Campbell Scientific PWS100 Present Weather Sensor - Manual (Page 113)

Appendix A. PWS100 Output Codes
Subjective Observations (Trappes, Paris, France, 14-16 May 1997) and the
Working Group on Surface Measurements (Geneva, Switzerland, 27-31 August
2001).
TABLE A-2. Light, moderate and heavy precipitation defined with respect to type of precipitation and to
intensity, i, with intensity values based on a three-minute measurement period.
Variable Range Intensity class
Drizzle i < 0.1 mm h -1
0.1 ≤ i < 0.5 mm h -1
i ≥ 0.5 mm h -1
Light
Moderate
Heavy
Rain
(also showers)
i < 2.5 mm h -1
2.5 ≤ i < 10.0 mm h -1
10.0 ≤ i < 50.0 mm h -1
≥ 50.0 mm h -1
Light
Moderate
Heavy
Violent
Snow
(also showers)
i < 1.0 mm h -1 (water equivalent)
1.0 ≤ i < 5.0 mm h -1 (water equivalent)
i ≥ 5.0 mm h -1 (water equivalent)
Light
Moderate
Heavy
Additionally the following points are given in the WMO No. 8 document:
Mixed precipitation of rain and snow: The same as for snow (since the ratio
rain/snow is not subject to any measurement, a simple choice should be made).
Note that this is altered in the PWS100 processing and additional mixed output
intensity modifiers have been developed – see further discussion below.
Hail: The same as for rain. This is implemented in the PWS100.
Ice-pellets: The same as for snow. This is implemented in the PWS100.
Freezing phenomena: The same as for the non-freezing phenomena. This is
implemented in the PWS100.
Further details are given in the WMO No. 8 guide relating to showers or
intermittent precipitation as follows:
The implementation here for the PWS100 uses a proportional mix of the
intensity values for the mixed types, i.e. if an event is of mixed rain and snow
then the rain and snow modifiers for light, moderate and heavy events are
added and divided by two, thus giving some mean value for the intensity
modifiers. There are some meteorological services which suggest using the
intensity value of the highest type (i.e. using the largest values of intensity so
for a mixed rain and drizzle event the intensity values for rain would be used).
Though this is scientifically inaccurate, it does follow the above WMO
guideline for mixed rain and snow. The implementation in the PWS100 as
outlined here serves as a reasonable compromise between altering intensity
ranges on particle distributions and choosing the largest intensity ranges.
A-7