Campbell Scientific PWS100 PWS100 Present Weather Sensor - Page 97
Visibility
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Section 8. Functional Description 8.6.4.2 Visibility Types Also by using the WMO SYNOP code table (4680) a visibility type can be defined. These types cover mist, fog, haze and smoke. See Appendix A for details. 8.6.4.3 Weather Classes Continuous and showers or intermittent classes can be defined when analyzing the time series of code output given by the PWS100 over a given time period. These are again given specific codes in the WMO 4680 table. Since the PWS100 stores historical data it can give codes relating to the conditions during the previous hour if no event is currently taking place. Again these events have specific WMO 4680 codes. 8.6.4.4 Weather Code Selection Weather code selection is based on a history of particle types determined for each particle falling through the detection volume and classified by the instrument. A table of particle types can be called from the history over a user specified period of time (or after a specific amount of time if the instrument runs in an automated mode). This table will consist of 11 numbers, one for each particle type as defined above, one for unknown type and one for errors observed from the signal (e.g., a size out of the range specified for the instrument). Logical processing is then used to determine the most prevalent particle type or to classify mixed types if present. No precipitation and unknown precipitation outputs are also possible. Unknown precipitation is classified if a certain number of the detected particles are individually classified as unknown types. Precipitation intensity is also given for the period specified. Output can then be given as ASCII (e.g., heavy snow) or as any of the set output standard codes, such as WMO SYNOP (WW73), METAR (+SN) and NWS (S+). Note that for some output types, certain particle types are not given an individual code. In these cases the most appropriate classification is given. METAR can support 3 types in one output, NWS output can be given with multiple codes if necessary and the WMO code has mixed types incorporated into its 80 specific codes (00 to 99 available but 20 codes reserved for future allocation). Although the PWS100 detects and classifies graupel (heavily rimed precipitation particles), for the purposes of WMO, 4680 coding graupel is classed as snow as there is no classification codes for this type of particle. The type classification output will still reflect the number of particles identified as snow grains, snowflakes and graupel. The PWS100 will output snow grains and snow flakes (snow) as separate outputs as per the WMO, 4680 code tables if these are dominant. 8.6.5 Visibility Visibility as defined in the British Standard 'BS 185-15:1972 Glossary of Aeronautical and Astronautical Terms - Section 15 : Meteorology' is "the greatest distance at which an object of specified characteristics can be seen and identified. At night, lights are observed and an equivalent day-light visibility is deduced". Because of the human perception of the environment, visibility is a subjective measurement dependant on the contrast response of the individual human eye. Therefore no instrument can truly measure visibility. It is only 8-11