Vaisala FD12P - Manual (Page 112)

User's Guide _______________________________________________________________________
112 _____________________________________________________ M210296en-A
a. Fog patches: Fog patches (code 31) are detected from
rapid changes in visibility across the one-kilometer fog
limit. If the visibility of the one-minute average is more
than twice or less than half of the two-minute average,
then the internal patch indicator is set for about 30
minutes.
b. Thinner fog: Fog is reported to become thinner (32) if
(latest 20-minute average - earliest 20-minute average) >
0, and the difference is greater than 30 % of the earliest
20-minute average.
c. Stable fog: Fog has been stable (33) if the Absolute (latest
20-minute average - earliest 20-minute average) is less
than 20 % of the lower average visibility.
d. Thicker fog: Fog is reported to become thicker (34) if
(latest 20-minute average - earliest 20-minute average) is
less than zero and the difference is more than 30 % of the
new 20-minute average.
Haze and Mist
If no precipitation is detected, a visibility code is generated. The
HAZE code (04) is used when the DRD12-estimated, relative
humidity is low and the visibility during the 10-minute average is less
than the Haze limit. If the estimated humidity is high, the MIST code
(10) is used instead of HAZE. If the estimated humidity is low and the
visibility during the 10-minute average is less than 1000 m, then code
05 (smoke, dust, or sand) is used.
When visibility increases above Haze limit × 1.2, the FD12P will
report CLEAR (code 00).
Weather Classes
The weather classes are continuous, showers, or intermittent.
The weather class is:
- Continuous, when there are less than two clear (no
precipitation) periods during the preceding hour, the
precipitation is continuous.
- Showers or intermittent, when there are more than two clear
periods during the preceding hour in the precipitation, the
weather type is intermittent.