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