Vaisala FD12P - Manual (Page 111)

Chapter 5 _______________________________________________________ Functional Description
VAISALA _______________________________________________________________________ 111
Mixed Precipitation
The FD12P 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 one and Snow limit. The intensity is determined by estimating
the water equivalent intensity and using the rain intensity limits.
a. 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.
b. Rain and snow: Rain and snow is reported when the TS is
below +8
°C (warm limit) and the criteria for ice pellets
are not met (see above).
Unknown Precipitation
Precipitation type is unknown in the following cases:
- When the default value is shown.
- When the TS is between -5 °C and +8 °C (warm limit).
- Before enough data is available for better analysis.
- When intensity is very low.
- When the intensity ratio is not reliable.
Visibility Types
When precipitation is not detected, the weather type is determined
from visibility. The visibility types for FD12P can be divided into two
main categories:
- Fog
- Haze and mist
The visibility types and their detection are listed below.
Fog
Fog codes are reported when there is no precipitation but the visibility
is less than 1 km in an average of 10 minutes. The fog trend is
calculated from the one-hour data. The average of the latest and
earliest 20 minutes the data are calculated. The change in these
averages determines the trend. Code 20 is reported, when fog has been
reported during the preceding hour but visibility is now better than 1
km and no fog patch detection is on.