Vaisala FD12P - Manual (Page 110)

User's Guide _______________________________________________________________________
110 _____________________________________________________ M210296en-A
minute average intensity limits in mm/h used to set the
rain intensity type.
b. Drizzle: Drizzle consists only of small droplets. The
maximum droplet size allowed for drizzle is set by the
Drizzle limit parameter. The Light drizzle limit and Heavy
drizzle limit are the number of droplets in 15 s (average)
that are used to determine the drizzle intensity. The
intensity ratio (optical/DRD12) 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). The FD12P reports the instant weather code
by the size information only. The 15-minute weather code in the
FD12P 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.
Frozen Precipitation
Precipitation is frozen, if the ratio of optical intensity measurement to
DRD12 is high.
a. Snow: For snow, the intensity ratio (optical/DRD12) must
be larger than the Snow limit parameter and crossarm
surface temperature (TS) less than
+8 °C (warm limit).
b. Snow grains: For snow grains, the droplet signal size must
be less than the Snow grains limit in size. Also the DRD12
intensity must be very small as the small snow grains
easily bounce off the sensing surfaces.
c. Snow pellets: Snow pellets are bigger than snow grains,
but also cause very little signal on the DRD12. The Snow
pellets limit sets the minimum of the biggest particle
detected within a period of about two minutes.
d. Ice crystals: Ice crystals are even smaller than snow
grains. The Ice crystals limit sets the maximum particle
size for ice crystals. The ice crystals are identified from
the snow grain class by size. Almost no signal is detected
from the DRD12.
e. Hail: In principle, the intensity ratio is very high for hail
because hail is not melted by the DRD12 surfaces. In
practice, there is usually heavy rain with hail, which
makes the hail detection a little uncertain. Hail limit sets
the minimum particle size that is required for hail.