In the United States, there are two weather reporting systems, namely, ASOS and AWOS. ASOS stands for Automated Surface Observing System and AWOS stand for Automated Weather Observing System. There is a huge difference between these two automated systems. While both have their inherent issues, ASOS, by far, is a superior system. This array of sensors is the primary surface weather observing system in the United States, which supports over 900 airports as the essential aviation observation programs of the National Weather Service (NWS), the Federal Aviation Administration (FAA), and the Department of Defense (DoD). As such, every ASOS makes a routine report once an hour, usually just before the top of the hour. But be careful with the ASOS blind spot that I discuss here.
Although many of the AWOS units are operated and controlled by the FAA they are among the oldest automated weather stations and predate ASOS. AWOS generally report at 20-minute intervals and, unlike ASOS, do not report special observations for rapidly changing weather conditions. From the perspective of the actual system requirements between the two systems, the ASOS has a more strict set of requirements including the installation which requires certified installers.
For instance, let's look at the AWOS-3 at Santee Cooper Regional Airport in Manning, South Carolina. If you are based in that area you will notice a thunderstorm (TS) is reported just about every day at 0555Z. And you may also find that it will report a thunderstorm at 1215Z and 1715Z as well as you can see below. Is there really a thunderstorm? This is an automated report so it's unlikely a thunderstorm is occurring here...so you have to be extremely careful when relying on reports from AWOS.
KMNI 061215Z AUTO 00000KT 10SM TS SCT012 SCT017 SCT025 11/11 A3035 RMK AO2
KMNI 060555Z AUTO 16003KT 10SM TS CLR 13/13 A3035 RMK AO2
KMNI 051715Z AUTO 00000KT 10SM TS SCT039 20/14 A3041 RMK AO2
KMNI 050555Z AUTO 00000KT 10SM TS OVC014 12/06 A3038 RMK AO2
KMNI 021715Z AUTO 00000KT 10SM TS CLR 21/M02 A3012 RMK AO2
Moreover, it is common to see some problems with cloud coverage and ceilings at times during the nighttime hours. For example, in the Charleston, South Carolina area, meteorologists often see issues with the AWOS at KMKS, KDYB, KJZI, KBNL, and occasionally KRBW, KMNI, KCKI and KLRO. The words of wisdom here...be careful when using the reports from AWOS sites.
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
Founder, EZWxBrief™
CFI & former NWS meteorologist
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