Vaisala FD12P - Manual (Page 113)

Chapter 5 _______________________________________________________ Functional Description
VAISALA _______________________________________________________________________ 113
However, if more than 30 minutes has elapsed from the last detected
precipitation, the PRECIPITATION DURING THE PRECEDING
HOUR code is used.
Weather Code Selection
The FD12P presents the weather type using the World Meteorological
Organization (WMO) code table 4680. Some code numbers have been
adopted from code table 4677 as these codes are not included in table
4680. Precipitation type is also reported using the United States
National Weather Service (NWS) abbreviations. The complete list of
the WMO and NWS codes used in the FD12P is presented in
Appendix A on page 143. The FD12P outputs present weather data
also in the METAR present weather codes (code table 4678).
The weather type is selected every 15 seconds using the principles
described earlier. The instant weather type will be selected from the
15-second types according to the WEATHER DELAY number of
samples by majority occurrences. The instant type is reported using
both the NWS and WMO codes.
For the 15-minute and one-hour WMO weather codes, the latest one
hour of instant precipitation types is stored in a buffer. The last
15-minute and one-hour parts of this table are then analyzed. The
number of each precipitation type in the buffer is counted.
The counts are stored in a table in decreasing order by the code value
(that is, the highest SYNOP code number first). This table is then
analyzed from top down by adding the counts together until a
minimum sum is obtained. The code number associated with the last
added count is the 15-minute or one-hour WMO code. The minimum
sum is different for the 15-minute code and the one-hour code.
Applications
In the SYNOP group 7W
a
W
a
W
1
W
2
, code W
a
W
a
is for weather at the
observation time, and W
1
and W
2
are for weather after the last main
observation (00, 06 ... ). The FD12P codes are directly intended to be
used for W
a
W
a
. The automactic weather station will generate W
1 and
W
2
.
The (automatic) weather station that uses the FD12P data, can in many
cases also use other weather parameters to select from the three codes
from the FD12P or even correct them. For example, the humidity