Chapter 3________________________________________________________ Functional Description
VAISALA ________________________________________________________________________ 27
Precipitation Accumulation
PWD22 calculates the accumulation of water (including the water
content of snow) and snow. The water sum is automatically reset
when it reaches 99.99 mm and the snow sum at 999 mm. There is also
a command for forced resetting by the operator or the system host
computer (
E
C
PW id C
C
R
).
Two methods are used in calculating the water sum. In rain the optical
intensity values are directly scaled to sum increments and added to the
accumulated sum. In other types of precipitation an internal scaling
factor is used to get the water increment. The scaling factor is
calculated from optical and RAINCAP
®
intensities.
The snow sum is accumulated from the optical intensity when snow is
detected. The snow sum is only a coarse estimate of the thickness of
snow. In a shorter period the accumulated snow value is a reasonable
measure of the new layer of snow above the previously fallen snow.
Present Weather
Precipitation Types
The ratio of the optical intensity and the RAINCAP
®
intensity
estimates are the key factors used in defining the precipitation type
(see section Precipitation Intensity on page 26). Some filtering is used
in calculating the intensity ratio to get the parameter that is used in the
type finding procedure. The Weather delay parameter is also used to
eliminate unrealistic changes in the precipitation type. The sensor
crossarm temperature TS is used in selecting the default precipitation
type. The default precipitation type is rain when the temperature is
above +6 °C. The default is snow when the temperature is below -0 °C
and between -0 °C and +6 °C the default is unknown (P or 40).
