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User's Guide _______________________________________________________________________
NOTE
NOTE
28____________________________________________________________________ M210543EN-B
Figure 7
Principle for Determining Precipitation Type
Liquid Precipitation
In liquid precipitation the optical and the RAINCAP
near each other, or the RAINCAP
drizzle are separated by the highest peak signal (largest droplet). If
liquid precipitation is detected below 0 °C (TS), it is indicated as
freezing rain or freezing drizzle (ZR, ZL)
PWD22 freezing precipitation detection is based on the practical
definition of "rain (or drizzle) freezing on surfaces", not on detecting
supercooled water.
Rain
If the maximum droplet size is bigger than the Drizzle limit, the
precipitation type is rain. Light rain limit and Heavy rain limit are the
(approx.) two-minute average intensity limits in mm/h, which are used
to set the rain intensity type.
Drizzle
Drizzle consists only of small droplets. The intensity ratio
®
(optical/RAINCAP
) must be similar to that of rain.
By definition, drizzle cannot be identified only by its size, but also by
its origin (stratus cloud). PWD22 reports the instant weather code by
the size information only. The 15-minute weather code in the PWD22
report is drizzle, when at least ten minutes of only drizzle
precipitation has been observed. The one hour code is generated by
the majority principle.
®
intensities are
®
intensity is higher. Rain and
0311-142
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