Visibility Types; Fog Codes; Table 6 Wmo Synop Code Limits - Vaisala PWD22 User Manual

User's Guide _______________________________________________________________________
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Visibility Types

The weather type is determined from visibility, when precipitation is
not detected. The visibility types for PWD22 are the following.
Table 6 WMO SYNOP Code Limits
Code 10-minute Visibility Average Weather Condition
CLEAR (00) Higher than or equal to 10 km. No precipitation
MIST (10) Higher than or equal to 1 km
but below 10 km.
HAZE, Higher than or equal to 1 km
SMOKE but below 10 km.
DUST, or
SAND
(04)
FOG Lower than 1 km.
(30)
HAZE, Lower than 1 km.
SMOKE,
DUST, or
SAND
(05)
WMO has not clearly defined the upper visibility limit for mist
reporting in SYNOP codes. PWD22 uses the 10000-meter upper limit
as this is the widely used limit by the Meteorological offices.

Fog Codes

The fog trend is calculated from the one-hour data. The average of the
last 20 minutes and the first 20 minutes of the data is 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.
Fog patches (code 31) are detected from rapid changes in visibility
across the one-kilometer fog limit. If 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.
No precipitation.
Reduction in visibility is
caused by moisture in the
air.
No precipitation.
Reduction in visibility is
caused by dry particles in
the air.
No precipitation.
Reduction in visibility is
caused by moisture in the
air.
No precipitation.
Reduction in visibility is
caused by moisture in the
air.