Independent Assessment of the Accuracy of ASOS Visibility and Sky Cover Observations

The NWS in cooperation with the FAA and the DoD developed ASOS. ASOS currently provides critical meteorological data at almost 1,000 locations. Concerns were raised over the representativeness of ASOS reports at many locations. At the request of the FAA, the Air Force Operational Test and Evaluation Center (AFOTEC) undertook an independent assessment addressing concerns with the visibility and sky conditions sensors of ASOS. Of special interest were times of MVFR and IFR weather conditions. In 1998 AFOTEC fielded teams of independent Air National Guard and contract weather observers (IWO) to three ASOS locations in the Northern Tier. For 90 days these observers took independent visibility and sky cover observations from the ASOS locations. The observations were then compared to the ASOS and the contract weather observers (CWO) normally located at those sites. AFOTEC quantified ASOS representativeness in two ways, at MVFR and IFR transition events and by minute-by-minute comparisons. Overall the IWO and ASOS agreed close to 90% of the time. The contract weather observer, located as much as a mile from the ASOS, agreed less often. AFOTEC considered the CWO and ASOS equivalent although the two observation sources disagreed with "truth" under different conditions. "Truth" was established in two ways, by agreement with the IWO and by two of the three observation sources agreeing within the reporting criteria established by FAA and NWS. AFOTEC also analyzed 25 years of climatology at the three sites to determine the overall relevance of the observations taken during 90 days to the representativeness of ASOS over long periods of time.