The first WMO International Visibility Intercomparison was held in the United Kingdom at Royal Air Force Finningley (WMO No 03360) from 17 October 1988 to 11 May 1989. Twenty-five visibility instruments of 18 different designs were entered by seven Members and comprised a backscatter meter, forward scatter meters and long and short baseline transmissometers. This report contains a complete description of the conduct of the Intercomparison, the part icipat ing instruments, the data recorded, and the principles, techniques and results of the analysis. The characteristics of each instrument are summarised under the headings: stability, contamination susceptibility, overall performance, performance in precipitation, and any additional comments. Also presented is a comparison with local visual estimates of MOR. An estimate is given of the uncertainty in the measurements of MOR produced by the instruments included in the Intercomparison and these results are discussed with reference to the stated accuracy requirements of WMO Technical Commissions. In addition a detailed case study of the spatial and temporal structure of MOR in conditions of low MOR is presented. The report also includes recommendations both for the specification of accuracy reqUirements and the operational use of the different types of instrument studied.
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