User's Guide _______________________________________________________________________
150 __________________________________________________________________ M211107EN-C
Drizzle
Drizzle consists only of small droplets. The intensity ratio
(optical/RAINCAP
®
) must be similar to that of rain.
NOTE
By definition, drizzle cannot be identified only by its size,
but also by its
origin (stratus cloud). PWD32
reports the instant weather code by the
size information only. The 15
-minute weather code in the PWD32 report
is drizzle, when at least ten minutes of only drizzle precipitati
on has been
observed. The one hour code is generated by the majority principle.
Frozen Precipitation
Precipitation is frozen if the optical-to-surface intensity ratio is high.
Snow
For snow, the intensity ratio (optical/RAINCAP
®
) must be larger than the
Snow limit parameter and the cross arm surface temperature (TS) less
than 6 °C.
Mixed Precipitation
PWD32 reports rain and snow (WMO codes 67 and 68) or ice pellets
(WMO codes 74, 75, and 76) when the intensity ratio is between 1 and
Snow limit. The intensity is determined by estimating the water
equivalent intensity and using the rain intensity limits.
Ice Pellets
Ice pellets are reported if the sensor cross arm temperature (TS) is below
+3 °C and the maximum droplet size in internal units is less than 50.
Rain and Snow
Rain and snow is reported when TS is below +6 °C and the criteria for
ice pellets are not met (see above).
Precipitation Type Unknown
Precipitation type is unknown when the default value is shown, when TS
is between -0 °C and +6 °C, and before enough data is available for
better analysis. The precipitation type is also unknown when the intensity
is very low and the intensity ratio is not reliable, or intensity ratio is
otherwise not clear.
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