Campbell Scientific PWS100 Present Weather Sensor - Manual (Page 97)

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
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