Comparison of ceiling and visibility observations for NWS manned observation sites and ASOS sites

Cornick, Jon C., author
McKee, Thomas B., author
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The National Weather Service modernization program involves, among other things, a shift from manned weather observation to automated, unmanned instrument sensing. The Automated Surface Observing System (ASOS) is the device that will replace the conventional manned weather observation in use today. ASOS observations of ceiling and visibility were compared to the standard manual observations at 16 sites having at least four months of overlap data. The 16 sites were located in the central plains states of Colorado, Nebraska, Missouri, Kansas, Oklahoma, and Texas. The period of study was confined to the precommissioning period of the sites when both conventional data and ASOS data were available. The study spans from mid-September of 1991 to late July 1992, with the greatest amount of data collected between February and June 1992. The overall results show that ASOS ceiling reports were within 1000 ft of conventional ceiling reports 92.7% of the time. Similarly, ASOS derived visibility was within one reportable category of conventionally derived visibility 93.7% of the time. These percentages were determined from a data base composed of approximately 64,000 observations. During periods of active weather that would require a weather type entry into the coded observation, the high level of equality is decreased. The percentage of visibility reports within one reportable category is 60.8% and the percentage of ceilings within 1000 ft of conventional reports is 76%. These percentages were determined from a data base of approximately 9,300 observations containing a current weather entry. There were 5,263 cases of conventionally observed weather that would be categorized as requiring IFR (Instrument Flight Rules) by the FAA (Federal Aviation Administration) for safe air travel. ASOS observations correctly identified 4,499 of these events for an 85.5% equivalency rate. ASOS observations indicated 5,129 IFR occurrences, or nearly the same amount as conventional observations. Fog is the most frequently reported weather phenomena when large discrepancies occur between conventional and ASOS ceiling or visibility reports. This investigation shows that ASOS reported visibilities in foggy conditions are generally higher than those reported by conventional means. Ceilings in foggy conditions as reported by ASOS are generally much lower than those reported conventionally.
May 1993.
Also issued as Jon C. Cornick's thesis (M.S.) -- Colorado State University, 1993.
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Meteorology -- Observations
Weather forecasting
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