Campbell Scientific PWS100 Present Weather Sensor - Manual (Page 95)

Section 8. Functional Description
The PWS100 has exceptional sensitivity to the start and end of precipitation
events.
To avoid falsely reporting precipitation due to insects etc., the
sensor will not report the detection of particles at the start of an
event until a threshold of ~3 particles per minute is exceeded. NOTE
Measurements of both intensity and accumulation are however subject to some
assumptions as is common with most optical rain detectors. These are that all
rain particles are distorted in a similar way as they fall and also that there is a
defined cut-off area for the optical sampling volume. Those assumptions break
down in windy conditions, when accuracy can be increasingly degraded.
During high winds from critical directions the optical housing or mounting
poles may also create shadows causing under-reading.
In terms of an intensity output through the SYNOP and METAR and NWS
codes the intensity is calculated from the precipitation rate over the
measurement period. As heavy drizzle can give less precipitation accumulation
(and thus precipitation rate) than moderate rain for example, it is necessary to
observe the precipitation rate and also take into consideration the particle type
distribution of those particles. Table 8-1 is a WMO standard table of intensity
definitions that define the intensity class of different types of precipitation.
The PWS100 defines various intensity classes for all types and combination of
types of precipitation and these are shown fully in Appendix A. Although the
WMO guidelines have been followed as far as possible with these intensity
ranges some have been added since the WMO guidelines do not cover all
possible eventualities.
TABLE 8-1. Precipitation intensities
Variable Range Intensity Class
i < 0.1 mmh -1 Light
0.1 mmh -1 < i < 0.5 mmh -1 Moderate
Drizzle
i > 0.5 mmh -1 Heavy
i < 2.5 mmh -1 Light
2.5 mmh -1 < i < 10.0 mmh -1 Moderate
Rain
i > 10.0 mmh -1 Heavy
i < 1.0 mmh -1 Light
1.0 mmh -1 < i < 5.0 mmh -1 Moderate
Snow
i > 5.0 mmh -1 Heavy
8-9