles of the ASOS, the ASOS will make a SPECI observation and carry “VCTS” (for thunderstorm in the vicinity) in the body of the observation in the present weather field. “VCTS” will continue to be carried in the present weather field in subsequent METAR observations until no cloud-to-ground lightning strikes are observed in the vicinity for a 15-minute period. At that time, the ASOS will make a SPECI and end the “VCTS.”
3. Independent from above, if the cloud-to-ground lightning strike is recorded between 10 miles and 30 miles of the ASOS, the ASOS will carry a “LTG DSNT xx” remark indi- cating distant lightning, with “xx” being the direction of the lightning in octants. This will be appended as appropriate on all SPECI and/or METAR observations. This means that many automated reports may carry TS or VCTS and still report distant lightning.
Of course, at locations with a human observer, the ASOS observation can be overridden, including adding remarks such as FRQ LTGICCG OHD TS OHD MOV NE – translated as frequent lightning overhead in clouds and cloud-to-ground with a thunderstorm overhead moving northeast.
What Will Automation Not Provide?
We can easily become complacent when it comes to auto- mation. We learn to trust automation and sometimes don’t acknowledge that it has real limitations. Therefore, to finish this discussion, it is just as important to know what automa- tion will not provide.
Automation systems can only report the weather that pass- es through the sensor array. They do not provide a horizon-to- horizon evaluation of the weather. This means that weather in the vicinity of the airport will not be measured. A rain shower that passes just north of the airport, for instance, may reduce visibility in that immediate area but will not be reported by the automated system.
Next, the automated system only reports clouds that are below 12,000 feet above ground level (AGL). This means
that an overcast cloud deck at 15,000 feet will be reported as clear. Effectively, a clear sky report from an automated station means clear below 12,000 feet. For airports with a human observer, this report can be augmented to include cloud decks above 12,000 feet.
Automated systems can only report one precipitation type at a time. For instance, if freezing rain and snow are detected, snow is reported. Certainly, weather observers can edit the observation before transmission to include additional pre- cipitation types.
Lastly, the system is not designed to report virga, tornadoes, funnel clouds, ice crystals, snow pellets, ice pellets, drizzle, freezing drizzle, and blowing obstructions such as dust or sand. All of these elements can be provided at locations that employ a trained observer. Often with drizzle, freezing driz- zle, ice pellets, or a mixture, you will see the automated system report an unknown precipitation type (UP). Nevertheless, even in the year 2021, automated reporting is in its infancy,
so new sensors will likely be added to measure some of these other weather elements in the future.
Dr. Scott Dennstaedt is a CFI and former NWS meteo- rologist and founder of EZWxBrief (ezwxbrief.com). He is the co-author of the newest weather book, entitled, Pilot Weather: From Solo to the Airlines.
42 MMOPA MAGAZINE JULY / AUGUST 2021