Vaisala FS11P - Fog Codes; Table 22 WMO SYNOP Code Limits; Visibility Types

Chapter 5 _________________________________________________________ Theory of Operation
VAISALA _______________________________________________________________________ 151
Visibility Types
The weather type is determined from visibility when precipitation is not
detected. The visibility types for PWD32 are shown in Table 24 on page
155.
Table 22 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.
No precipitation.
Reduction in visibility is
caused by moisture in the
air.
HAZE, SMOKE
DUST, or SAND
(04)
Higher than or equal to 1 km
but below 10 km.
No precipitation.
Reduction in visibility is
caused by dry particles in
the air.
FOG
(30)
Lower than 1 km.
No precipitation.
Reduction in visibility is
caused by moisture in the
air.
HAZE, SMOKE,
DUST, or SAND
(05)
Lower than 1 km.
No precipitation.
Reduction in visibility is
caused by dry particles in
the air.
WMO has not clearly defined the upper visibility limit for mist reporting
in SYNOP codes. PWD32 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, the internal
patch indicator is set for about 30 minutes.