Chapter 3________________________________________________________ Functional Description
VAISALA ________________________________________________________________________ 29
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 crossarm surface temperature (TS) less
than 6 °C.
Mixed Precipitation
PWD22 reports either 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 is reported if the sensor crossarm 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.
